Histology_Ross

Histology_Ross. Excessive sweating can lead to the loss of other electrolytes such as potassium and magnesium, as well as significant water loss. Normally, the body loses about 600 ml of water a day through evaporation from the lungs and skin. Under conditions of high ambient temperature, water loss can be increased in a regulated manner by an increased rate of sweating. This thermoregulatory sweating first occurs on the forehead and scalp, extends to the face and to the rest of the body, and occurs last on the palms and soles. Under conditions of emotional stress, however, the palms, soles, and axillae are the first surfaces to sweat. Control of thermoregulatory sweating is cholinergic, whereas emotional sweating may be stimulated by adrenergic portions of the sympathetic division of the autonomic nervous system. The secretory segment of the eccrine sweat gland contains three cell types.


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Histology_Ross. The secretory segment of the eccrine sweat gland contains three cell types. Three cell types are present in the secretory segment of the gland: clear cells and dark cells, both of which are secretory epithelial cells, and myoepithelial cells, which are contractile epithelial cells (Fig. 15.16 and Plate 45, page 521). All of the cells rest on the basal lamina; their arrangement is that of a pseudostratified epithelium.


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Histology_Ross.  Clear cells are characterized by abundant glycogen. The glycogen is conspicuous in Figure 15.16a because of its amount; it would stain intensely with the periodic acid–Schiff (PAS) method. In routine H&E preparations, the cytoplasm of clear cells stains poorly. Membranous organelles include numerous mitochondria, profiles of sER, and a relatively small Golgi apparatus. The plasma membrane is remarkably amplified at the lateral and apical surfaces by extensive cytoplasmic folds. In addition, the basal surface of the cell possesses infoldings, although they are considerably less complex than the cytoplasmic folds. The morphology of these cells indicates that they produce the watery component of sweat.


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Histology_Ross.  Dark cells are characterized by abundant rER and se cretory granules (see Fig. 15.16). The Golgi apparatus is relatively large, a feature consistent with the glyco protein secretion of these cells. The apical cytoplasm contains mature secretory granules and occupies most of the luminal surface (see Fig. 15.16a). Clear cells have considerably less cytoplasmic exposure to the lumen; their secretion is largely via the lateral surfaces of the cell, which are in contact with intercellular canaliculi that allow the watery secretion to reach the lumen. Here it mixes with the proteinaceous secretion of the dark cells.  Myoepithelial cells are limited to the basal aspect of the secretory segment. They lie between the secretory cells, with their processes oriented transversally to the tubule.


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Histology_Ross.  Myoepithelial cells are limited to the basal aspect of the secretory segment. They lie between the secretory cells, with their processes oriented transversally to the tubule. The cytoplasm contains numerous contractile filaments (actin) that stain deeply with eosin, thus making them readily identifiable in routine H&E specimens. Contraction of these cells is responsible for rapid expression of sweat from the gland. The duct segment of eccrine glands is lined by stratified cuboidal epithelium and lacks myoepithelial cells.


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Histology_Ross. The duct segment of the gland continues from the secretory portion with coiling. In histologic sections, multiple duct profiles typically appear among the secretory profiles. As the duct passes upward through the dermis, it takes a gentle spiral course until it reaches the epidermis, where it then continues in a tighter spiral to the surface. When the duct enters the epidermis, however, the duct cells end and the epidermal cells form the wall of the duct. The duct is composed of stratifed cuboidal epithelium, consisting of a basal cell layer and a luminal cell layer. The duct cells are smaller and appear darker than the cells of the secretory portion of the gland. Also, the duct has a smaller diameter than the secretory portion. In contrast to the secretory portion of the eccrine gland, the duct portion does not possess myoepithelial cells. These features are useful in distinguishing the duct from the secretory portion in a histologic section (see Fig. 15.15).


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Histology_Ross. The basal or peripheral cells of the duct have a rounded or ovoid nucleus and contain a prominent nucleolus. The cytoplasm is filled with mitochondria and ribosomes. The apical or luminal cells are smaller than the basal cells, but their nuclei are similar in appearance. The most conspicuous feature of the luminal cells is the deeply stained, glassy (hyalinized) appearance of the apical cytoplasm. The glassy appearance is attributable to the presence of large numbers of aggregated tonofilaments in the apical cytoplasm. Apocrine glands are large-lumen tubular glands associated with hair follicles. Apocrine sweat glands develop from the same down-growths of epidermis that give rise to hair follicles. The connection to the follicle is retained, allowing the secretion of the gland to enter the follicle, typically at a level just above the entry of the sebaceous duct. From here, the secretion makes its way to the surface.


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Histology_Ross. Like eccrine glands, apocrine glands are coiled tubular glands. They are sometimes branched. The secretory portion of the gland is located deep in the dermis or, more commonly, in the upper region of the hypodermis. The secretory portion of apocrine glands has a wider lumen than that of eccrine glands and is composed of a single cell type. The secretory portion of apocrine glands differs in several respects from that of eccrine glands. The most obvious difference, readily noted in the light microscope, is its very wide lumen (Fig. 15.17 and Plate 44, page 519). Unlike eccrine glands, apocrine glands store their secretory product in the lumen. The secretory portion of the gland is composed of simple


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Histology_Ross. FIGURE 15.16 • Electron micrographs of an eccrine sweat gland. a. This micrograph shows myoepithelial cells (My) and two distinctive gland cell types, dark cells (D) and clear cells (C). The apical portion of the dark cell is broad; it faces the lumen (L) of the gland and contains numerous secretory granules. The dashed line marks the boundary of one dark cell. The clear cell is more removed from the lumen of the gland. Its base rests on the myoepithelial cells or directly on the basal lamina. Most of the free surface of the clear cell faces an intercellular canaliculus (IC). Clear cells contain numerous mitochondria, extensive infoldings of the plasma membrane, and large numbers of electron-dense glycogen inclusions. 5,600. (Courtesy of Dr. John A. Terzakis.) b. At higher magnification, dark cells display rER (arrow) and a Golgi apparatus (G) in addition to secretory granules. Clear cells show large amounts of folded membrane, mitochondria, and glycogen. The myoepithelial cells (My)


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Histology_Ross. dark cells display rER (arrow) and a Golgi apparatus (G) in addition to secretory granules. Clear cells show large amounts of folded membrane, mitochondria, and glycogen. The myoepithelial cells (My) contain large numbers of contractile actin filaments. Short stubby arrows (upper right) mark the boundary of a clear cell. 17,500. (Courtesy of Dr. John A. Terzakis.)


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Histology_Ross. FIGURE 15.17 • Photomicrograph of an apocrine sweat gland. This section of adult skin from the area around the anus shows several apocrine (anal) sweat glands, which are easily identified by the large lumen of their secretory components. This apocrine sweat gland is close to a hair follicle (center of photomicrograph) and deep to the dense, irregular connective tissue of the dermis. 45. Inset. Higher magnification of secretory component shows the cell types of the apocrine gland. The gland consists of a simple epithelium whose cells are either cuboidal or columnar and myoepithelial cells located in the basal portion of the epithelial cell layer. 230.


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Histology_Ross. epithelium. Only one cell type is present, and the cytoplasm of the cell is eosinophilic. The apical part of the cell often exhibits a bleblike protrusion. It was once thought that this part of the cell pinched off and was discharged into the lumen as an apocrine secretion, thus the name of the gland. However, TEM studies confirm that the secretion is a merocrine type. The apical cytoplasm contains numerous small granules, the secretory component within the cell, which are discharged by exocytosis. Other features of the cell include numerous lysosomes and lipofuscin pigment granules. The latter represent secondary and tertiary lysosomes. Mitochondria are also numerous. During the refractory phase, after expulsion of the secretion, the Golgi apparatus enlarges in preparation for a new secretory phase.


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Histology_Ross. Myoepithelial cells are also present in the secretory portion of the gland and are situated between the secretory cells and the adjacent basal lamina. As in eccrine glands, contraction of the processes of myoid cells facilitates expulsion of the secretory product from the gland. The duct portion of apocrine glands is lined by stratified cuboidal epithelium and lacks myoepithelial cells. The duct of the apocrine gland is similar to that of the eccrine duct; it has a narrow lumen. However, it continues from the secretory portion of the gland in a relatively straight path to empty into the follicle canal. Because of its straight course, the probability of viewing both the duct and the secretory portion of an apocrine gland in the same histologic section is reduced. Also in contrast to the eccrine duct, resorption does not take place in the apocrine duct. The secretory product is not altered in its passage through the duct.


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Histology_Ross. The duct epithelium is stratified cuboidal, usually two but sometimes three cell layers thick. The apical cytoplasm of the luminal cells appears hyalinized, a consequence of the aggregated tonofilaments in the apical cytoplasm. In this aspect, they resemble the luminal cells of the eccrine duct. Apocrine glands produce a protein-rich secretion containing pheromones. Apocrine glands produce a secretion that contains protein, carbohydrate, ammonia, lipid, and certain organic compounds that may color the secretion. However, the secretions vary with the anatomic location. In the axilla, the secretion is milky and slightly viscous. When secreted, the fluid is odorless; through bacterial action on the skin surface, it develops an acrid odor.


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Histology_Ross. Apocrine glands become functional at puberty; as with axillary and pubic hair, their development depends on sex hormones. In the female, both axillary and areolar apocrine glands undergo morphologic and secretory changes that parallel the menstrual cycle. In many mammals, similar glands secrete pheromones, chemical signals used in marking territory, in courtship behavior, and in certain maternal and social behaviors. It is generally believed that apocrine secretions may function as pheromones in humans. Male pheromones (androstenol and androstenone) in the secretion of apocrine glands have a direct effect on the female menstruation cycle. Furthermore, female pheromones (copulins) inﬂuence male perception of females and may also induce hormonal changes in males. Innervation of sweat glands


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Histology_Ross. Innervation of sweat glands Both eccrine and apocrine sweat glands are innervated by the sympathetic portion of the autonomic nervous system. Eccrine sweat glands are stimulated by cholinergic transmitters (usually identified with the parasympathetic component of the autonomic system), whereas apocrine glands are stimulated by adrenergic transmitters. As described above, eccrine glands respond to heat and stress. Apocrine glands respond to emotional and sensory stimuli but not to heat. Nails are plates of keratinized cells containing hard keratin. The slightly arched fingernails and toenails, more properly referred to as nail plates, rest on nail beds. The nail bed consists of epithelial cells that are continuous with the stratum basale and stratum spinosum of the epidermis (Fig. 15.18 and Plate 47, page 525).


