Histology_Ross

Histology_Ross. The enamel organ is an epithelial formation that is derived from ectodermal epithelial cells of the oral cavity. The onset of tooth development is marked by proliferation of oral epithelium to form a horseshoe-shaped cellular band of tissue, the dental lamina, in the adjacent mesenchyme where the upper and lower jaws will develop. At the site of each future tooth, there is a further proliferation of cells that arise from the dental lamina, resulting in a rounded, cellular, budlike outgrowth, one for each tooth, that projects into the underlying mesenchymal tissue. This outgrowth, referred to as the bud stage, represents the early enamel organ (Fig. 16.10a). Gradually, the rounded cell mass enlarges and then develops a concavity at the site opposite where it arose from the dental lamina. The enamel organ is now referred to as being in the cap stage (Fig. 16.10b). Further growth and development of the enamel organ results in the bell stage (Fig. 16.10, c and d). At this stage the enamel


Histology_Ross

Histology_Ross. enamel organ is now referred to as being in the cap stage (Fig. 16.10b). Further growth and development of the enamel organ results in the bell stage (Fig. 16.10, c and d). At this stage the enamel organ consists of four recognizable cellular components:  Outer enamel epithelium, made up of a cell layer that forms the convex surface  Inner enamel epithelium, made up of a cell layer that forms the concave surface  Stratum intermedium, a cell layer that develops internal to the inner enamel epithelium Stellate reticulum, made up of cells that have a stellate ap pearance and occupy the inner portion of the enamel organ


Histology_Ross

Histology_Ross. The neural crest–derived preodontoblasts lined up within the “bell” adjacent to the inner enamel epithelial cells become columnar and have an epithelial-type appearance. They will become odontoblasts and form the dentin of the tooth. The inner enamel epithelial cells of the enamel organ will become ameloblasts. Along with the cells of the stratum intermedium, they will be responsible for enamel production. At the early stage, just before dentinogenesis and amelogenesis, the dental lamina degenerates, leaving the developing tooth primordium detached from its site of origin. Dental enamel is formed by a matrix-mediated biomineralization process known as amelogenesis. These are the major stages of amelogenesis:


Histology_Ross

Histology_Ross. Dental enamel is formed by a matrix-mediated biomineralization process known as amelogenesis. These are the major stages of amelogenesis: FIGURE 16.9 • Structure of young enamel. a. This electron micrograph shows enamel rods cut obliquely. Arrows indicate the boundaries between adjacent rods. 14,700. b. Parts of two adjacent rods are seen at higher magnification. Arrows mark the boundary between the two rods. The dark needlelike objects are young hydroxyapatite crystals; the substance between the hydroxyapatite crystals is the organic matrix of the developing enamel. As the enamel matures, the hydroxyapatite crystals grow, and the bulk of the organic matrix is removed. 60,000.


Histology_Ross

Histology_Ross.  Matrix production, or secretory stage. In the formation of mineralized tissues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix (Fig. 16.11) is deposited directly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are called secretory-stage ameloblasts. As do osteoblasts in bone, these cells produce an organic proteinaceous matrix by activity of the rough endoplasmic reticulum (rER), Golgi apparatus, and secretory granules. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved.


Histology_Ross

Histology_Ross.  Matrix maturation. Maturation of the partially mineralized enamel matrix involves the removal of organic material as well as continued influx of calcium and phosphate into the maturing enamel. Cells involved in this second stage of enamel formation are called maturation-stage ameloblasts. Maturation-stage ameloblasts differentiate from secretory-stage ameloblasts and function primarily as a transport epithelium, moving substances into and out of the maturing enamel. Maturation-stage ameloblasts undergo cyclical alterations in their morphology that correspond to cyclical entry of calcium into the enamel. Secretory-stage ameloblasts are polarized columnar cells that produce enamel.


Histology_Ross

Histology_Ross. The secretory-stage ameloblast lies directly adjacent to the developing enamel. At the apical pole of each ameloblast is a process, Tomes’s process, which is surrounded by the developing enamel (Fig. 16.12). A cluster of mitochondria in the base of the cell accounts for the eosinophilic staining of this region in hematoxylin and eosin (H&E)– stained paraffin sections (Fig. 16.13). Adjacent to the mitochondria is the nucleus; in the main column of cytoplasm are the rER, Golgi, secretory granules, and other cell elements. Junctional complexes are present at both apical and basal parts of the cell. They maintain the integrity and orientation of the ameloblasts as they move away from the dentoenamel junction. Actin filaments joined to these junctional complexes are involved in moving the secretory-stage ameloblast over the developing enamel. The rod produced by the ameloblast follows in the wake of the cell. Thus, in mature enamel, the direction of the enamel rod is a record of the path


Histology_Ross

Histology_Ross. ameloblast over the developing enamel. The rod produced by the ameloblast follows in the wake of the cell. Thus, in mature enamel, the direction of the enamel rod is a record of the path taken earlier by the secretory-stage ameloblast.


Histology_Ross

Histology_Ross. At their base, the secretory-stage ameloblasts are adjacent to a layer of enamel organ cells called the stratum intermedium (see Fig. 16.10, b, c, and g). The plasma membrane of these cells, especially at the base of the ameloblasts, contains alkaline phosphatase, an enzyme active in calcification. Stellate enamel organ cells are external to the stratum intermedium and are separated from the adjacent blood vessels by a basal lamina. Maturation-stage ameloblasts transport substances needed for enamel maturation. The histologic feature that marks the cycles of maturation-stage ameloblasts is a striated or ruffled border (Fig. 16.14) Maturation-stage ameloblasts with a striated border occupy approximately 70% of a specific cycle, and those that are smooth-ended, approximately 30% of a specific cycle. There is no stratum intermedium in the enamel organ during enamel maturation; stellate papillary cells are adjacent to the maturation-stage ameloblasts.


Histology_Ross

Histology_Ross. The maturation-stage ameloblasts and the adjacent papillary cells are characterized by numerous mitochondria. Their presence indicates cellular activity that requires large amounts of energy and reflects the function of maturation-stage ameloblasts and adjacent papillary cells as a transporting epithelium. Recent advances in the molecular biology of ameloblast gene products have revealed the enamel matrix to be highly heterogeneous. It contains proteins encoded by a number of different genes. Listed here are the principal proteins in the extracellular matrix of the developing enamel:  Amelogenins, important proteins in establishing and maintaining the spacing between enamel rods in early stages of enamel development.


Histology_Ross

Histology_Ross.  Ameloblastins, signaling proteins produced by ameloblasts from the early secretory to late maturation stages. Their function is not well understood; however, their developmental pattern suggests that ameloblastins play a much broader role in amelogenesis than the other proteins. Ameloblastins are believed to guide the enamel mineralization process by controlling elongation of the enamel crystals and to form junctional complexes between individual enamel crystals.  Enamelins, proteins distributed throughout the enamel layer. These proteins undergo proteolytic cleavage as the enamel matures. Low-molecular-weight products of this cleavage are retained in the mature enamel, often localized on the surface of enamel crystals.


Histology_Ross

Histology_Ross.  Tuftelins, the earliest detected proteins located near the dentinoenamel junction. Their acidic and insoluble nature aids in the nucleation of enamel crystals. Tuftelins are present in enamel tufts and account for hypomineralization, i.e., enamel tufts have a higher percentage of organic material than the remainder of the mature enamel. The maturation of the developing enamel results in its continued mineralization so that it becomes the hardest substance in the body. Amelogenins and ameloblastins are removed during enamel maturation. Thus, mature enamel contains only enamelins and tuftelins. The ameloblasts degenerate after the enamel is fully formed, at about the time of tooth eruption through the gum. Cementum covers the root of the tooth.


Histology_Ross

Histology_Ross. Cementum covers the root of the tooth. The root is the part of the tooth that fits into its alveolus, or socket in the maxilla or mandible. Cementum is a thin layer of bonelike material that is secreted by cementocytes, cells that closely resemble osteocytes. Like bone, cementum is 65% mineral. The lacunae and canaliculi in the cementum contain the cementocytes and their processes, respectively. They resemble those structures in bone that contain osteocytes and osteocyte processes. Unlike bone, cementum is avascular. Also, the canaliculi in cementum do not form an interconnecting network. A layer of cementoblasts (cells that resemble the osteoblasts of the surface of growing bone) is seen on the outer surface of the cementum, adjacent to the periodontal ligament.


Histology_Ross

Histology_Ross. Collagen fbers that project out of the matrix of the cementum and embed in the bony matrix of the socket wall form the bulk of the periodontal ligament. These fibers are another example of Sharpey’s fbers (Fig. 16.15). In addition, elastic fibers are also a component of the periodontal ligament. This mode of attachment of the tooth in its socket allows slight movement of the tooth to occur naturally. It also forms the basis of orthodontic procedures used to straighten teeth and reduce malocclusion of the biting and grinding surfaces of the maxillary and mandibular teeth. During corrective tooth movements, the alveolar bone of the socket is resorbed and resynthesized, but the cementum is not. Dentin is a calcified material that forms most of the tooth substance.


Histology_Ross

Histology_Ross. Dentin is a calcified material that forms most of the tooth substance. Dentin lies deep to the enamel and cementum. It contains less hydroxyapatite than enamel, about 70%, but more than is found in bone and cementum. Dentin is secreted by odontoblasts that form an epithelial layer over the inner surface of the dentin, i.e., the surface that is in contact with the pulp (Fig. 16.16). Like ameloblasts, odontoblasts are columnar cells that contain a well-developed rER, a large Golgi apparatus, and other organelles associated with the synthesis and secretion of large amounts of protein (Fig. 16.17). The apical surface of the odontoblast is in contact with the forming dentin; junctional complexes between the odontoblasts at that level separate the dentinal compartment from the pulp compartment. primordium of enamel primordium of pulp dental papilla dental papilla dental pulp


Histology_Ross

Histology_Ross. primordium of enamel primordium of pulp dental papilla dental papilla dental pulp FIGURE 16.11 • Diagram showing the cellular relationships during enamel formation. In the initial secretory stage, dentin is produced first by odontoblasts. Enamel matrix is then deposited directly on the surface of the previously formed dentin by secretory-stage ameloblasts. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved. (Adapted with permission from Schour I. The neonatal line in the enamel and dentin of the human deciduous teeth and first permanent molar. JADA 1936;23:1946. Copyright (c) 1936 American Dental Association. All rights reserved.)


Histology_Ross

Histology_Ross. The layer of odontoblasts retreats as the dentin is laid down, leaving odontoblast processes embedded in the dentin in narrow channels called dentinal tubules (see Fig. 16.15). The tubules and processes continue to elongate as the dentin continues to thicken by rhythmic growth. The rhythmic growth of dentin produces certain “growth lines” in the dentin (incremental lines of von Ebner and thicker lines of Owen) that mark significant developmental times such as birth (neonatal line) and when unusual substances such as lead are incorporated into the growing tooth. Study of growth lines has proved useful in forensic medicine.


Histology_Ross

Histology_Ross. Predentin is the newly secreted organic matrix, closest to the cell body of the odontoblast, which has yet to be mineralized. Although most of the proteins in the organic matrix are similar to those of bone, predentin contains two unique proteins:  Dentin phosphoprotein (DPP), a 45-kilodalton highly acidic phosphorylated protein, which is rich in aspartic acid and phosphoserine and binds large amounts of calcium. DPP is involved in the initiation of mineralization and in control of mineral size and shape.  Dentin sialoprotein (DSP), a 100-kilodalton proteogly can which is rich in aspartic and glutamic acids, serine, glycine, and chondroitin 6-sulfate. DSP is also involved in the mineralization process.


Histology_Ross

Histology_Ross. An unusual feature of the secretion of collagen and hydroxy-apatite by odontoblasts is the presence, in Golgi vesicles, of arrays of a formed filamentous collagen precursor. Granules believed to contain calcium attach to these precursors, giving rise to structures called abacus bodies (Figs. 16.17 and 16.18). Abacus bodies become more condensed as they mature into secretory granules. Dentin is produced by odontoblasts. Dentin is the first mineralized component of the tooth to be deposited. The outermost dentin, which is referred to as mantle dentin, is formed by subodontoblastic cells that produce small bundles of collagen fibers (von Korff ’s fibers). The odontoblasts differentiate from cells at the


Histology_Ross

Histology_Ross. FIGURE 16.10 • Diagrams and photomicrographs of a developing tooth. a. In this bud stage, the oral epithelium invaginates into the underlying mesenchyme, giving origin to the enamel organ (primordium of enamel). Mesenchymal cells adjacent to the tooth bud begin to differentiate, forming the dental papilla that protrudes into the tooth bud. b. Tooth bud in cap stage. In this stage, cells located in the concavity of the cap differentiate into tall, columnar cells (ameloblasts) forming the inner enamel epithelium. The condensed mesenchyme invaginates into the inner enamel epithelium, forming the dental papilla, which gives rise to the dentin and the pulp. c. In this bell stage, the connection with the oral epithelium is almost cut off. The enamel organ consists of a narrow line of outer enamel epithelium, an inner enamel epithelium formed by ameloblasts, several condensed layers of cells that form the stratum intermedium, and the widely spaced stellate reticulum. The dental papilla is


Histology_Ross

Histology_Ross. epithelium, an inner enamel epithelium formed by ameloblasts, several condensed layers of cells that form the stratum intermedium, and the widely spaced stellate reticulum. The dental papilla is deeply invaginated against the enamel organ. d. In this appositional dentin and enamel stage, the tooth bud is completely differentiated and independent from the oral epithelium. The relationship of the two mineralized tissues of the dental crown, enamel and dentin, is clearly visible. The surrounding mesenchyme has developed into bony tissue. e. In this stage of tooth eruption, the apex of the tooth emerges through the surface of the oral epithelium. The odontoblastic layer lines the pulp cavity. Note the developed periodontal ligaments that fasten the root of the tooth to the surrounding bone. The apex of the root is still open, but after eruption occurs, it becomes narrower. f. Functional tooth stage. Note the distribution of enamel and dentin. The tooth is embedded in surrounding bone and


Histology_Ross

Histology_Ross. apex of the root is still open, but after eruption occurs, it becomes narrower. f. Functional tooth stage. Note the distribution of enamel and dentin. The tooth is embedded in surrounding bone and gingiva. g. This photomicrograph of the developing tooth in the cap stage (comparable to b) shows its connection with the oral epithelium. The enamel organ consists of a single layer of cuboidal cells forming the outer enamel epithelium, the inner enamel epithelium has differentiated into columnar ameloblasts, and the layer of cells adjacent to the inner enamel epithelium has formed the stratum intermedium. The remainder of the structure is occupied by the stellate reticulum. The mesenchyme of the dental papilla has proliferated and pushed into the enamel organ. At this stage, the forming tooth is surrounded by condensed mesenchyme, called the dental sac, which gives rise to periodontal structures. 300. h. This photomicrograph shows the developing crown of an incisor, which is surrounded by


Histology_Ross

Histology_Ross. is surrounded by condensed mesenchyme, called the dental sac, which gives rise to periodontal structures. 300. h. This photomicrograph shows the developing crown of an incisor, which is surrounded by the outer enamel epithelium and remnants of the stellate reticulum. It is comparable to d. The underlying lighter-stained layer of dentin is a product of the odontoblasts. These tall columnar odontoblasts have differentiated from cells of the dental papilla. The pulp cavity is filled with dental pulp, and blood vessels permeate the pulp tissue. 40.


Histology_Ross

Histology_Ross. FIGURE 16.12 • Schematic diagrams of a partially formed tooth showing details of amelogenesis. a. The enamel is drawn to show the enamel rods extending from the dentinoenamel junction to the surface of the tooth. Although the full thickness of the enamel is formed, the full thickness of the dentin has not yet been established. The contour lines within the dentin show the extent to which the dentin has developed at a particular time, as labeled in the illustration. Note that the pulp cavity in the center of the tooth becomes smaller as the dentin develops. (Based on Schour I, Massler M. The neonatal line in the enamel and dentin of the human deciduous teeth and first permanent molar. J Am Dent Assoc 1936;23:1948.) b. During amelogenesis, enamel formation is influenced by the path of the ameloblasts. The rod produced by the ameloblast forms in the wake of the cell. Thus, in mature enamel, the direction of the enamel rod is a record of the path taken earlier by the secretory-stage


Histology_Ross

Histology_Ross. the ameloblasts. The rod produced by the ameloblast forms in the wake of the cell. Thus, in mature enamel, the direction of the enamel rod is a record of the path taken earlier by the secretory-stage ameloblast. c. At the apical pole of the secretory-stage ameloblasts are Tomes’ processes, surrounded by the developing enamel. Junctional complexes at the apical pole are also shown. Note the numerous matrix-containing secretory vesicles in the cytoplasm of the processes.


Histology_Ross

Histology_Ross. periphery of the dental papilla. The progenitor cells have the appearance of typical mesenchymal cells, i.e., they contain little cytoplasm. During their differentiation into odontoblasts, the cytoplasmic volume and organelles characteristic of collagen-producing cells increase. The cells form a layer at the periphery of the dental papilla, and they secrete the organic matrix of dentin, or predentin, at their apical end (away from the dental papilla; Fig. 16.19). As the predentin thickens, the odontoblasts move or are displaced centrally (see Fig. 16.12). A wave of mineralization follows the receding odontoblasts; this mineralized product is the dentin. As the cells move centrally, the odontoblastic


Histology_Ross

Histology_Ross. FIGURE 16.13 • Enamel organ cells and odontoblasts in a developing tooth. This photomicrograph of an unstained plastic thick section viewed with the phase contrast microscope shows enamel organ cells and odontoblasts as they begin to produce enamel (E) and dentin (D), respectively. Young enamel is deposited by secretory-stage ameloblasts (SA) onto the previously formed dentin. The enamel appears dark in the illustration. At the top, the enamel surface displays a characteristic picket-fence pattern because of the sharp contrast between the lightly stained Tomes’ processes (TP) of the secretory-stage ameloblasts and the darkly stained young enamel product that partly surrounds the cell processes. The nuclei (N) at the right belong to cells of the stratum intermedium. The nuclei (N) on the left belong to odontoblasts located in the basal part of the cells. The odontoblast cytoplasm extends to the dashed line. At this point, cytoplasmic processes (OP) extend into the dentin. 85.


Histology_Ross

Histology_Ross. CTPLMABVbCT PL MA BV ccbb CTPLMABVaCT PL MA BV a BVCTPLPLMAEBV CT PL PL MA E FIGURE 16.14 • Ameloblasts in different stages of maturation. a. This black and white photomicrograph of an H&E–stained specimen shows maturation-stage ameloblasts (MA) in demineralized tissue. The maturing enamel has been lost during slide preparation, and the space below the ameloblasts previously occupied by the enamel appears empty. Maturation-stage ameloblasts with a striated border account for 80% of the cell population in the maturation zone. BV, blood vessels; CT, connective tissue; PL, papillary layer. 650. b. This photomicrograph shows smooth-ended maturation-stage ameloblasts (MA), which account for 20% of the cell population in the mature zone. At the basal pole of the ameloblasts are the cells of the papillary layer (PL). A layer of stratum intermedium is no longer present during this stage of ameloblast maturation. 650. c. Colorized scanning electron micrograph of freeze fracture section of the


Histology_Ross

Histology_Ross. papillary layer (PL). A layer of stratum intermedium is no longer present during this stage of ameloblast maturation. 650. c. Colorized scanning electron micrograph of freeze fracture section of the tooth shows layer of smooth-ended maturation-stage ameloblasts (MA, green) on the enamel surface (orange). During slide preparation apical surfaces of ameloblasts were detached from the enamel. Basal surface of ameloblast is attached to connective tissue (CT) containing blood vessels 1,300. (Part C from SPL / Photo Researchers, Inc, with permission.) processes elongate; the longest are surrounded by the mineralized dentin. In newly formed dentin, the wall of the dentinal tubule is simply the edge of the mineralized dentin. With time, the dentin immediately surrounding the dentinal tubule becomes more highly mineralized; this more mineralized sheath of dentin is referred to as the peritubular dentin. The remainder of the dentin is called the intertubular dentin.


Histology_Ross

Histology_Ross. FIGURE 16.15 • Electron micrograph of Sharpey’s fibers. Sharpey’s fibers extend from the periodontal ligament (right) into the cementum. They consist of collagen fibrils. Sharpey’s fibers within the cementum are mineralized; those within the periodontal ligament are not mineralized. 13,000. The dental pulp cavity is a connective tissue compartment bounded by the tooth dentin. The central pulp cavity is the space within a tooth that is occupied by dental pulp, a loose connective tissue that is richly vascularized and supplied by abundant nerves. The pulp cavity takes the general shape of the tooth. The blood vessels and nerves enter the pulp cavity at the tip (apex) of the root, at a site called the apical foramen. (The designations apex and apical in this context refer only to the narrowed tip of the root of the tooth rather than to a luminal (apical) surface, as used in describing secretory and absorptive epithelia.)