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Histology_Ross. The proximal part of the nail, the nail root, is buried in a fold of epidermis and covers the cells of the germinative zone, or matrix. The matrix contains a variety of cells, including stem cells, epithelial cells, melanocytes, Merkel’s cells, and Langerhans’ cells. The stem cells of the matrix regularly divide, migrate toward the root of the nail, and there differentiate and produce the keratin of the nail. Nail keratin is a hard keratin, like that of the hair cortex. Unlike the soft keratin of the epidermis, it does not desquamate. It consists of densely packed keratin filaments embedded in a matrix of amorphous keratin with a high sulfur content, which is responsible for the hardness of the nail. The process of hard keratin formation, as with the hair cortex, does not involve keratohyalin granules. In addition, a cornified cell envelope contains proteins similar to those found in the epidermis.


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Histology_Ross. FIGURE 15.18 • Photomicrograph of a sagittal section of distal phalanx with a nail. A nail is a keratinized plate located on the dorsal aspect of the distal phalanges. Under the free edge of the nail is a boundary layer, the hyponychium, which is continuous with the stratum corneum of the adjacent epidermis. The proximal end, the root of the nail, is overlapped by skin, the eponychium, which is also continuous with the stratum corneum of the adjacent epidermis. Deep to the nail is a layer of epithelium with underlying dermis. The proximal portion of this epithelium is referred to as the nail matrix. The bone in this section represents a distal phalanx. Numerous Pacinian corpuscles are present in the connective tissue of the palmar side of the finger. Note that even at this low magnification, the stratum lucidum is visible in the epidermis of the fingertip. 10.


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Histology_Ross. The constant addition of new cells at the root and their keratinization account for nail growth. As the nail plate grows, it moves over the nail bed. On the microscopic level, the nail plate contains closely packed interdigitating corneocytes lacking nuclei and organelles. The crescent-shaped white area near the root of the nail, the lunula, derives its color from the thick, opaque layer of partially keratinized matrix cells in this region. When the nail plate becomes fully keratinized, it is more transparent and takes on the coloring of the underlying vascular bed. The edge of the skin fold covering the root of the nail is the eponychium, or cuticle. The cuticle is also composed of hard keratin; therefore, it does not desquamate. Because of its thinness, it tends to break off or, as with many individuals, it is trimmed and pushed back. A thickened epidermal layer, the hyponychium, secures the free edge of the nail plate at the fingertip.


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Histology_Ross.  FOLDER 15.6 Clinical Correlation: Skin Repair


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Histology_Ross. The process of wound healing of the skin is classically described as either a primary or secondary union. Healing by primary union (frst intention) occurs after surgical in-cisions in which wounds that are usually clean and unin-fected have their edges approximated by surgical sutures. The healing by secondary union (secondary intention) occurs in traumatic wounds with separated edges, which are characterized by more extensive loss of cells and tis-sues. Wound healing in such cases involves generating a large amount of granulation tissue, which represents a specialized type of tissue formed during the repair process. The repair of an incision or laceration of the skin requires stimulated growth of both the dermis and the epidermis. Der-mal repair involves (1) blood clot formation, (2) removal of damaged collagen fibers, primarily through the effort of macrophage activity that is associated with inflammation, (3) formation of granulation tissue, (4) reepithelialization of the exposed


Histology_Ross

Histology_Ross. of damaged collagen fibers, primarily through the effort of macrophage activity that is associated with inflammation, (3) formation of granulation tissue, (4) reepithelialization of the exposed surface, (5) proliferation and migration of fibroblasts and differentiation of myofibroblasts involved in wound contraction, and (6) deposition and remodeling of the extracellular matrix of underlying connective tissue. Healing by primary union following application of sutures reduces the extent of the repair area through maximal closure of a wound, minimizing scar formation. Surgical incisions are typically made along cleavage lines; the cut tends to parallel the collagen fibers, thus minimizing the need for excess collagen production and the inherent scarring that may occur. Repair of the epidermis involves the proliferation of the basal keratinocytes in the stratum basale in the undamaged site surrounding the wound (Fig. F15.6.1). Mitotic activity is markedly increased within the first 24


Histology_Ross

Histology_Ross. involves the proliferation of the basal keratinocytes in the stratum basale in the undamaged site surrounding the wound (Fig. F15.6.1). Mitotic activity is markedly increased within the first 24 hours. In a short time, the wound site is covered by a scab that represents dehydrated blood clot. The proliferating basal cells of the stratum basale begin migrating beneath the scab and across the wound surface. The migration rate may be as much as 0.5 mm/day, starting within 8 to 18 hours after wounding. Further proliferation and differentiation occur behind the migration front, leading to restoration of the mul-tilayered epidermis. As new cells ultimately keratinize and desquamate, the overlying scab is freed with the desqua-mating cells, which explains why a scab detaches from its periphery inward. In cases in which the full thickness of the epidermal layer is removed either by trauma or in surgery, the parts of hair follicles, the follicular bulge that contains niche of the epidernal


Histology_Ross

Histology_Ross. inward. In cases in which the full thickness of the epidermal layer is removed either by trauma or in surgery, the parts of hair follicles, the follicular bulge that contains niche of the epidernal stem cells, will produce cells that migrate over the exposed surface to reestablish a complete epithelial (epidermal) layer. Massive destruction of all of the epithelial structures of the skin, as in a third-degree burn or extensive full-thickness abrasion, prevents reepithelialization. Such wounds can be healed only by grafting epidermis to cover the wounded area. In the absence of a graft, the wound would, at best, reepithelialize slowly and imperfectly by ingrowth of cells from the margins of the wound. FIGURE F15.6.1 • Photomicrograph showing a late stage in the epidermal repair of a skin wound. The initial injury was caused by an incision through the full thickness of the skin and partially into the hypodermis, which contains adipose cells (A). The epidermis has re-formed beneath the


Histology_Ross

Histology_Ross. wound. The initial injury was caused by an incision through the full thickness of the skin and partially into the hypodermis, which contains adipose cells (A). The epidermis has re-formed beneath the scab. The asterisk marks an artifact where epithelium separated during specimen preparation. The scab, which contains numerous dead neutrophils in its inferior aspect, is close to the point of release. The dermis at this stage shows little change during the repair process but will ultimately reestablish itself to form a continuous layer. 110. LLLL L*scabL*scabL*scabL *scabA A * scab


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Histology_Ross. The skin, or integument, consists of two main layers: the epidermis, composed of stratified squamous epithelium that is keratinized, and the dermis, composed of connective tissue. Under the dermis is a layer of loose connective tissue called the hypodermis, which is also generally referred to as the subcutaneous tissue or, by gross anatomists, as the superficial fascia. Typically, the hypodermis contains large amounts of adipose tissue, particularly in an adequately nourished individual. The epidermis gives rise to nails, hairs, sebaceous glands, and sweat glands. On the palms of the hands and soles of the feet, the epider-mis has an outer keratinized layer that is substantially thicker than that over the other parts of the body. Accordingly, the skin over the palms and soles is referred to as thick skin, in contrast to the skin over other parts of the body, which is referred to as thin skin. There are no hairs in thick skin. In addition, the interface between the epidermis and the


Histology_Ross

Histology_Ross. to as thick skin, in contrast to the skin over other parts of the body, which is referred to as thin skin. There are no hairs in thick skin. In addition, the interface between the epidermis and the dermis is more complex in thick skin than in thin skin. The fingerlike projections of the dermis into the base of the epidermis, the dermal papillae, are much longer and more closely spaced in thick skin. This provides greater resistance to frictional forces acting on this skin.


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Histology_Ross. Thick skin, human, H&E ×45. In this sample of thick skin, the epidermis (Ep) is at the top; the remainder of the field consists of dermis, in which a large number of sweat glands (SW) can be observed. Although the layers of the epidermis are examined more advantageously at higher magnification, it is easy to see, even at this relatively low magnification, that about half of the thickness of the epidermis consists of a distinctive surface layer that stains more lightly than the remainder of the epidermis. This is the keratinized layer. The dome-shaped surface contours represent a cross section through the minute ridges on the surface of thick skin that produce the characteristic fingerprints of an individual.


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Histology_Ross. In addition to sweat glands, the dermis displays blood vessels (BV) and adipose tissue (AT). The ducts of the sweat glands (D) extend from the glands to the epidermis. One of the ducts is shown as it enters the epidermis at the bottom of an epithelial ridge. It will pass through the epidermis in a spiral course to open onto the skin surface. Thin skin, human, H&E ×60. Often, as in this tissue sample, the hair follicles and the glands, both sebaceous and sweat, extend beyond the dermis (De) and into the hypodermis. A sample of thin skin is shown here to compare with the thick Note the blood vessels (BV) and adipose tissue (AT) in the hypodermis. skin in the above figure. In addition to sweat glands, thin skin contains hair follicles (HF) and their associated sebaceous glands (SGI). Each sebaceous gland opens into a hair follicle. Epidermis, skin, human, H&E ×320; inset ×640.


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Histology_Ross. The layers of the epidermis of thin skin are shown here at higher magnification. The cell layer that occupies the deepest location is the stratum basale (SB). This is one cell deep. Just above this is a layer several cells in thickness, the stratum spinosum (SS). It consists of cells that have spinous processes on their surface. These processes meet with spinous processes of neighboring cells and, together, appear as intercellular bridges (arrows, inset). The next layer is the stratum granulosum (SGr), whose cells contain keratohyalin granules (arrowhead, inset). On the surface is the stratum corneum (SC). This consists of keratinized cells, i.e., cells that no longer possess nuclei. The keratinized cells are flat and generally adhere to other cells above and below without evidence of cell boundaries. In thick skin, a fifth layer, the stratum lucidum, is seen between the stratum granulosum and the stratum corneum. The pigment in the cells of the stratum basale is melanin; some of this


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Histology_Ross. boundaries. In thick skin, a fifth layer, the stratum lucidum, is seen between the stratum granulosum and the stratum corneum. The pigment in the cells of the stratum basale is melanin; some of this pigment (P) is also present in connective tissue cells of the dermis.