Histology_Ross

Histology_Ross. The blood vessels and nerves extend to the crown of the tooth, where they form vascular and neural networks beneath and within the layer of odontoblasts. Some bare nerve fibers also enter the proximal portions of the dentinal tubules and contact odontoblast processes. The odontoblast processes are believed to serve a transducer function in transmitting stimuli from the tooth surface to the nerves in the dental pulp. In teeth with more than one cusp, pulpal horns extend into the cusps and contain large numbers of nerve fibers. More of these fibers extend into the dentinal tubules than at other sites. Because dentin continues to be secreted throughout life, the pulp cavity decreases in volume with age.


Histology_Ross

Histology_Ross. FIGURE 16.16 • Dental pulp and structure of dentin. This photomicrograph of a decalcified tooth shows the centrally located dental pulp, surrounded by dentin on both sides. The dental pulp is a soft tissue core of the tooth that resembles embryonic connective tissue, even in the adult. It contains blood vessels and nerves. Dentin contains the cytoplasmic processes of the odontoblasts within dentinal tubules. They extend into the dentinoenamel junction. The cell bodies of the odontoblasts are adjacent to the unmineralized dentin called the predentin. 120. Left inset. Longitudinal profiles of the dentinal tubules. 240. Right inset. Cross-sectional profiles of dentinal tubules. The dark outline of the dentinal tubules, as seen in both insets, represents the peritubular dentin, which is the more mineralized part of the dentin. 240. Supporting Tissues of the Teeth


Histology_Ross

Histology_Ross. Supporting Tissues of the Teeth Supporting tissues of the teeth include the alveolar bone of the alveolar processes of the maxilla and mandible, periodontal ligaments, and gingiva. The alveolar processes of the maxilla and mandible contain the sockets or alveoli for the roots of the teeth. The alveolar bone proper, a thin layer of compact bone, forms the wall of the alveolus (see Fig. 16.7) and is the bone to which the periodontal ligament is attached. The rest of the alveolar process consists of supporting bone. The surface of the alveolar bone proper usually shows regions of bone resorption and bone deposition, particularly when a tooth is being moved (Fig. 16.20). Periodontal disease usually leads to loss of alveolar bone, as does the absence of functional occlusion of a tooth with its normal opposing tooth.


Histology_Ross

Histology_Ross. The periodontal ligament is the fibrous connective tissue joining the tooth to its surrounding bone. This ligament is also called the periodontal membrane, but neither term describes its structure and function adequately. The periodontal ligament provides for the following:  Tooth attachment (fixation) Tooth support  Bone remodeling (during movement of a tooth)


Histology_Ross

Histology_Ross. A histologic section of the periodontal ligament shows that it contains areas of both dense and loose connective tissue. The dense connective tissue contains collagen fibers and fibroblasts that are elongated parallel to the long axis of the collagen fibers. The fibroblasts are believed to move back and forth, leaving behind a trail of collagen fibers. Periodontal fibroblasts also contain internalized collagen fibrils that are digested by the hydrolytic enzymes of the cytoplasmic lysosomes. These observations indicate that the fibroblasts not only produce collagen fibrils but also resorb collagen fibrils, thereby adjusting continuously to the demands of tooth stress and movement.


Histology_Ross

Histology_Ross. The loose connective tissue in the periodontal ligament contains blood vessels and nerve endings. In addition to fibroblasts and thin collagenous fibers, the periodontal ligament also contains thin, longitudinally disposed oxytalan fbers. They are attached to bone or cementum at each end. Some appear to be associated with the adventitia of blood vessels. The gingiva is the part of the mucous membrane commonly called the gums. The gingiva is a specialized part of the oral mucosa located around the neck of the tooth. It is firmly attached to the teeth and to underlying alveolar bony tissue. An idealized diagram


Histology_Ross

Histology_Ross. The gingiva is a specialized part of the oral mucosa located around the neck of the tooth. It is firmly attached to the teeth and to underlying alveolar bony tissue. An idealized diagram FIGURE 16.17 • Electron micrograph of odontoblasts. The plasma membrane of one odontoblast has been marked with arrows. The cell contains a large amount of rough endoplasmic reticulum and a large Golgi apparatus. The odontoblast processes are not included in this image; one process would extend from the apical pole of each cell (top). The black objects in the Golgi region are abacus bodies. The tissue has been treated with pyroantimonate, which forms a black precipitate with calcium. 12,000.


Histology_Ross

Histology_Ross. of the gingiva is presented in Figure 16.20. The gingiva is composed of two parts:  Gingival mucosa, which is synonymous with the masticatory mucosa described above  Junctional epithelium, or attachment epithelium, which adheres firmly to the tooth. A basal lamina-like material is secreted by the junctional epithelium and adheres firmly to the tooth surface. The cells then attach to this material via hemidesmosomes. The basal lamina and the hemidesmosomes are together referred to as the epithelial attachment. In young individuals, this attachment is to the enamel; in older individuals, where passive tooth eruption and gingival recession expose the roots, the attachment is to the cementum. FIGURE 16.18 • Golgi apparatus in an odontoblast. This electron micrograph shows a region of the Golgi apparatus containing numerous large vesicles. Note the abacus bodies (arrows) that contain parallel arrays of filaments studded with granules. 52,000.


Histology_Ross

Histology_Ross. Above the attachment of the epithelium to the tooth, a shallow crevice called the gingival sulcus is lined with crevicular epithelium that is continuous with the attachment epithelium. The term periodontium refers to all the tissues involved in the attachment of a tooth to the mandible and maxilla. These include the crevicular and junctional epithelium, the cementum, the periodontal ligament, and the alveolar bone. The major salivary glands are paired glands with long ducts that empty into the oral cavity.


Histology_Ross

Histology_Ross. The major salivary glands are paired glands with long ducts that empty into the oral cavity. The major salivary glands, as noted above, consist of the paired parotid, submandibular, and sublingual glands. The parotid and the submandibular glands are actually located outside the oral cavity; their secretions reach the cavity by ducts. The parotid gland is located subcutaneously, below and in front of the ear in the space between the ramus of the mandible and the styloid process of the temporal bone. The submandibular gland is located under the floor of the mouth, in the submandibular triangle of the neck. The sub-lingual gland is located in the floor of the mouth anterior to the submandibular gland. The minor salivary glands are located in the submucosa of different parts of the oral cavity. They include the lingual, labial, buccal, molar, and palatine glands.


Histology_Ross

Histology_Ross. The minor salivary glands are located in the submucosa of different parts of the oral cavity. They include the lingual, labial, buccal, molar, and palatine glands. Each salivary gland arises from the developing oral cavity epithelium. Initially, the gland takes the form of a solid cord of cells that enters the mesenchyme. The proliferation of epithelial cells eventually produces highly branched epithelial cords with bulbous ends. Degeneration of the innermost FIGURE 16.19 • Odontoblast process of a young odontoblast. This electron micrograph shows a process of the odontoblast entering a dentinal tubule. The process extends into the predentin and, after passing the mineralization front (arrows), lies within the dentin. The collagen fibrils in the predentin are finer than the more mature, coarser fibrils of the mineralization front and beyond. 34,000.


Histology_Ross

Histology_Ross. FIGURE 16.20 • Schematic diagram of gingiva. This schematic diagram of gingiva corresponds to the rectangular area of the orientation diagram. The gingival epithelium is attached to the enamel of the tooth. Here, the junction between epithelium and connective tissue is smooth. Elsewhere, the gingival epithelium is deeply indented by connective tissue papillae, and the junction between the two is irregular. The black lines represent collagen fibers from the cementum of the tooth and from the crest of the alveolar bone that extend toward the gingival epithelium. Note the shallow papillae in the lining mucosa (alveolar mucosa) that contrast sharply with those of the gingiva. cells of the cords and bulbous ends leads to their canalization. The cords become ducts, and the bulbous ends become secretory acini. Secretory acini are organized into lobules.


Histology_Ross

Histology_Ross. cells of the cords and bulbous ends leads to their canalization. The cords become ducts, and the bulbous ends become secretory acini. Secretory acini are organized into lobules. The major salivary glands are surrounded by a capsule of moderately dense connective tissue from which septa divide the secretory portions of the gland into lobes and lobules. The septa contain the larger blood vessels and excretory ducts. The connective tissue associated with the groups of secretory acini blends imperceptibly into the surrounding loose connective tissue. The minor salivary glands do not have a capsule. Numerous lymphocytes and plasma cells populate the connective tissue surrounding the acini in both the major and minor salivary glands. Their significance in the secretion of salivary antibodies is described below. Acini are of three types: serous, mucous, or mixed.


Histology_Ross

Histology_Ross. Acini are of three types: serous, mucous, or mixed. The basic secretory unit of salivary glands, the salivon, consists of the acinus, intercalated duct, and excretory duct (Fig. 16.21). The acinus is a blind sac composed of secretory cells. The term acinus [L., berry or grape] refers to the secretory unit of the salivary glands. The acini of salivary glands contain either serous cells (protein secreting), mucous cells (mucin secreting), or both. The relative frequencies of the three types of acini are a prime characteristic by which the


Histology_Ross

Histology_Ross. Dental caries is an infectious microbial disease of teeth that results in the destruction of affected calcified tissues, i.e., enamel, dentin, and cementum. Carious lesions gener-ally occur under masses of bacterial colonies referred to as “dental plaque.” The onset of dental caries is primarily as-sociated with bacterial colonies of Streptococcus mutans, whereas lactobacilli are associated with active progression of the disease. These bacterial colonies metabolize carbo-hydrates, producing an acidic environment that demineral-izes the underlying tooth structure. Frequent sucrose ingestion is strongly associated with the development of these acidogenic bacterial colonies. Trace amounts of fluoride, from sources such as water supplies (0.5 to 1.0 ppm is optimal), toothpaste, and even diet, can improve resistance to the effects of cariogenic bacteria. Fluoride improves the acid resistance of the tooth structure, acts as an antimicrobial agent, and promotes remineralization of small


Histology_Ross

Histology_Ross. can improve resistance to the effects of cariogenic bacteria. Fluoride improves the acid resistance of the tooth structure, acts as an antimicrobial agent, and promotes remineralization of small carious lesions. Resis-tance to acid breakdown of enamel is facilitated by the substitution of fluoride ion for the hydroxyl ion in the hydroxyapatite crystal. This decreases enamel crystal solubility in acid. Treatment of cavitated lesions, or “tooth cavities” (Fig. F16.3.1), includes excavation of the infected tooth tis-sue and replacement with dental materials such as amal-gam, composite, and glass ionomer cements. Microbial invasion of tooth structure can reach the “pulp” of the tooth and elicit an inflammatory response. In this case, endodon-tic treatment, or a “root canal,” is generally recommended, with subsequent placement of a crown to add strength to the compromised coronal tooth structure.


Histology_Ross

Histology_Ross.  FOLDER 16.3 Clinical Correlation: Dental Caries aabba b EEEEDDDDEE CLCLCL D D DDD FIGURE F16.3.1 • Photomicrographs of carious lesions. a. Photomicrograph of an unstained ground section of a tooth showing a carious lesion (CL) that has penetrated the entire thickness of the enamel (E) and spread laterally at the amelodentinal junction. D, dentin. b. The lesion here is more advanced. The enamel (E) has been undermined and weakened, causing fracture and a resulting cavity. At this point, bacteria can invade and penetrate down the exposed dental tubules, resulting in destructive liquefaction foci in the dentin (D) and, ultimately, exposure of the pulp. 16. (From Eveson JW, Scully C. Color Atlas of Oral Pathology. London: Times Mirror International Publishers, 1995.) major salivary glands are distinguished. Thus, three types of acini are described:  Serous acini, which contain only serous cells and are generally spherical Mucous acini, which contain only mucous cells and are usually


Histology_Ross

Histology_Ross. are distinguished. Thus, three types of acini are described:  Serous acini, which contain only serous cells and are generally spherical Mucous acini, which contain only mucous cells and are usually more tubular  Mixed acini, which contain both serous and mucous cells. In routine H&E preparations, mucous acini have a cap of serous cells that are thought to secrete into the highly convoluted intercellular space between the mucous cells. Because of their appearance in histologic sections, such caps are called serous demilunes [Fr., half-moon].


Histology_Ross

Histology_Ross. Serous demilunes are artifacts of the traditional fixation method.


Histology_Ross

Histology_Ross. As noted above, each mixed acinus, such as those found in the sublingual and submandibular glands, contains serous and mucus-producing cells. In routine preparation for both light and electron microscopy, serous cells have traditionally been regarded as the structures that make up the demilune. Recent electron microscopic studies now challenge this classic interpretation of the demilune. Rapid freezing of the tissue in liquid nitrogen, followed by rapid freeze substitution with osmium tetroxide in cold acetone, reveals that both mucous and serous cells are aligned in the same row to surround the lumen of the secretory acinus. No serous demilune is found. Sections prepared from the same specimen by conventional methods show swollen mucous cells with enlarged secretory granules. The serous cells form typical demilunes and are positioned in the peripheral region of the acinus with slender cytoplasmic processes interposed between the mucous cells. These findings indicate that the demilune


Histology_Ross

Histology_Ross. form typical demilunes and are positioned in the peripheral region of the acinus with slender cytoplasmic processes interposed between the mucous cells. These findings indicate that the demilune observed in light or electron microscopy is an artifact of the routine fxation method (Fig. 16.22). The process of demilune formation can be explained by the expansion of mucinogen, a major component of secretory granules, during routine fixation. This expansion increases the volume of the mucous cells and displaces the serous cells from their original position, thus creating the demilune effect. A similar phenomenon is sometimes seen in the intestinal mucosa, in which swollen goblet cells displace adjacent absorptive cells.


Histology_Ross

Histology_Ross. FIGURE 16.21 • Diagram comparing the components of the salivon in the three major salivary glands. The four major parts of the salivon–the acinus, intercalated duct, striated duct, and excretory duct—are color-coded. The three columns on the right of the salivon compare the length of the different ducts in the three salivary glands. The red-colored cells of the acinus represent serous-secreting cells, and the yellow-colored cells represent mucus-secreting cells. The ratio of serous-secreting cells to mucus-secreting cells is depicted in the acini of the various glands. Serous cells are protein-secreting cells. Serous cells have a pyramidal shape, with a relatively wide basal surface facing the basal lamina and a small apical surface facing the lumen of the acinus. They contain large amounts of rER, free ribosomes, a prominent Golgi appara-


Histology_Ross

Histology_Ross. FIGURE 16.22 • Relationship of serous-secreting cells and mucus-secreting cells in the mixed acinus. a. This drawing indicates the relationship of the mucous and serous cells as observed in the electron microscope after the rapid-freezing method. The serous cells extend from the basal lamina to the lumen of the acinus. b. In this drawing, serous cells are shown occupying the periphery of the acinus to form the so-called serous demilune. This feature is visible in routine preparations using immersion fixation. The swollen mucous cells have forced out the serous cells, leaving small remnants of the cytoplasm between the mucous cells.


Histology_Ross

Histology_Ross. tus, and numerous spherical secretory granules (Fig. 16.23). As in most protein-secreting cells that store their secretions in zymogen granules, the granules are located in the apical cytoplasm. Most other organelles are located in the basal or perinuclear cytoplasm. In H&E sections, the basal cytoplasm of the serous cell stains with hematoxylin because of the rER and the free ribosomes, whereas the apical region stains with eosin, in large part because of the secretory granules. When examined with the transmission electron microscope (TEM), the base of the serous cell may display infoldings of the plasma membrane and basolateral folds in the form of processes that interdigitate with similar processes of adjacent cells. The serous cells are joined near their apical surface by junctional complexes to neighboring cells of the acinus (see Fig. 16.23). Mucous cells are mucin-secreting cells.


Histology_Ross

Histology_Ross. As in other mucus-secreting epithelia, the mucous cells of the mucous salivary acini undergo cyclic activity. During part of the cycle, mucus is synthesized and stored within the cell as mucinogen granules. When the product is discharged after hormonal or neural stimulation, the cell begins to resynthesize mucus. After discharge of most or all of the mucinogen granules, the cell is difficult to distinguish from an inactive serous cell. However, most mucous cells contain large numbers of mucinogen granules in their apical cytoplasm, and because the mucinogen is lost in H&E–stained paraffin sections, the apical portion of the cell usually appears empty. In TEM preparation, the rER, mitochondria, and other components are seen chiefly in the basal portion of the cell; this part of the cell also contains the nucleus, which is typically flattened against the base of the cell (Fig. 16.24). In rapid-freeze preparations (Fig. 16.25), cells are rounded and clearly isolated from each other. The


Histology_Ross

Histology_Ross. contains the nucleus, which is typically flattened against the base of the cell (Fig. 16.24). In rapid-freeze preparations (Fig. 16.25), cells are rounded and clearly isolated from each other. The nuclei are round and centrally located. The apical portion of the mucous cell contains numerous mucinogen granules and a large Golgi apparatus, in which large amounts of carbohydrate are added to a protein base to synthesize the glycoprotein of the mucin. Mucous cells possess apical junctional complexes, the same as those seen between serous cells.


Histology_Ross

Histology_Ross. Myoepithelial cells are contractile cells that embrace the basal aspect of the acinar secretory cells. Myoepithelial cells are contractile cells with numerous processes. They lie between the basal plasma membrane of the epithelial cells and the basal lamina of the epithelium (Fig. 16.26). Myoepithelial cells also underlie the cells of the proximal portion of the duct system. In both locations, the myoepithelial cells are instrumental in moving secretory products toward the excretory duct. Myoepithelial cells are sometimes difficult to identify in H&E sections. The nucleus of the cell is often seen as a small round profile near the basement membrane. The contractile filaments stain with eosin and are sometimes recognized as a thin eosinophilic band adjacent to the basement membrane.


Histology_Ross

Histology_Ross. FIGURE 16.23 • Electron micrograph of the apical portion of parotid gland serous cells. The cells are polarized, with their product package within the secretory vesicles (SV ) near the lumen (L) of the acinus. The cells display rough endoplasmic reticulum (rER) and several profiles of the Golgi apparatus (G). Immature secretory vesicles (IV ) are present close to the Golgi apparatus. At the apical pole of the cells are junctional complexes (JC). The intercellular space (IC) is dilated, and profiles of sectioned lateral plications are seen. M, mitochondria. 15,000. The lumen of the salivary acinus is continuous with that of a duct system that may have as many as three sequential segments:  Intercalated duct, which leads from the acinus  Striated duct, so-called because of the presence of “stria tions,” the infoldings of the basal plasma membrane of the columnar cells that form the duct  Excretory ducts, which are the larger ducts that empty into the oral cavity


Histology_Ross

Histology_Ross. The degree of development of the intercalated ducts and striated ducts varies, depending on the nature of the acinar secretion (see Fig. 16.21). Serous glands have well-developed intercalated ducts and striated ducts that modify the serous secretion by both absorption of specific components from the secretion and secretion of additional components to form the final product. Mucous glands, in which the secretion is not modified, have very poorly developed intercalated ducts that may not be recognizable in H&E sections. Moreover, they do not display striated ducts. Intercalated ducts are located between a secretory acinus and a larger duct.


Histology_Ross

Histology_Ross. Intercalated ducts are located between a secretory acinus and a larger duct. Intercalated ducts are lined by low cuboidal epithelial cells that usually lack any distinctive feature to suggest a function other than that of a conduit. However, the cells of intercalated ducts possess carbonic anhydrase activity. In serous-secreting glands and mixed glands, they have been shown to  secrete HCO3 into the acinar product.  absorb Cl from the acinar product. As noted above, intercalated ducts are most prominent in those salivary glands that produce a watery serous secretion. In mucus-secreting salivary glands, the intercalated ducts, when present, are short and difficult to identify. Striated duct cells have numerous infoldings of the basal plasma membrane.


Histology_Ross

Histology_Ross. Striated duct cells have numerous infoldings of the basal plasma membrane. FIGURE 16.24 • Low-magnification electron micrograph of a mucous acinus. The mucous cells contain numerous mucinogen granules. Many of the granules have coalesced to form larger irregular masses that will ultimately discharge into the lumen (L) of the acinus. Myoepithelial cell processes (MyC) are evident at the periphery of the acinus. 5,000.


Histology_Ross

Histology_Ross. Striated ducts are lined by a simple cuboidal epithelium that gradually becomes columnar as it approaches the excretory duct. The infoldings of the basal plasma membrane are seen in histologic sections as “striations.” Longitudinally oriented, elongated mitochondria are enclosed in the infoldings. Basal infoldings associated with elongated mitochondria are a morphologic specialization associated with reabsorption of fluid and electrolytes. The striated duct cells also have numerous basolateral folds that are interdigitated with those of adjacent cells. The nucleus typically occupies a central (rather than basal) location in the cell. Striated ducts are the sites of  reabsorption of Na from the primary secretion.  secretion of K and HCO3 into the secretion.


Histology_Ross

Histology_Ross. More Na is resorbed than K is secreted, so the secretion becomes hypotonic. When secretion is very rapid, more Na and less K appear in the final saliva because the reabsorption and secondary secretion systems cannot keep up with the rate of primary secretion. Thus, the saliva may become isotonic to hypertonic. The diameter of striated ducts often exceeds that of the secretory acinus. Striated ducts are located in the parenchyma of the glands (they are intralobular ducts) but may be surrounded by small amounts of connective tissue in which blood vessels and nerves can be seen running in parallel with the duct. Excretory ducts travel in the interlobular and interlobar connective tissue.