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Histology_Ross. KEY AT, adipose tissue BV, blood vessels D, duct of sweat glands De, dermis Ep, epidermis HF, hair follicle P, pigment SB, stratum basale SC, stratum corneum SGl, sebaceous gland SGr, stratum granulosum SS, stratum spinosum SW, sweat gland arrowhead, granules in cell of stratum granulosum arrows, intercellular bridges


Histology_Ross

Histology_Ross. The epidermis contains four distinctive cell types: keratinocytes, melanocytes, Langerhans’ cells, and Merkel’s cells. Keratinocytes are the most numerous of these cells; they are generated in the stratum basale and move toward the surface. As they do so, they produce the intracellular protein keratin and the special extracellular lipid that serves as a water barrier in the upper layers of the epidermis. Histologically, the keratinocytes are the cells that show spinous processes in the stratum spinosum. The other three cell types are not readily identified in H&E–stained paraffin sections. The product of the melanocyte is, however, evident in H&E sections, and this is considered in the first two figures of this plate. The skin contains a pigment, melanin, which protects the tissue against the harmful effects of ultraviolet light. It is formed by the melanocytes that then pass the pigment to the keratinocytes. More pigment is present in dark skin than in light skin; this can be seen by


Histology_Ross

Histology_Ross. the harmful effects of ultraviolet light. It is formed by the melanocytes that then pass the pigment to the keratinocytes. More pigment is present in dark skin than in light skin; this can be seen by com-paring light skin (top figure) and dark skin (middle figure). The epidermis and a small amount of the dermis are shown in each figure. Whereas the deep part of the dark skin contains considerable pigment, the amount of pigment in light skin is insufficient to be noticeable at this magni-fication. Cells for producing the pigment are present in both skin types and in approximately equal numbers. The difference is due to more rapid digestion of the pigment by lysosomes of keratinocytes in light skin. After prolonged exposure to sunlight, pigment is also produced in sufficient amounts to be seen in light skin.


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Histology_Ross. Light skin, human, H&E ×300. may also appear as clear cells, but they are located more superficially in the stratum spinosum. Merkel’s cells may also appear as clear cells, thus mak-In routine H&E–stained paraffin sections of light skin, such ing it difficult to identify these three cell types with certainty. as this sample, the melanocytes are among the cells that appear as small, rounded, clear cells (CC) mixed with the other cells of the stratum basale. However, not all clear cells of the epidermis are melanocytes. For example, Langerhans’ cells Dark skin, human, H&E ×300. corneum. In light skin, the melanin is broken down before it leaves the upper part of the stratum spinosum. Thus, pigment is not seen in the upper In dark skin, most of the pigment is in the basal portion of the layers of the epidermis.


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Histology_Ross. In dark skin, most of the pigment is in the basal portion of the layers of the epidermis. epidermis, but it is also present in cells progressing toward the surface and within the nonnucleated cells of the keratinized layer. The arrows indicate the melanin pigment in keratinocytes of the stratum spinosum and in the stratum Dermis, skin, human, H&E and elastin stain ×200; inset ×450.


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Histology_Ross. Dermis, skin, human, H&E and elastin stain ×200; inset ×450. This figure is included because it shows certain features of the dermis, the connective tissue layer of the skin. The dermis is divided into two layers: the papillary layer (PL) of loose connective tissue and the reticular layer (RL) of more dense connective tissue. The papillary layer is immediately under the epidermis. It includes the connective tissue papillae that project into the undersurface of the epidermis. The reticular layer is deep to the papillary layer. The boundary between these two layers is not demarcated by any specific structural feature except for the change in the histologic composition of the two layers.


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Histology_Ross. This specimen was stained with H&E and also with a procedure to display elastic fibers (EF). They are relatively thick and conspicuous in the reticular layer (see also inset), where they appear as the dark-blue profiles, some of which are elongate, whereas others are short. In the papillary layer, the elastic fibers are thinner and relatively sparse (arrows). The inset shows the typical eosinophilic staining of the thick collagenous fibers in the reticular layer. Although the collagenous fibers at the lower magnification of this figure are not as prominent, it is nevertheless possible to note that they are thicker in the reticular layer than in the papillary layer. The papillary layer is evidently more cellular than the reticular layer. Many of the small dark-blue profiles in the reticular layer represent oblique and cross sections through elastic fibers (see inset) and not nuclei of cells.


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Histology_Ross. KEY CC, clear cells EF, elastic fibers PL, papillary layer RL, reticular layer arrows, middle figure pigment in different layers of epidermis; Lower figure delicate elastic fibers


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Histology_Ross. Skin possesses three types of glands: eccrine, apocrine and sebaceous. Eccrine sweat glands are distributed over the entire body surface except for the lips, glans penis, prepuce, clitoris, and labia minora. They are especially numerous in the thick skin of the hands and feet. Evaporation of the secreted sweat on the skin surface cools the body. Apocrine sweat glands are localized in the axilla, areolae, perineal and perianal area, prepuce, scrotum, mons pubis, and labia majora. Many of the epithelial cells in the secretory segment of these glands exhibit an apical bleb-like protrusion that was earlier thought to represent their mode of secretion, i.e., pinching off of the bleb as the secretory product, thus the name apocrine. It is now known that secretion occurs as a merocrine process. The secretion is a clear, viscous product that becomes odife-rous through the action of resident microbes on the skin surface. In the human, its role is unclear, but it is generally believed that the


Histology_Ross

Histology_Ross. The secretion is a clear, viscous product that becomes odife-rous through the action of resident microbes on the skin surface. In the human, its role is unclear, but it is generally believed that the secretion may act as a sex attractant (pheromone). Apocrine glands are present at birth but do not fully develop and become functional until puberty. In the female, these glands undergo changes that parallel the menstrual cycle. ORIENTATION MICROGRAPH: The adjacent orientation micrograph of the skin of the axilla shows both the large, branched tubular apocrine glands (A) and the smaller, simple tubular eccrine glands (E) in the hypodermis (H). Also evident is a tangentially cut hair follicle (HF). The overlying dermis (D) consists of dense connec-tive tissue and includes part of a sebaceous gland (SG). AAEESGDHFHA A E E SG D HF H


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Histology_Ross. Apocrine sweat gland, skin, human, H&E, ×33. image are two sweat glands (SwG) also surrounded by dense connective tissue. Note the considerable difference in diameter and lumen size of the Low-power micrograph showing the secretory segment of an two types of glands. apocrine sweat gland. The sectioned profiles seen here represent several coiled branches of a single gland surrounded by dense connective tissue (DCT). In the upper part of this Apocrine sweat gland, skin, human, H&E, ×256. The epithelium (Ep) of the apocrine sweat gland from the boxed area to the left is simple columnar. The individual cells vary in height, and some show the bleb-like protrusions (B). At the base of the epithelium are the spindle-shaped myoepithelial cells. In some regions of the tubule, these cells have been cut longitudinally and thus appear as a deeply stained eosin band (EB). At other sites, the cells have been sectioned tangentially and appear as a series of parallel linear profiles (MyC).


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Histology_Ross. Eccrine sweat gland, skin, human, H&E, ×512. At this very high magnification, two cross-sectioned profiles of the secretory segment (SS) and one profile of the duct segment (DS) from the boxed area to the left are shown. When the tubule wall of the secretory segment is cut in a perpendicular plane, the simple columnar nature of the epithelium (Ep) is evident. Because the tubule is so tortuous, more often the epithelium appears to be multilayered. The myoepithelial cells of the secretory segment appear here both as a circumferential band (CB) and in a cross-sectioned array (CA) in which they resemble the teeth of a saw blade. Occasionally, myoepithelial cell nuclei (MyN) are present. Such profiles give the appearance of a pseudostratified epithelium. The duct segment (DS) lacks the myoepithelium and also differs in that it is stratified cuboidal. See next plate.


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Histology_Ross. KEY A, apocrine gland B, bleb-like protrusions CA, cross-sectioned array CB, circumferential band D, dermis DCT, dense connective tissue DS, duct segment E, eccrine gland EB, eosin band Ep, epithelium H, hypodermis HF, hair follicle MyC, linear profiles of myoepithelial cells MyN, myoepithelial cell nuclei SG, sebaceous gland SS, secretory segment SwG, sweat glands


Histology_Ross

Histology_Ross. Normally, the body loses ~600 mL of water a day through evaporation from the lungs and skin. Under conditions of high ambient temperature, water loss is increased by an increased rate of sweating. This thermoregulatory sweating first occurs on the forehead and scalp, extends to the face and the rest of the body, and occurs last on the palms and soles. Emotional sweating, however, occurs first on the palms and soles and in the axillae. Sweating is under both nervous control through the autonomic nervous system and hormonal control. Sebaceous glands secrete sebum, an oily substance that coats the hair and skin surface. Sebaceous secretion is a holocrine secretion; the entire cell produces, and becomes filled with, the fatty secretory product while it simultaneously undergoes progressive disruption, fol-lowed by necrosis, as the product fills the cell. Both secretory product and cell debris are discharged into the pilosebaceous canal. Sweat gland, skin, human, H&E ×1000.


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Histology_Ross. Sweat gland, skin, human, H&E ×1000. This section through a sweat gland shows five profiles of the ductal portion (D) and two profiles of the secretory portion (SG). The larger secretory segment is through a region either just below or above where a U turn was made; therefore, it shows two luminal profiles. The lumina of both the ductal 520 and the secretory units are marked by asterisks. The glandular unit of the eccrine sweat gland contains two epithelial cell types and myoepithelial cells (M). Arrowheads show small cross sections of myoepithelial cell cytoplasm; large arrows show where more elongate profiles of myoepithelial cytoplasm are evident. The epithelial cells are of Sebaceous gland, skin, human, H&E ×160.


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Histology_Ross. Sebaceous gland, skin, human, H&E ×160. Sebaceous glands develop from the epithelial cells of the hair follicle and discharge their secretion into the follicle, from where it reaches the skin surface. The sebaceous secretion is rich in lipid, and this is reflected in the cells of the sebaceous gland. A section of a sebaceous gland and its related hair folli cle is shown in this figure. At this level, the hair follicle consists of the ex- Sebaceous gland, skin, human, H&E ×320.


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Histology_Ross. The same sebaceous gland as in figure above is shown here at higher magnification. Numbers 1 to 4 show a series of cells filled with an increasingly greater amount of lipid and progressively closer to the opening of the gland into the hair follicle. The sebaceous secretion includes the entire cell, and two types, designated dark cells and clear cells. Unfortunately, the characteristic dark cytoplasmic staining of the dark cells is not evident unless special precautions are taken to preserve the secretory granules in their apical cytoplasm. Nevertheless, note that the dark cells are closer to the lumen, whereas the clear cells are closer to the base of the epithelial layer, making contact with either the basal lamina or, more frequently, the myoepithelial cells. In addition, the clear cells are in contact with intercellular canaliculi. Several such intercellular canaliculi are shown in the secretory units (small arrows). This figure also shows that the duct consists of two layers of


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Histology_Ross. are in contact with intercellular canaliculi. Several such intercellular canaliculi are shown in the secretory units (small arrows). This figure also shows that the duct consists of two layers of small cuboidal cells.