Histology_Ross

Histology_Ross. Excretory ducts travel in the interlobular and interlobar connective tissue. Excretory ducts constitute the principal ducts of each of the major glands. They ultimately connect to the oral cavity. The epithelium of small excretory ducts is simple cuboidal. It gradually changes to pseudostratified columnar or stratified cuboidal. As the diameter of the duct increases, stratified columnar epithelium is often seen, and as the ducts approach the oral epithelium, stratified squamous epithelium may be present. The parotid duct (Stensen’s duct) and the submandibular duct (Wharton’s duct) travel in the connective tissue of the face and neck, respectively, for some distance from the gland before penetrating the oral mucosa. The parotid glands are completely serous. The paired serous parotid glands are the largest of the major salivary glands. The parotid duct travels from the gland,


Histology_Ross

Histology_Ross. FIGURE 16.25 • Electron micrographs of mixed acini. a. Low-magnification electron micrograph of the sublingual gland, prepared by the rapid freezing and freeze-substitution method, shows the arrangement of the cells within a single acinus. The mucous cells have well-preserved round mucinogen granules. The mucous and serous cells are aligned to surround the acinus lumen. Serous demilunes are not evident. 6,000. b. Electron micrograph of the sublingual gland prepared by traditional fixation in formaldehyde. Note the considerable expansion and coalescence of the mucinogen granules and the formation of a serous demilune. 15,000. (Courtesy of Dr. Shohei Yamashina.) which is located below and in front of the ear, to enter the oral cavity opposite the second upper molar tooth. The secretory units in the parotid are serous and surround numerous, long, narrow intercalated ducts. Striated ducts are large and conspicuous (Fig. 16.27a).


Histology_Ross

Histology_Ross. Large amounts of adipose tissue often occur in the parotid gland; this is one of its distinguishing features (Plate 52, page 565). The facial nerve (cranial nerve VII) passes through the parotid gland; large cross sections of this nerve may be encountered in routine H&E sections of the gland and are useful in identifying the parotid. Mumps, a viral infection of the parotid gland, can damage the facial nerve. The submandibular glands are mixed glands that are mostly serous in humans. The large, paired, mixed submandibular glands are located under either side of the floor of the mouth, close to the mandible. A duct from each of the two glands runs forward and medially to a papilla located on the floor of the mouth just lateral to the frenulum of the tongue. Some mucous acini capped by serous demilunes are generally found among the predominant serous acini. Intercalated ducts are less extensive than in the parotid gland (Fig. 16.27b and Plate 51, page 563).


Histology_Ross

Histology_Ross. The small sublingual glands are mixed glands that are mostly mucous secreting in humans. The sublingual glands, the smallest of the paired major salivary glands, are located in the floor of the mouth anterior to the submandibular glands. Their multiple small sublingual ducts empty into the submandibular duct as well as directly onto the floor of the mouth. Some of the predominant mucous acini exhibit serous demilunes, but purely serous acini are rarely present (Fig. 16.27c and Plate 53, page 567). Intercalated ducts and striated ducts are short, difficult to locate, or sometimes absent. The mucous secretory units may be more tubular than purely acinar. Saliva includes the combined secretions of all the major and minor salivary glands. Most saliva is produced by the salivary glands. A smaller amount is derived from the gingival sulcus, tonsillar crypts, and general transudation from the epithelial lining of the oral


Histology_Ross

Histology_Ross. Most saliva is produced by the salivary glands. A smaller amount is derived from the gingival sulcus, tonsillar crypts, and general transudation from the epithelial lining of the oral FIGURE 16.26 • Electron micrograph of the basal portion of an acinus. This electron micrograph shows the basal portion of two secretory cells from a submandibular gland. A myoepithelial cell process is also evident. Note the location of the myoepithelial cell process on the epithelial side of the basal lamina. The cytoplasm of the myoepithelial cell contains contractile filaments and densities (arrows) similar to those seen in smooth muscle cells. The cell on the left with the small nucleus is a lymphocyte. Having migrated through the basal lamina, it is also within the epithelial compartment. Arrowheads, cell boundaries; asterisks, basolateral folds. 15,000.


Histology_Ross

Histology_Ross. cavity. One of the unique features of saliva is the large and variable volume produced. The volume (per weight of gland tissue) of saliva exceeds that of other digestive secretions by as much as 40 times. The large volume of saliva produced is undoubtedly related to its many functions, only some of which are concerned with digestion. Saliva performs both protective and digestive functions.


Histology_Ross

Histology_Ross. Saliva performs both protective and digestive functions. The salivary glands produce about 1,200 mL of saliva a day. Saliva has numerous functions relating to metabolic and nonmetabolic activities, including these:  Moistening the oral mucosa Moistening dry foods to aid swallowing Providing a medium for dissolved and suspended food ma terials that chemically stimulate taste buds Buffering the contents of the oral cavity, because of its high concentration of bicarbonate ions  Digesting carbohydrates with the digestive enzyme -amylase, which breaks one to four glycosidic bonds and continues to act in the esophagus and stomach  Controlling the bacterial flora of the oral cavity by use of lysozyme (muramidase), an enzyme that lyses the muramic acid in certain bacteria (e.g., staphylococci) The unique composition of saliva is summarized in Table 16.1. Saliva is a source of calcium and phosphate ions essential for normal tooth development and maintenance.


Histology_Ross

Histology_Ross. The unique composition of saliva is summarized in Table 16.1. Saliva is a source of calcium and phosphate ions essential for normal tooth development and maintenance. Calcium and phosphate in the saliva are essential for the mineralization of newly erupted teeth and for repair of precarious lesions of the enamel in erupted teeth. In addition, saliva serves several other roles in protecting the teeth. Proteins in saliva cover the teeth with a protective coat called the acquired pellicle. Antibodies and other antibacterial agents retard bacterial action that would otherwise lead to tooth decay. Patients whose salivary glands are irradiated, as in the treatment of salivary gland tumors, fail to produce normal amounts of saliva; these patients typically develop rampant caries. Anticholinergic drugs used to treat some forms of heart disease also significantly reduce salivary secretion, leading to dental caries. Saliva performs immunologic functions.


Histology_Ross

Histology_Ross. Saliva performs immunologic functions. As noted, saliva contains antibodies, salivary immunoglobulin A (IgA). IgA is synthesized by plasma cells in the connective tissue surrounding the secretory acini of the salivary glands, and both dimeric and monomeric forms are released into the connective tissue matrix (Fig. 16.28).


Histology_Ross

Histology_Ross. FIGURE 16.27 • Photomicrographs of the three major salivary glands. a. The parotid gland in the human is composed entirely of serous acini and their ducts. Typically, adipose cells are also distributed throughout the gland. The lower portion of the figure reveals an excretory duct within a connective tissue septum. 120. Inset. Higher magnification of the serous acinar cells. 320. b. The submandibular glands contain both serous and mucous acini. In humans, the serous components predominate. The mucus-secreting acini are readily discernible at this low magnification because of their light staining. The remainder of the field is composed largely of serous acini. Various ducts—excretory, striated, and intercalated—are evident in the field. 120. Left inset. Higher magnification of an acinus revealing a serous demilune surrounding mucus-secreting cells. 360. Right inset. Higher magnification of a striated duct. These ducts have columnar epithelium with visible basal striations. 320. c. The


Histology_Ross

Histology_Ross. a serous demilune surrounding mucus-secreting cells. 360. Right inset. Higher magnification of a striated duct. These ducts have columnar epithelium with visible basal striations. 320. c. The sublingual gland also contains both serous and mucous elements. Here, the mucous acini predominate. The mucous acini are conspicuous because of their light staining. Critical examination of the mucous acini at this relatively low magnification reveals that they are not spherical structures but, rather, elongate or tubular structures with branching outpockets. Thus, the acinus is rather large, and much of it is usually not seen within the plane of a single section. The ducts of the sublingual gland that are observed with the greatest frequency in a section are the interlobular ducts. 120. Inset. The serous component of the gland is composed largely of demilunes (asterisks), artifacts of conventional fixation. 320.


Histology_Ross

Histology_Ross. TABLE Composition of Unstimulated Saliva16.1 Organic constituents Mean (mg/mL) Protein 220.0 Amylase 38.0 Mucin 2.7 Muramidase (lysozyme) 22.0 Lactoferrin 0.03 ABO group markers 0.005 EGF 3.4 sIgA 19.0 IgG 1.4 IgM 0.2 Glucose 1.0 Urea 20.0 Uric acid 1.5 Creatinine 0.1 Cholesterol 8.0 cAMP 7.0 Inorganic constituents Sodium 15.0 Potassium 80.0 Thiocyanate Smokers 9.0 Nonsmokers 2.0 Calcium 5.8 Phosphate 16.8 Chloride 50.0 Fluoride Traces (according to intake) Modified from Jenkins GN. The Physiology and Biochemistry of the Mouth, 4th ed. Oxford: Blackwell Scientific Publications, 1978. sIgA, secretory IgA; EGF, epithelial growth factor. A polymeric immunoglobulin receptor (pIgR) protein is synthesized by the salivary gland cells and inserted into the basal plasma membrane, where it serves as a receptor for dimeric IgA.


Histology_Ross

Histology_Ross. A polymeric immunoglobulin receptor (pIgR) protein is synthesized by the salivary gland cells and inserted into the basal plasma membrane, where it serves as a receptor for dimeric IgA. When the dimeric IgA binds to the receptor, the pIgR-dIgA complex is internalized by receptor-mediated endocytosis and carried through the acinar cell to the apical plasma membrane. FIGURE 16.28 • Diagram of different forms of immunoglobulin A (IgA). This drawing shows the monomer of IgA (top). The dimer of IgA is a product of the plasma cell and contains a J chain (J ) connecting two monomers (middle). The secretory component (SC), a product of proteolytically cleaved pIgR is added to the dimer to form secretory IgA (sIgA) (sIgA, bottom).


Histology_Ross

Histology_Ross. Here, pIgR is proteolytically cleaved and the extracellular part of the receptor that is bound to dIgA is released into the lumen as secretory IgA (sIgA). This process of synthesis and secretion of IgA is essentially identical to that which occurs in the more distal parts of the gastrointestinal tract, where sIgA is transported across the absorptive columnar epithelium of the small intestine and colon (see page 596). Saliva contains water, various proteins, and electrolytes. Saliva contains chiefly water, proteins and glycoproteins (enzymes and antibodies), and electrolytes. It has a high potassium concentration that is approximately seven times that of blood, a sodium concentration approximately one tenth that of blood, a bicarbonate concentration approximately three times that of blood, and significant amounts of calcium, phosphorus, chloride, thiocyanate, and urea. Lysozyme and -amylase are the principal enzymes present (see Table 16.1).


Histology_Ross

Histology_Ross.  FOLDER 16.4 Clinical Correlation: Salivary Gland Tumors chapter 16 Digestive System I SALIVARY G LAN DS 555 Tumors of salivary glands usually occur in the major salivary glands (parotid, submandibular, and sublingual); however, a small percentage occur in the minor glands located within the oral mucosa, palate, uvula, floor of mouth, tongue, pharynx, larynx, and paranasal sinuses. Approxi-mately 80% of salivary gland tumors are benign. Most orig-inate in the parotid gland (Fig. F16.4.1a). The palate is the most common site of minor salivary gland tumors. The most common benign tumor is the pleomorphic adenoma, which accounts for 65% of all salivary gland tu-mors. It is characterized by epithelial tissue containing duc-tal and myoepithelial cells intermingled with areas resembling ground substance of connective tissues (e.g., in cartilage). These connective-like tissues are produced by myoepithelial cells (Fig. F16.4.1b). Most patients with benign tumors present with painless


Histology_Ross

Histology_Ross. ground substance of connective tissues (e.g., in cartilage). These connective-like tissues are produced by myoepithelial cells (Fig. F16.4.1b). Most patients with benign tumors present with painless swelling of the involved gland. Because of nerve involvement, signs such as numbness or weakness of innervated muscle are also reported. For instance, paralysis of facial muscles or persistent facial pain may be present in some individuals with parotid tumors. The most common treatment is surgical removal of the tumor. For parotid gland tumors, a total parotidectomy (ex-cision of parotid gland) is often necessary. Postoperative radiation therapy is also used when the tumor is cancer-ous. Complications of surgical treatment of parotid gland tumors include facial nerve dysfunction and Frey’s syn-drome (also known as gustatory sweating). a bb FIGURE F16.4.1 • Pleomorphic adenoma of the parotid gland. a. This photograph shows a patient with a parotid mass located near the angle of the


Histology_Ross

Histology_Ross. syn-drome (also known as gustatory sweating). a bb FIGURE F16.4.1 • Pleomorphic adenoma of the parotid gland. a. This photograph shows a patient with a parotid mass located near the angle of the mandible. b. This low magnification photomicrograph shows the features of a pleomorphic adenoma. Note that normal parotid tissue (basophilic-stained areas in upper right) is infiltrated by nodules containing connective-like tissue that resembles the extracellular matrix of cartilage (lighter eosinophilic-stained regions). 120. (From Rubin E, Gorstein F, Schwarting R, Strayer DS. Rubin’s Pathology, 4th ed. Baltimore: Lippincott Williams & Wilkins, 2004, Figs 25–20 and 25–22.)


Histology_Ross

Histology_Ross. The lips are the entry point of the alimentary canal. Here, the thin keratinized epithelium of face skin changes to the thick parakeratinized epithelium of the oral mucosa. At the mucocutaneous junction, the red portion of the lips, is characterized by deep pene-tration of connective tissue papillae into the base of the stratified squamous kera-tinized epithelium. The blood vessels and nerve endings in these papillae are responsible for both the color and the exquisite touch sensitivity of the lips. ORIENTATION MICROGRAPH: An H&E–stained sagittal section through the upper lip in this low-power orientation photomicrograph to the right (×8) reveals the skin of the face, the red margin of the lip, and the transition to the oral mucosa (OM). The marked rectangles indicate representative areas of each of these sites, shown at higher magnifications in upper, middle and lower rows of figures, on the adjacent plate. Note the change in thickness of the epithelium from the exterior or facial


Histology_Ross

Histology_Ross. of each of these sites, shown at higher magnifications in upper, middle and lower rows of figures, on the adjacent plate. Note the change in thickness of the epithelium from the exterior or facial portion of the lip (the vertical surface on the right) to the interior surface of the oral cavity (the surface beginning with rectangle marked lower and continuing down the left surface of the lip) in this micrograph. lower OM skin middle top


Histology_Ross

Histology_Ross. Keratinized epithelium, lip, human, H&E ×120. The keratinized epithelium (EP) of the face is relatively thin and has the general features of thin skin found in other sites. Associated with it are hair follicles (HF) and sebaceous glands (SGl). Red margin, lip, human, H&E ×120. The epithelium of the red margin of the lip is much thicker than that of the face. The stratum granulosum is still present; thus, the epithelium is keratinized. The feature that accounts for the coloration of the red margin is the deep penetration of the connective tissue papillae into the epithelium (arrowheads). The thinness of the epithelium combined with the extensive vascularity of the underlying connective tissue, particularly the extensive venous blood vessels (BV), allows the color of the blood to show through. Keratinized epithelium, lip, human, H&E ×380.


Histology_Ross

Histology_Ross. Keratinized epithelium, lip, human, H&E ×380. The circled area in figure on left is shown at higher magnification here. The reddish brown material in the basal cells is the pigment melanin (M), and the dark blue near the surface is the stratum granulosum (SG) with its deep-blue-stained keratohyalin granules. Red margin, lip, human, H&E ×380. The sensitivity of the red margin to stimuli such as light touch is due to the presence of an increased number of sensory receptors. In fact, each of the two deep papillae seen in figure on left contains a Meissner’s corpuscle, one of which (MC) is more clearly seen in this figure. Mucocutaneous junction, lip, human, H&E ×120. The transition from the keratinized red margin to the fairly thick stratified squamous parakeratinized epithelium of the oral mucosa is evident in this figure. Note how the stratum granulosum suddenly ends. This is more clearly shown at higher magnification in figure on right.


Histology_Ross

Histology_Ross. Mucocutaneous junction, lip, human, H&E ×380. Beyond the site where the stratum granulosum cells disappear, nuclei are seen in the superficial cells up to the surface (arrows). The epithelium is also much thicker at this point and remains so throughout the oral cavity. PLATE 48 KEY BV, venous blood vessels EP, epithelium HF, hair follicle M, melanin pigment MC, Meissner’s corpuscle OM, oral mucosa SG, stratum granulosum SGl, sebaceous gland arrowheads, connective tissue papillae arrows, nuclei of superficial cells up to surface


Histology_Ross

Histology_Ross. The tongue is a muscular organ projecting into the oral cavity from its inferior surface. It is covered with a mucous membrane that consists of stratified squamous epithelium, keratinized in parts, resting on a loose connective tissue. The undersurface of the tongue is relatively uncomplicated. The mucosa of the dorsal surface, however, is modified to form three types of papillae: filiform, fungiform, and circumvallate. The circumvallate papillae form a V-shaped row that divides the tongue into a body and a root; the dorsal surface of the body, i.e., the portion anterior to the circumvallate papillae, contains filiform and fungiform papillae. Parallel ridges bearing taste buds are found on the sides of the tongue and are particularly evident in infants. When sectioned at right angles to their long axis, they appear as papillae and, although not true papillae, are called foliate papillae. The tongue contains both intrinsic and extrinsic voluntary striated muscle. The striated muscles


Histology_Ross

Histology_Ross. their long axis, they appear as papillae and, although not true papillae, are called foliate papillae. The tongue contains both intrinsic and extrinsic voluntary striated muscle. The striated muscles of the tongue are arranged in three inter-weaving planes, with each arrayed at right angles to the other two. This arrangement is unique to the tongue. It provides enormous flexibility and precision in the movements of the tongue that are essential to human speech as well as to its role in digestion and swallowing. The ar-rangement also allows easy identification as lingual muscle.


Histology_Ross

Histology_Ross. Dorsal surface, tongue, monkey, H&E ×65; inset ×130.


Histology_Ross

Histology_Ross. This figure shows the dorsal surface of the tongue with the filiform papillae (Fil P). They are the most numerous of the three types of papillae. Structurally, they are bent, conical projections of the epithelium, with the point of the projection directed posteriorly. These papillae do not possess taste buds and are composed of stratified squamous keratinized epithelium. The fungiform papillae are scattered about as isolated, slightly rounded, elevated structures situated among the filiform papillae. A fungiform papilla is shown in the inset. A large connective tissue core (primary connective tissue papilla) forms the center of the fungiform papilla, and smaller connective tissue papillae (secondary connective tissue papillae) project into the base of the surface epithelium (arrowhead). The connective tissue of the papillae is highly vascularized. Because of the deep penetration of connective tissue into the epithelium, combined with a very thin keratinized surface, the fungiform


Histology_Ross

Histology_Ross. The connective tissue of the papillae is highly vascularized. Because of the deep penetration of connective tissue into the epithelium, 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.


Histology_Ross

Histology_Ross. Ventral surface, tongue, monkey, H&E ×65. The ventral surface of the tongue is shown in this figure. The smooth surface of the stratified squamous epithelium (Ep) contrasts with the irregular surface of the dorsum of the tongue. Moreover, the epithelial surface on the ventral surface of the tongue is usually not keratinized. The connective tissue (CT) is immediately deep to the epithelium; deeper still is the striated muscle (M). The numerous connective tissue papillae that project into the base of the epithelium of both ventral and dorsal surfaces give the epithelial– connective tissue junction an irregular profile. Often, these connective tissue papillae are cut obliquely and then appear as small islands of connective tissue within the epithelial layer (see figure above).


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Histology_Ross. The connective tissue extends as far as the muscle without changing character, and no submucosa is recognized. The muscle (M) is striated and is unique in its organization; i.e., the fibers travel in three planes. Therefore, most sections will show bundles of muscle fibers cut longitudinally, at right angles to each other, and in cross section. Nerves (N) that innervate the muscle are also frequently observed in the connective tissue septa between the muscle bundles. The surface of the tongue behind the vallate papillae (the root of the tongue) contains lingual tonsils (not shown). These are similar in structure and appearance to the palatine tonsils illustrated in Plate 36. KEY CT, connective tissue Ep, epithelium Fil P, filiform papillae M, striated muscle bundles N, nerves arrowhead (inset), secondary connective tissue papilla


Histology_Ross

Histology_Ross. The papillae and their associated taste buds constitute the specialized mucosa of the oral cavity. Although filiform papillae do not have taste buds, the other three types, viz., foliate, fungiform, and circumvallate, contain taste buds in their epithelium. The fungiform (i.e., mushroom-shaped) papillae (see inset on Plate 49) are most numerous near the tip of the tongue. Taste buds are present in the epithelium on their dor-sal surface. The taste buds in the epithelium covering the circumvallate and foliate papillae are located in deep clefts that separate the papillae from adjacent mucosa or from each other, respectively. Ducts of lingual salivary glands (von Ebner’s glands; a component of the minor salivary glands) empty their serous secretions into the moat surrounding each circumvallate papilla. The secretions flush material from the moat to allow the taste buds to respond to new stimuli. Similarly, ducts of small serous glands empty into the clefts between foliate papillae.