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Histology_Ross. PLATE 45 • SW EAT AN D S E BACEOUS G LAN DS KEY BC, basal cells CT, connective tissue D, duct of eccrine sweat gland eRS, junction between sebaceous gland and external root sheath M, myoepithelial cell RS, external root sheath of hair follicle Seb, sebaceous gland SG, secretory component of eccrine sweat gland arrowheads, myoepithelial cell cytoplasm (cross section) asterisks, lumina of glands and ducts large arrows, myoepithelial cell cytoplasm (longitudinal section) numbers 1 to 4 (lower right image), see text small arrows, intercellular canaliculi ternal root sheath (RS) surrounding the hair shaft. The sebaceous gland (Seb) appears as a cluster of cells, most of which display a washed-out or finely reticulated cytoplasm. This is because these cells contain numerous lipid droplets and the lipid is lost by dissolution in fat solvents during the routine preparation of the H&E–stained paraffin section. The opening of the sebaceous gland through the external root sheath (eRS) and into


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Histology_Ross. the lipid is lost by dissolution in fat solvents during the routine preparation of the H&E–stained paraffin section. The opening of the sebaceous gland through the external root sheath (eRS) and into the hair follicle is shown in the lower right.


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Histology_Ross. therefore, cells need to be replaced constantly in the functional gland. Cells at the periphery of the gland are basal cells (BC). Dividing cells in the basal layer replace those that are lost with the secretion.


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Histology_Ross. The skin is endowed with numerous sensory receptors of various types. These are the peripheral terminals of sensory nerves whose cell bodies are in the dorsal root ganglia. The receptors in the skin are described as free nerve endings and encapsulated nerve endings. Free nerve endings are the most numerous. They subserve fine touch, heat, and cold and are found in the basal layers of the epidermis and as a network around the root sheath of hair follicles. Encapsulated nerve endings include Pacinian corpuscles (deep pressure), Meissner’s corpuscles (touch, especially in the lips and thick skin of fingers and toes), and Ruffini endings (sustained mechanical stress on the dermis). Motor endings of the autonomic nervous system supply the blood vessels, the arrector pili muscles, and the apocrine and eccrine sweat glands. Skin, fngertip, human, H&E ×20.


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Histology_Ross. Skin, fngertip, human, H&E ×20. This specimen is a section of thick skin from the finger tip, showing the epidermis (Ep) and the dermis (De) and, under the skin, a portion of the hypodermis (Hy). The thickness of the epidermis is largely due to the thickness of the stratum corneum. This layer is more lightly stained than the deeper 522 portions of the epidermis. Note, even at this low magnification, the thick collagenous fibers in the reticular layer of the dermis. Sweat glands (SG) are present in the upper part of the hypodermis, and several sweat ducts (D) are seen passing through the epidermis. A feature of this specimen is that it depicts those sensory receptors that can be recognized in a routine low-power H&E–stained paraffin section. Meissner's corpuscle, skin, human, H&E ×190.


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Histology_Ross. Meissner's corpuscle, skin, human, H&E ×190. This high-magnification micrograph shows portions of the upper left field of figure above in which two Meissner’s corpuscles (MC) are in direct proximity to the undersurface of the epidermis in adjacent dermal papillae. The section shows the long axis of the corpuscles. A Meissner’s corpuscle consists of an axon (sometimes two) taking a zigzag or flat spiral course They are Meissner’s corpuscles and Pacinian corpuscles (PC). Several nerve bundles (N) are seen in proximity to the Pacinian corpuscles. Meissner’s corpuscles are in the upper part of the dermis, in the dermal papillae immediately under the epidermis. These corpuscles are small and difficult to identify at this low magnification; however, their location is characteristic. Knowing where they are located is a major step in finding Meissner’s corpuscles in a tissue section; they are shown at higher magnification in figure below.


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Histology_Ross. Pacinian corpuscles are seen in the lower part of the hypodermis. These corpuscles are large, slightly oval structures, and even at low magnification, a layered or lamellated pattern can be discerned. Pacinian corpuscle, skin, human, H&E ×320. At this higher magnification, the concentric layers or lamellae of the Pacinian corpuscle can be seen to be due to flat cells. These are fibroblast-like cells, and although not evident within the tissue section, these cells are continuous with the perineurium of the nerve fiber. The space between the cellular lamellae contains mostly fluid. The neural portion of the Pacinian corpuscle travels longitudinally through the center of the corpuscle. In this specimen, the corpuscle has been cross-sectioned; an arrowhead points to the centrally located nerve fiber.


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Histology_Ross. from one pole of the corpuscle to the other. The nerve fiber terminates at the superficial pole of the corpuscle. Consequently, as seen here, the nerve fibers and supporting cells are oriented approximately at right angles to the long axis of the corpuscle. Meissner’s corpuscles are particularly numerous near the tips of the fingers and toes. Meissner's corpuscle, skin, human, H&E ×550. seen) and its supporting cells is evident here, as is the fibrous capsule (FC) that surrounds the ending. At the even higher magnification of this figure, the close apposition of Meissner’s corpuscle to the undersurface of the epidermis is well demonstrated throughout the entire area of the dermal papilla. The flat spiral path of the neuron (not PLATE 46KEY D, ducts of sweat glands De, dermis Ep, epidermis FC, fibrous capsule Hy, hypodermis MC, Meissner’s corpuscles. N, nerve bundles PC, Pacinian corpuscles SG, sweat glands arrowhead, nerve fiber in center of Pacinian corpuscle


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Histology_Ross. Hairs are composed of keratinized cells that develop from hair follicles. Hairs are present over almost the entire body, being conspicuously absent only from the sides and palmar surfaces of the hands, from the sides and plantar surfaces of the feet, from the lips, and from the skin around the urogenital orifices. Coloration of the hair is due to the content and type of melanin that it contains. The follicle varies in appear-ance, depending on whether it is in a growing or a resting phase; the growing follicle is the more elaborate. The skin appendages (adnexa), especially hair follicles and sweat glands, are particularly important in healing of skin wounds. They serve as the source of new epithelial cells when there is extensive loss of epidermis, as in deep abrasions and second-degree burns. Hair follicle, skin, human, H&E ×300; inset ×440.


Histology_Ross

Histology_Ross. The growing end of a hair follicle consists of an expanded bulb of epithelial cells that is invaginated by a papilla (HP) of connective tissue. The epithelial cells surrounding the papilla at the very tip of the follicle are not yet specialized; they constitute the matrix, the region of the hair follicle where cell division occurs. As the cells leave the matrix, they form cell layers that will 524 become the shaft of the hair and the inner and outer root sheaths of the hair follicle. The cells that will develop into the shaft of the hair are seen just to the right of the expanded bulb. They constitute the cortex (C), medulla (M), and cuticle (asterisks) of the hair. The cells of the cortex become keratinized. This layer will come to constitute most of the hair shaft as a thick cylinder. The medulla forms the centrally located axis of the hair shaft; it does not always extend through the entire length of the hair and is absent from some hairs. The cuticle consists of overlapping cells


Histology_Ross

Histology_Ross. medulla forms the centrally located axis of the hair shaft; it does not always extend through the entire length of the hair and is absent from some hairs. The cuticle consists of overlapping cells that ultimately lose their nuclei and become filled with keratin. The cuticle covers the hair shaft like a layer of overlapping shingles.


Histology_Ross

Histology_Ross. The root sheath (RS) has two parts: the outer root sheath, which is continuous with the epidermis of the skin, and the inner root sheath, which extends only as far as the level at which sebaceous glands enter the hair follicle. The inner root sheath is further divided into three layers: Henle’s layer, Huxley’s layer, and the cuticle of the inner root sheath. These layers are seen in the growing hair follicle and are shown at higher magnification in the inset with numbers 1 to 5: 1, cells of the outer root sheath; 2, Henle’s layer; 3, Huxley’s layer; 4, cuticle of the inner root sheath; and 5, future cuticle of the hair. Many of the cells of the growing hair follicle contain pigment that contributes to the color of the hair. Most of this pigment is inside the cell (inset); however, in very dark hair some pigment is also extracellular. The connective tissue surrounding the hair follicle forms a distinct layer referred to as the sheath, or dermal sheath (DS), of the hair follicle.


Histology_Ross

Histology_Ross. The connective tissue surrounding the hair follicle forms a distinct layer referred to as the sheath, or dermal sheath (DS), of the hair follicle. Nail, skin, human, H&E ×12. A nail is a keratinized plate located on the dorsal aspect of the distal phalanges. A section through a nail plate is shown here. The nail itself (N) is difficult to stain. Under the free edge of the nail is a boundary layer, the hyponychium (Hypon), which is continuous with the stratum corneum of the adjacent epidermis. The proximal end of the nail is overlapped by skin; here, the junctional region is called the eponychium (Epon) and is also continuous with the stratum corneum of the adjacent epidermis. Under the nail is a layer of epithelium, the proximal portion of which is referred to as the nail matrix (NM). The cells of the nail matrix function in the growth of the nail.


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Histology_Ross. Together, the epithelium under the nail and the underlying dermis (D) constitute the nail bed. The proximal portion of the nail, covered by the fold of the skin, is the root of the nail (NR). The relationship of the nail to other structures in the fingertip is also shown in this figure. The bone (B) in the specimen represents a distal phalanx. Note that in this bone there is an epiphyseal growth plate (EP) at the proximal extremity of the bone but not at the distal extremity. Numerous Pacinian corpuscles (PC) are present in the connective tissue of the palmar side of the finger. Also seen to advantage in this section is the stratum lucidum (SL) in the epidermis of the thick skin of the fingertip.