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Histology_Ross. papilla. The secretions flush material from the moat to allow the taste buds to respond to new stimuli. Similarly, ducts of small serous glands empty into the clefts between foliate papillae. Taste buds in section appear as oval, pale-staining bodies that extend through the thickness of the epithelium. A small opening at the epithe-lial surface is called the taste pore. Taste buds react to only five stimuli: sweet, salty, bitter, sour, and umami. These modalities appear to be more concentrated in the specific areas of the tongue; taste buds at the tip of the tongue detect sweet stimuli, those immediately posterolat-eral to the tip detect salty stimuli, those on the circumvallate papillae detect bitter and umami stimuli.


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Histology_Ross. Foliate papillae, tongue, human, H&E, ×50.


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Histology_Ross. Foliate papillae consist of a series of parallel ridges that are separated by narrow, deep mucosal clefts (see orientation photograph, pg 75). They are aligned at right angles to the long axis of the tongue on its posterior lateral edge. In younger individuals, they are readily observed by gross inspection. However, with age, foliate papillae may not be recognized. This micrograph shows three papillae, each is separated from its neighbor by a narrow cleft (C). The surface of these papillae is covered by a thick stratified non keratinized epithelium (SE). The basal surface of the epithelium is extremely uneven due to the presence of deep, penetrating connective tissue papillae (CTP). In contrast, the epithelium lining the clefts (Ep) is relatively thin and uniform. It contains numerous taste buds. These are the light-staining objects seen in the cleft epithelium. Underlying the epithelium is a layer of loose connective tissue (LCT) and a central core of dense connective tissue. Within


Histology_Ross

Histology_Ross. buds. These are the light-staining objects seen in the cleft epithelium. Underlying the epithelium is a layer of loose connective tissue (LCT) and a central core of dense connective tissue. Within this core and between bundles of muscle fibers beneath the papillae are lingual serous glands (LSG). These glands, like the serous glands associated with the circumvalate papillae, have ducts (D) that empty into the base of the clefts between papillae.


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Histology_Ross. Taste bud, tongue, human, H&E, ×1100. This micrograph shows to advantage the taste pore (TP), the cells of the taste bud, and its associated nerve fibers (NF). The cells with the large, round nuclei are neuroepithelial sensory cells (NSC). They are the most numerous cells of the taste bud. At their apical surface, they possess microvilli that extend into the taste pore. At their basal surface, they form a synapse with the afferent sensory fibers that make up the underlying nerve. Among the sensory cells are supporting cells (SC). These cells contain microvilli on their apical surface. Also present in the taste bud at its base are small cells simply referred to as basal cells (BC), one of which is identified here. They are the stem cells for the supporting and neuroepithelial cells which have a turnover life of about 10 days.


Histology_Ross

Histology_Ross. KEY BC, basal cells C, cleft CTP, connective tissue papillae D, ducts Ep, epithelium lining the clefts LCT, loose connective tissue LSG, lingual serous glands NF, nerve fibers NSC, neuroepithelial sensory cells SC, supporting cells SE, stratified nonkeratinized epithelium TP, taste pore


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Histology_Ross. Like the parotid glands, the submandibular glands are located outside of the oral cav-ity. They are located under either side of the floor of the mouth near the mandible. A duct runs forward and medially from each of the two glands to a papilla located on the floor of the mouth just lateral to the frenulum of the tongue. The secretory component of the submandibular glands are the acini which are of three types, namely serous acini that are protein secreting like those of the parotid gland, mucous acini that secrete mucin, and third, acini containing both serous and mucous secreting cells. In the case of the mixed acini, the mucous cells are capped by serous cells, which are typically described as demilumes. Recent studies suggest that the demilume is an artifact of tissue preparation and that all of the cells are aligned to secrete into the acinus lumen. Traditional fixation in formaldehyde appears to expand the mucous cells with the consequent squeezing of the serous cells to form


Histology_Ross

Histology_Ross. that all of the cells are aligned to secrete into the acinus lumen. Traditional fixation in formaldehyde appears to expand the mucous cells with the consequent squeezing of the serous cells to form their cap like position. ORIENTATION MICROGRAPH: This micrograph reveals a portion of the sub-mandibular gland. A single well-defined lobe (L) is seen in the upper part of the micro-graph. Within the central portion of the gland, there is a dense connective tissue core (DCT) containing the larger arteries (A), veins (V), and excretory ducts (ED) of the gland. The submandibular gland is a mixed gland; those regions containing serous acini (SA) are darkly stained whereas, regions containing mucous acini (MA) are lighter in appearance. SASASASAMAMASASADCTDCTVVEDEDEDEDAASASAMAMALLSASAMASADCTVEDEDASAMAL


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Histology_Ross. Submandibular gland, human, H&E, ×175. This micrograph reveals the various components of the sub-mandibular gland. The serous acini (SA) are darkly stained in contrast to the lighter staining mucous acini (MA). Furthermore, the serous acini are generally spherical in shape. The mucous acini are more tubular or elongate and sometimes can be seen to branch. The secretion from the acini enters an intercalated duct. They are the smallest ducts and are of relatively short length. They reside within the lobule, but are often difficult to find because of their shortness. Submandibular gland, human, H&E, ×725.


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Histology_Ross. The boxed area in the above micrograph is shown here at higher magnification. It includes several mucous acini (MA) on the left side of the micrograph, a number of serous acini (SA) on the right side of the micrograph, and in the center, two mixed acini (MxA) consisting of mucous-secreting cells and serous-secreting cells. Characteristically, the mucous-secreting cells have a pale-staining cytoplasm with their nuclei flattened at the base of the cell. In contrast, the serous-secreting cells are deeply stained and exhibit round nuclei. In addition, the lumen (Lu) of the acini associated with the mucous se-These ducts, in turn, empty into the larger striated duct (SD). This type of duct is better demonstrated in the micrograph below. Their contents then empty into an excretory duct (ED) which is recognized by a stratified or pseudostratified epithelium. Other features of note in this micrograph are arteries (A) and veins (V) which are found coursing in the connective tissue with the


Histology_Ross

Histology_Ross. which is recognized by a stratified or pseudostratified epithelium. Other features of note in this micrograph are arteries (A) and veins (V) which are found coursing in the connective tissue with the larger ducts. Also evident in this micrograph is an area containing an accumulation of lymphocytes and plasma cells (LP).


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Histology_Ross. creting cells is relatively wide; the lumen of the serous acini is relatively narrow and difficult to find. An additional point that should be made is that the serous cells of the mixed acini generally appear as a cap in relation to the mucous cells. These cells are referred to as demilumes. In evaluating some of those acini that appear to be serous in nature, it is possible that they simply represent a tangential section of a demilume. A striated duct (SD) is also included in the micrograph. It is so named because of the faint striations that can be seen in the basal cytoplasm. These ducts, as noted, receive secretion from the intercalated ducts and empty into the larger excretory ducts. KEY A, arteries DCT, dense connective tissue core ED, excretory ducts L, lobe LP, lymphocytes and plasma cells Lu, lumen MA, mucous acini MxA, mixed acini SA, serous acini SD, striated duct V, veins


Histology_Ross

Histology_Ross. KEY A, arteries DCT, dense connective tissue core ED, excretory ducts L, lobe LP, lymphocytes and plasma cells Lu, lumen MA, mucous acini MxA, mixed acini SA, serous acini SD, striated duct V, veins The parotid glands are the largest of the major salivary glands. They are composed of alveoli containing only serous secretory cells. Adipose tissue often occurs in the parotid gland and may be one of its distinguishing features. The facial nerve (cranial nerve VII) passes through the parotid gland; large cross sections of this nerve, often found in routine H&E sections of the gland, may also be of help in identifying the parotid. Mumps, a viral infection of the parotid gland, can damage the facial nerve. Parotid gland, human, H&E ×160.


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Histology_Ross. Parotid gland, human, H&E ×160. The parotid gland in the human is composed entirely of serous acini (A) and their ducts. However, numerous adipose cells (AC) are usually distributed throughout the gland. Both the serous acini and their duct system in the parotid gland are comparable in structure and arrangement to the same components in the submandibular gland. Within the lobule, the striated ducts Parotid gland, monkey, glutaraldehyde-osmium tetroxide fxed, H&E ×640.


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Histology_Ross. Parotid gland, monkey, glutaraldehyde-osmium tetroxide fxed, H&E ×640. The serous cells are optimally preserved in this specimen and reveal their secretory (zymogen) granules. The granules appear as fine dot-like objects within the cytoplasm. The acinus in the upper right of the figure has been cut in cross section and reveals the acinar lumen (AL). The small rectangle drawn in the acinus represents an area comparable to the electron micrograph shown as Figure 16.23. The large acinar profile to the left of the striated duct (StD) shows that the acini are not simple spheres but, rather, irregular elongate structures. Because of the small size of the acinar lumen and the variability in sectioning an acinus, the lumen is seen infrequently.


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Histology_Ross. (StD) are readily observed. They exhibit a simple columnar epithelium. The intercalated ducts are smaller; at the low magnification of this figure, they are difficult to recognize. A few intercalated ducts (ID) are indicated. The lower portion of the figure reveals an excretory duct (ED) within a connective tissue septum (CT). The epithelium of this excretory duct exhibits two layers of nuclei and is either pseudostratified or, possibly, already true stratified epithelium.