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Histology_Ross. PLATE 47 KEY B, bone C, cortex D, dermis DS, dermal sheath EP, epiphyseal plate Epon, eponychium HP, dermal papilla of hair follicle Hypon, hyponychium M, medulla N, nail or nail plate NM, nail matrix NR, nail root PC, Pacinian corpuscles RS, root sheath SL, stratum lucidum asterisks, cuticle of hair numbers: 1, external root sheath; 2, Henle’s layer; 3, Huxley’s layer; 4, cuticle of inner root sheath; 5, future cuticle of the hair Digestive System I: Oral Cavity and Associated Structures


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Histology_Ross. Digestive System I: Oral Cavity and Associated Structures OVERVIEW OF THE DIGESTIVE SYSTEM / 526 ORAL CAVITY / 527 TONGUE / 529 TEETH AND SUPPORTING TISSUES / 534 Enamel / 536 Cementum / 539 Dentin / 539 Dental Pulp and Central Pulp Cavity (Pulp Chamber) / 543 Supporting Tissues of the Teeth / 544 SALIVARY GLANDS / 545 Secretory Gland Acini / 546 Salivary Ducts / 549 Major Salivary Glands / 550 Parotid Gland / 550 Submandibular Gland / 551 Sublingual Gland / 551 Saliva / 551 Folder 16.1 Clinical Correlation: The Genetic Basis of Taste / 533 Folder 16.2 Clinical Correlation: Classification of Permanent (Secondary) and Deciduous (Primary) Dentition / 534 Folder 16.3 Clinical Correlation: Dental Caries / 547 Folder 16.4 Clinical Correlation: Salivary Gland Tumors / 555 The digestive system consists of the alimentary canal and its principal associated organs, namely, the tongue, teeth, salivary glands, pancreas, liver, and gallbladder.


Histology_Ross

Histology_Ross. The digestive system consists of the alimentary canal and its principal associated organs, namely, the tongue, teeth, salivary glands, pancreas, liver, and gallbladder. The lumen of the alimentary canal is physically and functionally external to the body. As it passes through the alimentary canal, food is broken down physically and chemically so that the degraded products can be absorbed into the body. The various segments of the alimentary canal are morphologically specialized for specific aspects of digestion and absorption.


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Histology_Ross. After preliminary maceration, moistening, and formation into a bolus by the actions of the structures of the oral cavity and salivary glands, food passes rapidly through the pharynx to the esophagus. The rapid passage of food through the pharynx keeps it clear for the passage of air. The food passes more slowly through the gastrointestinal tract, and during its transit through the stomach and small intestine, the major alterations associated with digestion, solubilization, and absorption occur. Absorption occurs chiefly through the wall of the small intestine. Undigested food and other substances within the alimentary canal, such as mucus, bacteria, desquamated cells, and bile pigments, are excreted as feces. The alimentary mucosa is the surface across which most substances enter the body.


Histology_Ross

Histology_Ross. The alimentary mucosa is the surface across which most substances enter the body. The alimentary mucosa performs numerous functions in its role as an interface between the body and the environment. These functions include the following:  Secretion. The lining of the alimentary canal secretes, at specific sites, digestive enzymes, hydrochloric acid, mucin, and antibodies.  Absorption. The epithelium of the mucosa absorbs metabolic substrates (e.g., the breakdown products of digestion) as well as vitamins, water, electrolytes, recyclable materials such as bile components and cholesterol, and other substances essential to the functions of the body.  Barrier. The mucosa serves as a barrier to prevent the entry of noxious substances, antigens, and pathogenic organisms. Immunologic protection. Lymphatic tissue within the mucosa serves as the body’s first line of immune defense.


Histology_Ross

Histology_Ross. The functions listed above are discussed at the beginning of the next chapter. The digestive system is considered in three chapters that deal, respectively, with the oral cavity and pharynx (this chapter); the esophagus and gastrointestinal tract (Chapter 17); and the liver, gallbladder, and pancreas (Chapter 18). The oral cavity consists of the mouth and its structures, which include the tongue, teeth and their supporting structures (periodontium), major and minor salivary glands, and tonsils. The oral cavity is divided into a vestibule and the oral cavity proper. The vestibule is the space between the lips, cheeks, and teeth. The oral cavity proper lies behind the teeth and is bounded by the hard and soft palates superiorly, the tongue and the floor of the mouth inferiorly, and the entrance to the oropharynx posteriorly.


Histology_Ross

Histology_Ross. Each of the three major salivary glands are paired structures; they include the following:  Parotid gland, the largest of the three glands, located in the infratemporal region of the head. Its excretory duct, the parotid (Stensen’s) duct, opens at the parotid papilla, a small elevation on the mucosal surface of the cheek opposite the second upper molar tooth.  Submandibular gland, located in the submandibular triangle of the neck. Its excretory duct, the submandibular (Wharton’s) duct, opens at a small fleshy prominence (the sublingual caruncle) on each side of the lingual frenulum on the floor of the oral cavity.  Sublingual gland, lying inferior to the tongue within the sublingual folds at the floor of the oral cavity. It has a number of small excretory ducts; some enter the submandibular duct, and others enter individually into the oral cavity.


Histology_Ross

Histology_Ross. The parotid and submandibular glands have relatively long ducts that extend from the secretory portion of the gland to the oral cavity. The sublingual ducts are relatively short. The minor salivary glands are located in the submucosa of the oral cavity. They empty directly into the cavity via short ducts and are named for their location (i.e., buccal, labial, lingual, and palatine). The tonsils consist of aggregations of lymphatic nodules that are clustered around the posterior opening of the oral and nasal cavities.


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Histology_Ross. The tonsils consist of aggregations of lymphatic nodules that are clustered around the posterior opening of the oral and nasal cavities. Lymphatic tissue is organized into a tonsillar (Waldeyer’s) ring of immunologic protection located at the shared entrance to the digestive and respiratory tracts. This lymphatic tissue surrounds the posterior orifice of the oral and nasal cavities and contains aggregates of lymphatic nodules that include the following:  Palatine tonsils, or simply the tonsils, which are located at either side of the entrance to the oropharynx between the palatopharyngeal and palatoglossal arches  Tubal tonsils, which are located in the lateral walls of the nasopharynx posterior to the opening of the auditory tube  Pharyngeal tonsil, or adenoid, which is located in the roof of the nasopharynx  Lingual tonsil, which is located at the base of the tongue


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Histology_Ross. The oral cavity is lined by the oral mucosa that consists of masticatory mucosa, lining mucosa, and specialized mucosa. The masticatory mucosa is found on the gingiva (gums) and the hard palate (Fig. 16.1). It has a keratinized and, in some areas, a parakeratinized stratifed squamous epithelium (Fig. 16.2). Parakeratinized epithelium is similar to keratinized FIGURE 16.1 • Roof of oral cavity. The hard palate, which contains bone, is bisected into right and left halves by a raphe. Anteriorly, in the fatty zone, the submucosa of the hard palate contains adipose tissue; posteriorly, in the glandular zone, there are mucous glands within the submucosa. Neither the raphe nor the gingiva contains a submucosa; instead, the mucosa is attached directly to the bone. The soft palate has muscle instead of bone, and its glands are continuous with those of the hard palate in the submucosa. (Based on Bhaskar SN, ed. Orban’s Oral Histology and Embryology. St. Louis: CV Mosby, 1991.)


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Histology_Ross. FIGURE 16.2 • Stratified squamous epithelium of the hard palate. This photomicrograph shows a transition in the oral mucosa from a stratified squamous epithelium (on the right) to a stratified squamous parakeratinized epithelium (on the left). The flattened surface cells of the keratinized epithelium are devoid of nuclei. The layer of keratohyalin granule–containing cells is clearly visible in this type of epithelium. The flattened surface cells of the parakeratinized epithelium display the same characteristics as the keratinized cells, except they retain their nuclei, i.e., they are parakeratinized. In addition, note the paucity of keratohyalin granules present in the subsurface cells. 380.


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Histology_Ross. epithelium except that the superficial cells do not lose their nuclei and their cytoplasm does not stain intensely with eosin (Plate 48, page 557). The nuclei of the parakeratinized cells are pyknotic (highly condensed) and remain until the cell is exfoliated (see Fig. 16.2). The keratinized epithelium of the masticatory mucosa resembles that of the skin but lacks a stratum lucidum. The underlying lamina propria consists of a thick papillary layer of loose connective tissue that contains blood vessels and nerves, some of which send bare axon endings into the epithelium as sensory receptors, and some of which end in Meissner’s corpuscles. Deep to the lamina propria is a reticular layer of more-dense connective tissue.


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Histology_Ross. As in the skin, the depth and number of connective tissue papillae contribute to the relative immobility of the masticatory mucosa, thus protecting it from frictional and shearing stress. At the midline of the hard palate, in the palatine raphe, the mucosa adheres firmly to the underlying bone. The reticular layer of the lamina propria blends with the periosteum, and thus there is no submucosa. The same is true of the gingiva. Where there is a submucosa underlying the lamina propria on the hard palate (see Fig. 16.1), it contains adipose tissue anteriorly (fatty zone) and mucous glands posteriorly (glandular zone) that are continuous with those of the soft palate. In the submucosal regions, thick collagenous bands extend from the mucosa to the bone.


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Histology_Ross. Lining mucosa is found on the lips, cheeks, alveolar mucosal surface, floor of the mouth, inferior surfaces of the tongue, and soft palate. At these sites it covers striated muscle (lips, cheeks, and tongue), bone (alveolar mucosa), and glands (soft palate, cheeks, inferior surface of the tongue). The lining mucosa has fewer and shorter papillae so that it can adjust to the movement of its underlying muscles.


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Histology_Ross. Generally, the epithelium of the lining mucosa is nonkeratinized, although in some places it may be parakeratinized. The epithelium of the vermilion border of the lip (the reddish portion between the moist inner surface and the facial skin) is keratinized. The nonkeratinized lining epithelium is thicker than keratinized epithelium. It consists of only three layers:  Stratum basale, a single layer of cells resting on the basal lamina  Stratum spinosum, which is several cells thick  Stratum superfciale, the most superficial layer of cells, also referred as the surface layer of the mucosa The cells of the mucosal epithelium are similar to those of the epidermis of the skin and include keratinocytes, Langerhans’ cells, melanocytes, and Merkel’s cells.


Histology_Ross

Histology_Ross. The cells of the mucosal epithelium are similar to those of the epidermis of the skin and include keratinocytes, Langerhans’ cells, melanocytes, and Merkel’s cells. The lamina propria contains blood vessels, nerves that send bare axon endings into the basal layers of the epithelium, and encapsulated sensory endings in some papillae. The sharp contrast between the numerous deep papillae of the alveolar mucosa and the shallow papillae in the rest of the lining mucosa allows easy identification of the two different regions in a histologic section.


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Histology_Ross. A distinct submucosa underlies the lining mucosa except on the inferior surface of the tongue. This layer contains large bands of collagen and elastic fibers that bind the mucosa to the underlying muscle; it also contains the many minor salivary glands of the lips, tongue, and cheeks. Occasionally, sebaceous glands not associated with a hair follicle are found in the submucosa just lateral to the corner of the mouth and in the cheeks opposite the molar teeth. They are visible to the eye and are called Fordyce spots. The submucosa contains the larger blood vessels, nerves, and lymphatic vessels that supply the subepithelial neurovascular networks in the lamina propria throughout the oral cavity. Specialized mucosa is associated with the sensation of taste and is restricted to the dorsal surface of the tongue. It contains papillae and taste buds responsible for generating the chemical sensation of taste.