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Histology_Ross. A cross-sectional profile of an intercalated duct (ID) appears on the left of the micrograph; note its simple cuboidal epithelium. A single flattened nucleus is present at the top of the duct and may represent one of the myoepithelial cells that are associated with the beginning of the duct system as well as with the acini (A). The large duct occupying the center of the micrograph is a striated duct (StD). It is composed of columnar epithelium. The striations (S) that give the duct its name are evident. Also of significance is the presence of plasma cells (PC) within the connective tissue surrounding the duct. These cells produce the immunoglobulins taken up and resecreted by the acinar cells, particularly secretory IgA (sIgA). PLATE 52 • PAROTI D G LAN D KEY A, acinus AC, adipose cell AL, acinar lumen CT, connective tissue ED, excretory duct ID, intercalated duct PC, plasma cells S, striations of duct cells StD, striated duct


Histology_Ross

Histology_Ross. The sublingual glands are the smallest of the paired major salivary glands. Their multiple small ducts empty into the submandibular ducts as well as directly onto the floor of the mouth. The sublingual gland resembles the submandibular gland, in that it contains both serous and mu-cous elements. In the sublingual gland, however, the mucous acini predominate. Some of the predominant mucous acini have serous de-milunes, but purely serous acini are rarely present. Saliva includes the combined secretions of all the major and minor salivary glands. The functions of saliva include moistening dry foods to aid swallowing, dissolving and suspending food materials that chemically stimulate taste buds, buffering the contents of the oral cavity through its high concentration of bicarbonate ion, digestion of carbohydrates by the digestive enzyme -amylase (which breaks the 1–4 gly-coside bonds and continues to act in the esophagus and stomach), and controlling the bacterial flora of the oral cavity


Histology_Ross

Histology_Ross. of carbohydrates by the digestive enzyme -amylase (which breaks the 1–4 gly-coside bonds and continues to act in the esophagus and stomach), and controlling the bacterial flora of the oral cavity because of the pres-ence of the antibacterial enzyme lysozyme. Saliva is a source of calcium and phosphate ions essential for normal tooth development and maintenance. It also contains antibodies, no-tably salivary sIgA. Salivation is part of a reflex arc that is normally stimulated by the ingestion of food, although sight, smell, or even thoughts of food can also stimulate salivation.


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Histology_Ross. Sublingual gland, human, H&E ×160. This figure shows a sublingual gland at low power. The mucous acini (MA) are conspicuous because of their light staining. Critical examination of the mucous acini at this relatively low magnification reveals that they are not spherical structures but, rather, elongate or tubular structures with branch ing outpockets. Thus, the acinus is rather large, and much of it is usually not seen within the plane of a single section. The serous component of the gland is composed largely of demilunes, but occasional serous acini are present. As noted earlier, some of the serous Sublingual gland, human, H&E ×400.


Histology_Ross

Histology_Ross. Sublingual gland, human, H&E ×400. Note that through a fortuitous plane of section the lumen of a mucous acinus (MA) (upper right) is seen joining an intercalated duct (ID). The juncture between the acinus and the beginning of the intercalated duct is marked by an arrowhead. The intercalated duct is composed of a flattened or low columnar epithelium similar to that seen in the other salivary glands. The intercalated ducts of the sublingual gland are extremely short, however, and thus are usually difficult to find. The intercalated duct seen in this micrograph joins with one or more other intercalated ducts to become the intralobular duct (InD), which is identified by its columnar epithelium and relatively large lumen. The point of transition from intercalated to intralobular duct is not recognizable in the micrograph, however, because the duct wall has only been grazed and the shape of the cells cannot be determined.


Histology_Ross

Histology_Ross. demilunes may be sectioned in a plane that does not include the mucous component of the acinus, thus giving the appearance of a serous acinus. The ducts of the sublingual gland that are observed most frequently in a section are the intralobular ducts. They are the equivalent of the striated duct of the submandibular and parotid glands but lack the extensive basal infoldings and mitochondrial array that creates the striations. One of the intralobular ducts (InD) is evident in this figure (upper right). The area within the rectangle includes part of this duct and is shown at higher magnification in figure below.


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Histology_Ross. Examination of the acini at this higher magnification also reveals the serous demilunes (SD). Note how they form a caplike addition to the mucous end pieces. The cytologic appearance of the mucous cells (MC) and serous cells is essentially the same as that described for the submandibular gland. The area selected for this higher magnification also reveals isolated cell clusters that bear some resemblance to serous acini. It is likely, however, that these cells are actually mucous cells that either have been cut in a plane parallel to their base and do not include the mucinogen-containing portions of the cell or are in a state of activity in which, after depletion of their granules, the production of new mucinogen granules does not yet suffice to give the characteristic “empty” mucous cell appearance.


Histology_Ross

Histology_Ross. An additional important feature of the connective tissue stroma is the presence of numerous lymphocytes and plasma cells. Some of the plasma cells are indicated by the arrows. The plasma cells are associated with the production of salivary IgA and are also present in the other salivary glands. KEY MA, mucous acinus MC, mucous cells ID, intercalated duct InD, intralobular duct SD, serous demilune arrowhead, mucous acinus joining intercalated duct arrows, plasma cells Digestive System II: Esophagus and Gastrointestinal Tract


Histology_Ross

Histology_Ross. OVERVIEW OF THE ESOPHAGUS AND GASTROINTESTINAL TRACT / 569 Mucosa / 569 Submucosa / 571 Muscularis Externa / 571 Serosa and Adventitia / 571 ESOPHAGUS / 572 STOMACH / 574 Gastric Mucosa / 574 Fundic Glands of the Gastric Mucosa / 577 Cardiac Glands of the Gastric Mucosa / 583 Pyloric Glands of the Gastric Mucosa / 583 Epithelial Cell Renewal in the Stomach / 584 Lamina Propria and Muscularis Mucosae / 584 Gastric Submucosa / 585 Gastric Muscularis Externa / 585 Gastric Serosa / 586 SMALL INTESTINE / 586 Submucosa / 596 Muscularis Externa / 597 Serosa / 597 Epithelial Cell Renewal in the Small Intestine / 597 LARGE INTESTINE / 597 Mucosa / 599 Epithelial Cell Renewal in the Large Intestine / 600 Lamina Propria / 600 Muscularis Externa / 601 Submucosa and Serosa / 601 Cecum and Appendix / 601 Rectum and Anal Canal / 603 Folder 17.1 Clinical Correlation: Pernicious Anemia and Peptic Ulcer Disease / 578 Folder 17.2 Clinical Correlation: Zollinger-Ellison Syndrome / 580 Folder 17.3


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Histology_Ross. / 601 Rectum and Anal Canal / 603 Folder 17.1 Clinical Correlation: Pernicious Anemia and Peptic Ulcer Disease / 578 Folder 17.2 Clinical Correlation: Zollinger-Ellison Syndrome / 580 Folder 17.3 Functional Considerations: The Gastrointestinal Endocrine System / 581 Folder 17.4 Functional Considerations: Digestive and Absorptive Functions of Enterocytes / 587 Folder 17.5 Functional Considerations: Immune Functions of the Alimentary Canal / 595 Folder 17.6 Clinical Correlation: The Pattern of Lymph Vessel Distribution and Diseases of the Large Intestine / 602  Serosa, a serous membrane consisting of a simple squa-GASTROINTESTINAL TRACT mous epithelium, the mesothelium, and a small amount of


Histology_Ross

Histology_Ross. The portion of the alimentary canal that extends from the proximal part of the esophagus to the distal part of the anal canal is a hollow tube of varying diameter. This tube has the same basic structural organization throughout its length. Its wall is formed by four distinctive layers. From the lumen outward (Fig. 17.1), they are as follows:  Mucosa, consisting of a lining epithelium, an underlying connective tissue called the lamina propria, and the muscularis mucosae, composed of smooth muscle  Submucosa, consisting of dense irregular connective tissue  Muscularis externa, consisting in most parts of two layers of smooth muscle underlying connective tissue. An adventitia consisting only of connective tissue is found where the wall of the tube is directly attached or fixed to adjoining structures (i.e., body wall and certain retroperitoneal organs).


Histology_Ross

Histology_Ross. The structure of the esophagus and gastrointestinal tract varies considerably from region to region; most of the variation occurs within the mucosa. The epithelium differs throughout the alimentary canal and is adapted to the specific function of each part of the tube. The mucosa has three principal functions: protection, absorption, and secretion.


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Histology_Ross. FIGURE 17.1 • Diagram of general organization of the alimentary canal. This composite diagram shows the wall structure of the alimentary canal in four representative organs: esophagus, stomach, small intestine, and large intestine. Note that villi, a characteristic feature of the small intestine, are not present in other parts of the alimentary canal. Mucosal glands are present throughout the length of the alimentary canal but sparingly in the esophagus and oral cavity. Submucosal glands are present in the esophagus and duodenum. The extramural glands (liver and pancreas) empty into the duodenum. Diffuse lymphatic tissues and nodules are found in the lamina propria throughout the entire length of the alimentary canal (shown here only in the large intestine). Nerves, blood vessels, and lymphatic vessels reach the alimentary canal via the mesenteries or via the adjacent connective tissue (as in the retroperitoneal organs).


Histology_Ross

Histology_Ross. The histologic characteristics of these layers are described below in relation to specific regions of the digestive tube. The epithelium of the mucosa serves as a barrier that separates the lumen of the alimentary canal from the rest of the organism. The epithelial barrier separates the external luminal environment of the tube from the tissues and organs of the body. The barrier aids in protection of the individual from the entry of antigens, pathogens, and other noxious substances. In the esophagus, a stratified squamous epithelium provides protection from physical abrasion by ingested food. In the gastrointestinal portion of the alimentary tract, tight junctions between the simple columnar epithelial cells of the mucosa serve as a selectively permeable barrier. Most epithelial cells transport products of digestion and other essential substances such as water through the cell and into the extracellular space beneath the tight junctions.


Histology_Ross

Histology_Ross. The absorptive function of the mucosa allows the movement of digested nutrients, water, and electrolytes into the blood and lymph vessels. The absorption of digested nutrients, water, and electrolytes is possible because of projections of the mucosa and submucosa into the lumen of the digestive tract. These surface projections greatly increase the surface area available for absorption, and vary in size and orientation. They consist of the following structural specializations (see Fig. 17.1):  Plicae circulares are circumferentially oriented submu cosal folds present along most of the length of the small intestine.  Villi are mucosal projections that cover the entire surface of the small intestine, the principal site of absorption of the products of digestion.  Microvilli are tightly packed, microscopic projections of the apical surface of intestinal absorptive cells. They fur ther increase the surface available for absorption.


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Histology_Ross. In addition, the glycocalyx consists of glycoproteins that project from the apical plasma membrane of epithelial absorptive cells. It provides additional surface for adsorption and includes enzymes secreted by the absorptive cells that are essential for the final steps of digestion of proteins and sugars. The epithelium selectively absorbs the products of digestion both for its own cells and for transport into the vascular system for distribution to other tissues. The secretory function of the mucosa provides lubrication and delivers digestive enzymes, hormones, and antibodies into the lumen of the alimentary tube.


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Histology_Ross. The secretory function of the mucosa provides lubrication and delivers digestive enzymes, hormones, and antibodies into the lumen of the alimentary tube. Secretion is carried out largely by glands distributed throughout the length of the digestive tube. The various secretory products provide mucus for protective lubrication, as well as buffering of the tract lining and substances that assist in digestion, including enzymes, hydrochloric acid, peptide hormones, and water (see Fig. 17.1). The mucosal epithelium also secretes antibodies that it receives from the underlying connective tissue.