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Histology_Ross. Oral mucosa forms an important protective barrier between the external environment of the oral cavity and internal environments of the surrounding tissues. It is resistant to the pathogenic organisms that enter the oral cavity and to indigenous microorganisms residing there as microbial ﬂora. Epithelial cells, migratory neutrophils, and saliva all contribute to maintaining the health of the oral cavity and protecting the oral mucosa from bacterial, fungal, and viral infections. The protective mechanisms include several salivary antimicrobial peptides, the -defensins expressed in the epithelium, the -defensins expressed in neutrophils, and the secretory immunoglobin A (sIgA). However, in individuals with immunodeficiency or undergoing antibiotic therapy, in which the balance between microorganisms and protective mechanisms is disrupted, oral infections are rather common.


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Histology_Ross. The tongue is a muscular organ projecting into the oral cavity from its inferior surface. Lingual muscles (i.e., muscles of the tongue) are both extrinsic (having one attachment outside of the tongue) and intrinsic (confined entirely to the tongue, without external attachment). The striated muscle of the tongue is arranged in bundles that generally run in three planes, with each arranged at right angles to the other two. This arrangement of muscle fibers allows enormous flexibility and precision in the movements of the tongue, which are essential to human speech as well as to its role in digestion and swallowing. This form of muscle organization is found only in the tongue, which allows easy identification of this tissue as lingual muscle. Variable amounts of adipose tissue are found among the muscle fiber groups.


Histology_Ross

Histology_Ross. Grossly, the dorsal surface of the tongue is divided into an anterior two thirds and a posterior one third by a V-shaped depression, the sulcus terminalis (Fig. 16.3). The apex of the V points posteriorly and is the location of the foramen cecum, the remnant of the site from which an evagination of the floor of the embryonic pharynx occurred to form the thyroid gland. Papillae cover the dorsal surface of the tongue. Numerous mucosal irregularities and elevations called lingual papillae cover the dorsal surface of the tongue anterior to the sulcus terminalis. The lingual papillae and their associated taste buds constitute the specialized mucosa of the oral cavity. Four types of papillae are described: filiform, fungiform, circumvallate, and foliate.


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Histology_Ross.  Filiform papillae are the smallest and most numerous in humans. They are conical, elongated projections of connective tissue that are covered with highly keratinized stratified squamous epithelium (Fig. 16.4a and Plate 49, page 559). This epithelium does not contain taste buds. The papillae serve only a mechanical role. Filiform papillae are distributed over the entire anterior dorsal surface of the tongue, with their tips pointing backward. They appear to form rows that diverge to the left and right from the midline and that parallel the arms of the sulcus terminalis.


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Histology_Ross.  Fungiform papillae, as the name implies, are mushroom-shaped projections located on the dorsal surface of the tongue (Fig. 16.4b). They project above the filiform papillae, among which they are scattered, and are just visible to the unaided eye as small spots (see Fig. 16.3 and Plate 50, page 561). They tend to be more numerous near the tip of the tongue. Taste buds are present in the stratified squamous epithelium on the dorsal surface of these papillae.  Circumvallate papillae are the large, dome-shaped structures that reside in the mucosa just anterior to the sulcus terminalis (see Fig. 16.3). The human tongue has 8 to 12 of these papillae. Each papilla is surrounded by a moatlike invagination lined with stratified squamous epithelium that contains numerous taste buds (Fig. 16.4d). Ducts of lingual salivary (von Ebner’s) glands empty their


Histology_Ross

Histology_Ross. FIGURE 16.3 • Human tongue. Circumvallate papillae are positioned in a V configuration, separating the anterior two thirds of the tongue from the posterior third. Fungiform and filiform papillae are on the anterior portion of the dorsal tongue surface. The uneven contour of the posterior tongue surface is attributable to the lingual tonsils. The palatine tonsil is at the junction between the oral cavity and the pharynx. serous secretion into the base of the moats. This secretion presumably flushes material from the moat to enable the taste buds to respond rapidly to changing stimuli.


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Histology_Ross. serous secretion into the base of the moats. This secretion presumably flushes material from the moat to enable the taste buds to respond rapidly to changing stimuli.  Foliate papillae consist of parallel low ridges separated by deep mucosal clefts (see Fig. 16.4c and Plate 50, page 561), which are aligned at right angles to the long axis of the tongue. They occur on the lateral edge of the tongue. In aged individuals, the foliate papillae may not be recognized; in younger individuals, they are easily found on the posterior lateral surface of the tongue and contain many taste buds in the epithelium of the facing walls of neighboring papillae (Fig. 16.3e). Small serous glands empty into the clefts. In some animals, such as the rabbit, foliate papillae constitute the principal site of aggregation of taste buds. The dorsal surface of the base of the tongue exhibits smooth bulges that reflect the presence of the lingual tonsil in the lamina propria (see Fig. 16.3).


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Histology_Ross. FIGURE 16.4 • Lingual papillae. a. Structurally, the filiform papillae are posteriorly bent conical projections of the epithelium. These papillae do not possess taste buds and are composed of stratified squamous keratinized epithelium. 45. b. Fungiform papillae are slightly rounded, elevated structures situated among the filiform papillae. A highly vascularized connective tissue core forms the center of the fungiform papilla and projects into the base of the surface epithelium. Because of the deep penetration of connective tissue into the epithelium (arrows), combined with a very thin keratinized surface, the fungiform papillae appear as small red dots when the dorsal surface of the tongue is examined by gross inspection. 45. c. In a section, foliate papillae can be distinguished from fungiform papillae because they appear in rows separated by deep clefts (arrows). The foliate papillae are covered by stratified squamous nonkeratinized epithelium containing numerous taste buds on their


Histology_Ross

Histology_Ross. papillae because they appear in rows separated by deep clefts (arrows). The foliate papillae are covered by stratified squamous nonkeratinized epithelium containing numerous taste buds on their lateral surfaces. The free surface epithelium of each papilla is thick and has a number of secondary connective tissue papillae projecting into its undersurface. The connective tissue within and under the foliate papillae contains serous glands (von Ebner’s glands) that open via ducts into the cleft between neighboring papillae. 45. d. Circumvallate papillae are covered by stratified squamous epithelium that may be slightly keratinized. Each circumvallate papilla is surrounded by a trench or cleft. Numerous taste buds are on the lateral walls of the papillae. The dorsal surface of the papilla is smooth. The deep trench surrounding the circumvallate papillae and the presence of taste buds on the sides rather than on the free surface are features that distinguish circumvallate from fungiform


Histology_Ross

Histology_Ross. smooth. The deep trench surrounding the circumvallate papillae and the presence of taste buds on the sides rather than on the free surface are features that distinguish circumvallate from fungiform papillae. The connective tissue near the circumvallate papillae also contains many serous-type glands that open via ducts into the bottom of the trench. 25.


Histology_Ross

Histology_Ross. Taste buds are present on fungiform, foliate, and circumvallate papillae. In histologic sections, taste buds appear as oval, pale-staining bodies that extend through the thickness of the epithelium (Fig. 16.5). A small opening onto the epithelial surface at the apex of the taste bud is called the taste pore. Three principal cell types are found in taste buds:  Neuroepithelial (sensory) cells are the most numerous cells in the taste bud. These elongated cells extend from the basal lamina of the epithelium to the taste pore, through which the tapered apical surface of each cell extends microvilli (see Fig. 16.5). Near their apical surface they are connected to neighboring neuroepithelial or supporting cells by tight junctions. At their base they form a synapse with the processes of afferent sensory neurons of the facial (cranial nerve VII), glossopharyngeal (cranial nerve IX), or vagus (cranial nerve X) nerves. The turnover time of neuroepithelial cells is about 10 days.


Histology_Ross

Histology_Ross.  Supporting cells are less numerous. They are also elongated cells that extend from the basal lamina to the taste pore. Like neuroepithelial cells, they contain microvilli on FIGURE 16.5 • Diagram and photomicrograph of a taste bud. a. This diagram of a taste bud shows the neuroepithelial (sensory), supporting, and basal cells. One of the basal cells is shown in the process of dividing. Nerve fibers have synapses with the neuroepithelial cells. (Based on Warwick R, Williams PL, eds. Gray’s Anatomy, 35th ed. Edinburgh: Churchill Livingstone, 1973.) b. This high-magnification photomicrograph shows the organization of the cells within the taste bud. The sensory and supporting cells extend through the full length of the taste bud. The apical surface of these cells contains microvilli. The basal cells are located at the bottom of the taste bud. Note that the taste bud opens at the surface by means of a taste pore. 640.


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Histology_Ross. their apical surface and possess tight junctions, but they do not synapse with the nerve cells. The turnover time of sup porting cells is also about 10 days. Basal cells are small cells located in the basal portion of the taste bud, near the basal lamina. They are the stem cells for the two other cell types. In addition to those associated with the papillae, taste buds are also present on the glossopalatine arch, the soft palate, the posterior surface of the epiglottis, and the posterior wall of the pharynx down to the level of the cricoid cartilage. Taste is a chemical sensation in which various chemicals elicit stimuli from neuroepithelial cells of taste buds.


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Histology_Ross. Taste is a chemical sensation in which various chemicals elicit stimuli from neuroepithelial cells of taste buds. Taste is characterized as a chemical sensation in which various tastants (taste-stimulating substances) contained in food or beverages interact with taste receptors located at the apical surface of the neuroepithelial cells. These cells react to five basic stimuli: sweet, salty, bitter, sour, and umami [Jap. delicious]. The molecular action of tastants can involve opening and passing through ion channels (i.e., salt and sour), closing ion channels (sour), or acting on a specific taste G protein–coupled receptors (i.e., bitter, sweet, and umami). Stimulation of bitter, sweet, and umami receptors activates G protein–coupled taste receptors that belong to T1R and T2R chemosensory receptors families.


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Histology_Ross. Stimulation of bitter, sweet, and umami receptors activates G protein–coupled taste receptors that belong to T1R and T2R chemosensory receptors families. Bitter, sweet, and umami tastes are detected by a variety of receptor proteins encoded by the two taste receptor genes (T1R and T2R). Their products are all characterized as being G protein–coupled taste receptors.


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Histology_Ross.  Bitter taste is detected by about 30 different types of T2R chemosensory receptors. Each receptor represents a single transmembrane protein coupled to its own G protein. After receptor activation by the tastant, the G protein stimulates the enzyme phospholipase C, leading to increased intracellular production of inositol 1,4,5-trisphosphate (IP3), a second messenger molecule. IP3 in turn activates taste-specifc Na channels causing influx of Na ions, thus depolarizing the neuroepithelial cell. Depolarization of the plasma membrane causes voltage-gated Ca2 channels in neuroepithelial cells to open. Increasing the concentration of intracellular Ca2 levels either by influx of extracellular Ca2 into the cell (the effect of depolarization) or by its release from intracellular stores (direct IP3 stimulation) results in the release of neurotransmitter molecules,


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Histology_Ross. FIGURE 16.6 • Diagram of taste receptors and their signaling mechanism. a. This diagram shows the signaling mechanism of bitter, sweet and umami receptors in the neuroepithelial cells. These cells selectively express only one class of receptor proteins; for simplicity all three taste receptors are depicted in the apical cell membrane. See text for details. PLC – phospholipase C, IP2–inositol-1,4-diphosphate, IP3 – inositol 1,4,5-trisphosphate (IP3). b. Signaling mechanism in sour sensation is generated by H protons that primary blocks K channels. The H protons enter the cell via amiloride-sensitive Na channels and through taste-specific H channels (PKD1L3 and PKD2L1) exclusively expressed in cells involved in sour taste transduction. c. Salty sensation derives from Na ions that enter the neuroepithelial cells through the amiloride-sensitive Na channels. Intracellular Na causes a depolarization of membrane and activation of additional voltage-sensitive Na and Ca2 channels. Calcium


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Histology_Ross. the neuroepithelial cells through the amiloride-sensitive Na channels. Intracellular Na causes a depolarization of membrane and activation of additional voltage-sensitive Na and Ca2 channels. Calcium mediated release of neurotransmitters from synaptic vesicles results in stimulating gustatory nerve fiber.


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Histology_Ross. which generate nerve impulses along the gustatory afferent nerve fiber (Fig 16.6a).  Sweet taste receptors are also G protein–coupled receptors. In contrast to the bitter taste receptors, they have two protein subunits, T1R2 and T1R3. The sweet tastants bound to these receptors activate the same second messanger system cascade of reactions that the bitter receptors do (see Fig 16.6a).


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Histology_Ross.  Umami taste is linked to certain amino acids (e.g., L-glutamate, aspartate, and related compounds) and is common to asparagus, tomatoes, cheese, and meat. Umami taste receptors are very similar to sweet receptors; they are also comprised of two subunits. One subunit, T1R3, is identical to that in the sweet receptor, but the second subunit formed by the T1R1 protein is unique for umami receptors (see Fig 16.6a). The transduction process is identical to that described previously for bitter taste pathways. Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of soy sauce), stimulates the umami receptors. The mechanism of transtuction can be similar to several tastes (i.e., bitter or sweet); however, it is important to remember that neuroepithelial cells selectively express only one class of receptor proteins. Therefore, the messages about bitterness or sweetness from eating food are transferred to the CNS along different nerve fibers.


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Histology_Ross. Sodium ions and hydrogen protons, which are responsible for salty and sour taste respectively, act directly on ion channels. Signaling mechanisms, in the case of sour and salty tastes, are similar to other signaling mechanisms found in synapses and neuromuscular junctions.  Sour taste is generated by H protons that are formed by hydrolysis of acidic compounds. The H primary blocks K channels that are responsible for generating the cell membrane potential that causes depolarization of the cell membrane. In addition, H protons enter the cell through amiloride-sensitive Na channels and through specification channels, called PKD1L3 and PKD2L1, found in neuroepithelial cells exclusively involved in sour taste transduction. The entry of H into receptor cell activates the voltage-sensitive Ca2 channels. Influx of Ca2 triggers migration of synaptic vesicles, their fusion, and transmitter release, which results in generating action potentials in apposed sensory nerve fiber (Fig 16.6b).


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Histology_Ross.  Salty taste that is stimulated by table salt (NaCl) is essentualy derived from the taste of the sodium ions. The Na enter the neuroepithelial cells through the specific amiloride-sensitive Na channels (the same that are involved in sour taste transmission). These channels are different from voltage-sensitive Na channels that generate action potentials in nerve or muscle cells. The entry of Na into receptor cell causes a depolarization of its membrane and activation of additional voltage-sensitive Na channels and voltage-sensitive Ca2 channels. As previously described, influx of Ca2 triggers migration and release of neurotransmitter from synaptic vesicles, which results in stimulating gustatory nerve fiber (Fig 16.6c). Some areas of the tongue are more responsive to certain tastes than others.


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Histology_Ross. Some areas of the tongue are more responsive to certain tastes than others. In general, taste buds at the tip of the tongue detect sweet stimuli, those immediately posterolateral to the tip detect salty stimuli, and those more posterolateral detect sour-tasting stimuli. Taste buds on the circumvallate papillae detect bitter and umami stimuli. However, studies with thermal stimulation of the tongue have shown that the classic taste maps as described above represent an oversimplified view of the distribution of taste receptors. Sensitivity to all tastes is distributed across the entire tongue, but some areas are indeed more responsive to certain tastes than others. The lingual tonsil consists of accumulations of lymphatic tissue at the base of the tongue.


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Histology_Ross. The lingual tonsil consists of accumulations of lymphatic tissue at the base of the tongue. The lingual tonsil is located in the lamina propria of the root or base of the tongue. It is found posterior to the sulcus terminalis (see Fig. 16.3). The lingual tonsil contains diffuse lymphatic tissue with lymphatic nodules containing germinal centers. These structures are discussed in Chapter 14, Lymphatic Tissues and Organs. Epithelial crypts usually invaginate into the lingual tonsil. However, the structure of the epithelium may be difficult to distinguish because of the extremely large number of lymphocytes that normally invade it. Between nodules, the lingual epithelium has the characteristics of lining epithelium. Mucous lingual salivary glands may be seen within the lingual tonsil and may extend into the muscle of the base of the tongue. The complex nerve supply of the tongue is provided by cranial nerves and the autonomic nervous system.


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Histology_Ross. The complex nerve supply of the tongue is provided by cranial nerves and the autonomic nervous system.  General sensation for the anterior two thirds of the tongue (anterior to the sulcus terminalis) is carried in the mandibular division of the trigeminal nerve (cranial nerve V). General sensation for the posterior one third of the tongue is carried in the glossopharyngeal nerve (cranial nerve IX) and the vagus nerve (cranial nerve X).  Taste sensation is carried by the chorda tympani, a branch of the facial nerve (cranial nerve VII) anterior to the sulcus terminalis, and by the glossopharyngeal  FOLDER 16.1 Clinical Correlation: The Genetic Basis of Taste


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Histology_Ross. The general ability to taste as well as the ability to sense specific tastes is genetically determined. Studies in large populations demonstrate that taste variation is common. About 25% of the population, referred to as “super-tasters,” have more than the normal number of lingual papillae and a high density of taste buds. Rare individuals in this group, such as wine, brandy, coffee, or tea tasters, have prodigious taste discrimination and taste memory. These individuals are characterized by their extreme sensi-tivity to the chemical phenylthiocarbamide (PTC) and its derivative 6-N-propylthiouracil (PROP); they typically re-port an intensely bitter taste after a drop of PTC/PROP solution is placed on the tip of their tongue. On the other side of the spectrum (about 25% of the population) are in-dividuals known as “nontasters,” with a smaller than normal number of lingual papillae and a lower density of taste buds. When tested with PTC/PROP solution, these individuals are unaware of


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Histology_Ross. are in-dividuals known as “nontasters,” with a smaller than normal number of lingual papillae and a lower density of taste buds. When tested with PTC/PROP solution, these individuals are unaware of its bitter taste. Many clinical conditions can affect taste perception. They include lesions in the nerves that transmit the taste sensa-tion to the central nervous system; inflammations of the oral cavity; mucosal disorders including radiation-induced in-flammation of the lingual mucosa; nutritional deficiencies; endocrine disorders such as diabetes mellitus, hypogo-nadism, and pseudohypoparathyroidism; and hormonal fluctuations during menstruation and pregnancy. Some rare genetic disorders also affect taste sensation. Type I familial dysautonomia (Riley-Day syndrome) causes se-vere hypogeusia (decreased ability to detect taste) be-cause of the developmental absence of taste buds and fungiform papillae. This sensory and autonomic neuropathy is an autosomal recessive disorder caused by a


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Histology_Ross. (decreased ability to detect taste) be-cause of the developmental absence of taste buds and fungiform papillae. This sensory and autonomic neuropathy is an autosomal recessive disorder caused by a mutation in the DYS gene (also referred to as the IKBKAP gene) lo-cated on chromosome 9. In addition to hypogeusia, these individuals experience other symptoms related to develop-mental defects in the peripheral and autonomic nervous systems, including diminished lacrimation, defective ther-moregulation, orthostatic hypotension, excessive sweating, loss of pain and temperature sensation, and absent re-flexes. A test that detects the causative mutation in the DYS gene has recently been developed to confirm the di-agnosis of familial dysautonomia.


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Histology_Ross. nerve (cranial nerve IX) and vagus nerve (cranial nerve X) posterior to the sulcus.  Motor innervation for the musculature of the tongue is supplied by the hypoglossal nerve (cranial nerve XII).  Vascular and glandular innervation is provided by the sympathetic and parasympathetic nerves. They supply blood vessels and small salivary glands of the tongue. Ganglion cells are often seen within the tongue. These cells belong to postsynaptic parasympathetic neurons and are destined for the minor salivary glands within the tongue. The cell bodies of sympathetic postsynaptic neurons are located in the superior cervical ganglion.


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Histology_Ross. Teeth are a major component of the oral cavity and are essential for the beginning of the digestive process. Teeth are embedded in and attached to the alveolar processes of the maxilla and mandible. Children have 10 deciduous (primary, milk) teeth in each jaw, on each side:  A medial (central) incisor, the first tooth to erupt (usually in the mandible) at approximately 6 months of age (in some infants, the first teeth may not erupt until 12 to 13 months of age)  A lateral incisor, which erupts at approximately 8 months  A canine tooth, which erupts at approximately 15 months  Two molar teeth, the first of which erupts at 10 to 19 months and the second of which erupts at 20 to 31 months


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Histology_Ross. During a period of years, usually beginning at about age 6 and ending at about age 12 to 13, deciduous teeth are gradually replaced by 16 permanent (secondary) teeth in each jaw (Folder 16.2). Each side of both upper and lower jaws consists of the following:  A medial (central) incisor, which erupts at age 7 to 8  A lateral incisor, which erupts at age 8 to 9  A canine tooth, which erupts at age 10 to 12  Two premolar teeth, which erupt between ages 10 and 12  Three molar teeth, which erupt at different times; the first molar usually erupts at age 6, the second molar in the early teens, and the third molar (wisdom teeth) during the late teens or early twenties Incisors, canines, and premolars have one root each, except for the first premolar of the maxilla, which has two roots. Molars have either two roots (lower jaw) or three (upper jaw) and, on rare occasions, four roots. All teeth have the same basic structure, however.


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Histology_Ross.  FOLDER 16.2 Clinical Correlation: Classification of Permanent (Secondary) and Deciduous (Primary) Dentition


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Histology_Ross. Three systems are currently used to classify permanent and deciduous teeth (Fig. F16.2.1):  Palmer system, which was the most commonly used notation worldwide. In this system, uppercase letters are used for the deciduous teeth, and arabic numerals are used for the permanent teeth. Each quadrant in this sys-tem is designated by angled lines: for upper right (UR), for upper left (UL), for lower right (LR), and for lower left (LL). For example, permanent canines are called number 3 in each quadrant, and the quadrant is desig-nated by its angled line.  International system, which uses two arabic numer-als to designate the individual tooth. In this system, the first numeral indicates the location of the tooth in a spe-cific quadrant. The permanent quadrants are designated UR 1, UL 2, LL 3, and LR 4; the deciduous quadrants are designated UR 5, UL 6, LL 7, and LR 8. The second numeral designates the individual tooth, which is numbered beginning from the dental mid-line. For example, in


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Histology_Ross. and LR 4; the deciduous quadrants are designated UR 5, UL 6, LL 7, and LR 8. The second numeral designates the individual tooth, which is numbered beginning from the dental mid-line. For example, in this system, the permanent canines are named 13, 23, 33, and 43, and the deciduous ca-nines would be 53, 63, 73, and 83.  American (Universal) system, which is the most commonly used notation in North America. In this system, the permanent dentition is designated by arabic numer-als, and the deciduous dentition is designated with up-percase letters. For permanent dentition, numbering begins in the UR quadrant, with the UR third molar des-ignated number 1. Numbering continues across the maxillary arch to the UL third molar, designated tooth number 16. Tooth number 17 is the third molar located in the LL quadrant inferior and opposite to tooth number 16. Then, the numbering progresses across the mandibular arch and terminates with tooth number 32, the LR third molar. In this system, the


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Histology_Ross. in the LL quadrant inferior and opposite to tooth number 16. Then, the numbering progresses across the mandibular arch and terminates with tooth number 32, the LR third molar. In this system, the sum of the num-bers of opposing teeth adds up to 33. For the decidu-ous dentition, the same pattern is followed, but the letters A to T are used to designate the individual teeth. Thus, in this system, the permanent canines are desig-nated 6, 11, 22, and 27, and the deciduous canines, C, H, M, and R. Also note that in Figure F16.2.1 the color outline demonstrates the relationship of the deciduous and per-manent dentitions. Examination of the table reveals that de-ciduous molars are replaced with permanent premolars after exfoliation and that the permanent molars have no de-ciduous precursors. continued next page


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Histology_Ross. FOLDER 16.2 Clinical Correlation: Classification of Permanent (Secondary) and Deciduous (Primary) Dentition (Cont.)


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Histology_Ross. FIGURE F16.2.1 • Classification of permanent and deciduous teeth. Three systems of tooth classification are used. The central panel of the diagram shows the permanent teeth, whereas the upper and lower panels show the deciduous teeth. Dentition is divided into four quadrants: upper left (UL), upper right (UR), lower left (LL), and lower right (LR). Each quadrant includes 8 permanent teeth or 5 deciduous teeth. In the American (Universal) system (blue), permanent teeth are designated with Arabic numerals. The numbering begins from the wisdom tooth in the upper right quadrant designated as tooth number 1 and continues along all the teeth in the maxilla to tooth number 16, which designates the third upper left molar. The numbering progresses to the mandible, beginning at the third left lower molar designated as number 17 to the third lower right molar designated as number 32. In the American system, deciduous teeth are marked with capital letters designated for each tooth. The pattern is


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Histology_Ross. molar designated as number 17 to the third lower right molar designated as number 32. In the American system, deciduous teeth are marked with capital letters designated for each tooth. The pattern is the same as that for permanent teeth, so the numbering begins from the second upper right molar and finishes with the second lower right molar. In the International system (red), also referred to as the Two-Digit System, each tooth is designated with two numbers: The first number indicates the dentition quadrant, which is marked from 1 to 4 and from 5 to 8 in clockwise direction beginning from the upper right quadrant for permanent and deciduous teeth, respectively. The second number specifies individual teeth in each quadrant beginning from the midline where the medial incisors are designated as number 1 and third molars are designated as number 8. In the Palmer system (yellow), the dentition is divided into four quadrants with a right-angle bracket. The vertical line of the bracket


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Histology_Ross. as number 1 and third molars are designated as number 8. In the Palmer system (yellow), the dentition is divided into four quadrants with a right-angle bracket. The vertical line of the bracket divides the dentition into a right and a left side beginning at the midline. The horizontal line of the bracket divides the dentition into the upper and lower parts to designate teeth in the maxilla and mandible. In the Palmer system, permanent teeth are numbered with Arabic numerals beginning from the midline. The deciduous teeth are marked with capital letters also starting from the midline. To mark a particular tooth with the Palmer system, two lines (vertical and horizontal) and the correct number or letter of the tooth are needed. (Table design courtesy of Dr. Wade T. Schultz.)


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Histology_Ross. Teeth consist of several layers of specialized tissues. Teeth are made up of three specialized tissues:  Enamel, a hard, thin, translucent layer of acellular mineralized tissue that covers the crown of the tooth.  Dentin, the most abundant dental tissue; it lies deep to the enamel in the crown and cementum in the root. Its unique tubular structure and biochemical composition support the more rigid enamel and cementum overlying the surface of the tooth.  Cementum, a thin, pale-yellowish layer of bone like calcified tissue covering the dentin of the root of the teeth. Cementum is softer and more permeable than dentin and is easily removed by abrasion when the root surface is exposed to the oral environment. Enamel is the hardest substance in the body; it consists of 96 to 98% calcium hydroxyapatite. Enamel is an acellular mineralized tissue that covers the crown of the tooth. Once formed it cannot be replaced. lines of Retzius


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Histology_Ross. Enamel is an acellular mineralized tissue that covers the crown of the tooth. Once formed it cannot be replaced. lines of Retzius Enamel is a unique tissue because, unlike bone, which is formed from connective tissue, it is a mineralized material derived from epithelium. Enamel is more highly mineralized and harder than any other mineralized tissue in the body; it consists of 96 to 98% of calcium hydroxyapatite. The enamel that is exposed and visible above the gum line is called the clinical crown; the anatomic crown describes all of the tooth that is covered by enamel, some of which is below the gum line. Enamel varies in thickness over the crown and may be as thick as 2.5 mm on the cusps (biting and grinding surfaces) of some teeth. The enamel layer ends at the neck, or cervix, of the tooth at the cementoenamel junction (Fig. 16.7); the root of the tooth is then covered by cementum, a bonelike material.


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Histology_Ross. Enamel is composed of enamel rods that span the entire thickness of the enamel layer. The nonstoichiometric carbonated calcium hydroxyapatite enamel crystals that form the enamel are arranged as rods that measure 4 m wide and 8 m high. Each enamel rod spans the full thickness of the enamel layer from the dentin showing dentinal tubules interglobular spaces odontoblasts gingival sulcus epithelium of gingiva pulp chamber granular layer of Tomes fibers of periodontal membrane alveolar bone with marrow pulp canal cellular cementum apical foramen


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Histology_Ross. FIGURE 16.7 • Diagram of a section of an incisor tooth and surrounding bony and mucosal structures. The three mineralized components of the tooth are dentin, enamel, and cementum. The central soft core of the tooth is the pulp. The periodontal ligament (membrane) contains bundles of collagenous fibers that bind the tooth to the surrounding alveolar bone. The clinical crown of the tooth is the portion that projects into the oral cavity. The anatomic crown is the entire portion of the tooth covered by enamel.


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Histology_Ross. dentinoenamel junction to the enamel surface. When examined in cross section at higher magnification, the rods reveal a keyhole shape (Fig. 16.8); the ballooned part, or head, is oriented superiorly, and the tail is directed inferiorly toward the root of the tooth. The enamel crystals are primarily oriented parallel to the long axis of the rod within the head, and in the tail they are oriented more obliquely (Figs. 16.8 and 16.9). The limited spaces between the rods are also filled with enamel crystals. Striations observed on enamel rods (contour lines of Retzius) may represent evidence of rhythmic growth of the enamel in the developing tooth. A wider line of hypomineralization is observed in the enamel of the deciduous teeth. This line, called the neonatal line, marks the nutritional changes that take place between prenatal and postnatal life.


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Histology_Ross. Although the enamel of an erupted tooth lacks cells and cell processes, it is not a static tissue. It is influenced by the secretion of the salivary glands, which are essential to its maintenance. The substances in saliva that affect teeth include digestive enzymes, secreted antibodies, and a variety of inor FIGURE 16.8 • Diagram showing the basic organization and structure of enamel rods. The enamel rod is a thin structure extending from the dentinoenamel junction to the surface of the enamel. Where the enamel is thickest, at the tip of the crown, the rods are longest, measuring up to 2,000 m. On cross section, the rods reveal a keyhole shape. The upper ballooned part of the rod, called the head, is oriented superiorly, and the lower part of the rod, called the tail, is directed inferiorly. Within the head, most of the enamel crystals are oriented parallel to the long axis of each rod. Within the tail, the crystals are oriented more obliquely. ganic (mineral) components.


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Histology_Ross. ganic (mineral) components. Mature enamel contains very little organic material. Despite its hardness, enamel can be decalcified by acid-producing bacteria acting on food products trapped on the enamel surface. This is the basis of the initiation of dental caries. Fluoride added to the hydroxyapatite complex makes the enamel more resistant to acid demineralization. The widespread use of fluoride in drinking water, toothpaste, pediatric vitamin supplements, and mouthwashes significantly reduces the incidence of dental caries. Enamel is produced by ameloblasts of the enamel organ, and dentin is produced by neural crest–derived odontoblasts of the adjacent mesenchyme.