Histology_Ross

Histology_Ross.  Paraptosis is an alternative, nonapoptotic cell death that may be induced by growth factor receptors (i.e., insulin growth factor [IGF-1] receptor). In contrast to apoptosis, cell death is not mediated by caspases but by mitogenactivated protein kinases (MAPKs). On a cellular level, paraptosis is characterized by the formation of multiple large vacuoles within the cell cytoplasm along with mitochondrial swellings.  Pyroptosis is a form of cell death induced by infection with certain microorganisms that generate intense inflammatory reactions. This pathway is uniquely dependent on the caspase-1 enzyme, which is not involved in caspase cascade in apoptotic cell death. Caspase-1 activates the inflammatory cytokines such as IL-1 and IL-18 that mediate intense inflammatory reactions in surrounding tissue.


Histology_Ross

Histology_Ross.  Necroptosis is a regulated caspase-independent cell death mechanism that can be induced in different cell types. It is initiated by the activation of the tumor necrosis factor receptors (TNFRs or death receptors) and Fas signaling pathway. Although it occurs under regulated conditions, necroptotic cell death is characterized by the same morphologic features as unregulated necrotic death. Necrostatin-1 is a specific inhibitor of necroptosis that significantly reduces ischemic damage in affected tissues. FIGURE 3.20 • Schematic drawing of mechanisms leading to apoptosis. Both external and internal stimuli can trigger apoptosis by activating the enzymatic caspase cascade. Many external activators act on the cell to initiate signals leading to apoptosis; note that TNF and TGFact through a “death receptor.” Controlled release of cytochrome c from mitochondria is an important internal step in the activation of apoptosis.


Histology_Ross

Histology_Ross. Microscopic studies of dying cells in the tissue reveals that different forms of cell death can occur simultaneously and that dying cells can share features of different types of cell death. Tissues: Concept and Classification OVERVIEW OF TISSUES / 98 EPITHELIUM / 99 CONNECTIVE TISSUE / 99 MUSCLE TISSUE / 100 NERVE TISSUE / 101 HISTOGENESIS OF TISSUES / 102 Ectodermal Derivatives / 102 Mesodermal Derivatives / 102 Endodermal Derivatives / 102 IDENTIFYING TISSUES / 102 Folder 4.1 Clinical Correlation: Ovarian Teratomas / 103 Tissues are aggregates or groups of cells organized to perform one or more specific functions.


Histology_Ross

Histology_Ross. Tissues are aggregates or groups of cells organized to perform one or more specific functions. At the light microscope level, the cells and extracellular components of the various organs of the body exhibit a recognizable and often distinctive pattern of organization. This organized arrangement reflects the cooperative effort of cells performing a particular function. Therefore, an organized aggregation of cells that function in a collective manner is called a tissue [Fr. tissu, woven; L. texo, to weave].


Histology_Ross

Histology_Ross. Although it is frequently said that the cell is the basic functional unit of the body, it is really the tissues, through the collaborative efforts of their individual cells, that are responsible for maintaining body functions. Cells within tissues communicate through specialized intercellular junctions (gap junctions, page 131), thus facilitating this collaborative effort and allowing the cells to operate as a functional unit. Other mechanisms that permit the cells of a given tissue to function in a unified manner include specific membrane receptors and anchoring junctions between cells. Despite their disparate structure and physiologic properties, all organs are made up of only four basic tissue types.


Histology_Ross

Histology_Ross. Despite their disparate structure and physiologic properties, all organs are made up of only four basic tissue types. The tissue concept provides a basis for understanding and recognizing the many cell types within the body and how they interrelate. Despite the variations in general appearance, structural organization, and physiologic properties of the various body organs, the tissues that compose them are classified into four basic types.  Epithelium (epithelial tissue) covers body surfaces, lines body cavities, and forms glands. Connective tissue underlies or supports the other three basic tissues, both structurally and functionally.  Muscle tissue is made up of contractile cells and is responsible for movement.  Nerve tissue receives, transmits, and integrates infor mation from outside and inside the body to control the activities of the body.


Histology_Ross

Histology_Ross.  Nerve tissue receives, transmits, and integrates infor mation from outside and inside the body to control the activities of the body. Each basic tissue is defined by a set of general morphologic characteristics or functional properties. Each type may be further subdivided according to specific characteristics of its various cell populations and any special extracellular substances that may be present.


Histology_Ross

Histology_Ross. In classifying the basic tissues, two different definitional parameters are used. The basis for definition of epithelium and connective tissue is primarily morphologic; for muscle and nerve tissue, it is primarily functional. Moreover, the same parameters exist in designating the tissue subclasses. For example, whereas muscle tissue itself is defined by its function, it is subclassified into smooth and striated categories, a purely morphologic distinction, not a functional one. Another kind of contractile tissue, myoepithelium, functions as muscle tissue but is typically designated epithelium because of its location. For these reasons, tissue classification cannot be reduced to a simple formula. Rather, students are advised to learn the features or characteristics of the different cell aggregations that define the four basic tissues and their subclasses. Epithelium is characterized by close cell apposition and presence at a free surface.


Histology_Ross

Histology_Ross. Epithelium is characterized by close cell apposition and presence at a free surface. Epithelial cells, whether arranged in a single layer or in multiple layers, are always contiguous with one another. In addition, they are usually joined by specialized cell-to-cell junctions that create a barrier between the free surface and the adjacent connective tissue. The intercellular space between epithelial cells is minimal and devoid of any structure except where junctional attachments are present. Free surfaces are characteristic of the exterior of the body, the outer surface of many internal organs, and the lining of the body cavities, tubes, and ducts, both those that ultimately communicate with the exterior of the body and those that are enclosed. The enclosed body cavities and tubes include the pleural, pericardial, and peritoneal cavities as well as the cardiovascular system. All of these are lined by epithelium.


Histology_Ross

Histology_Ross. Subclassifications of epithelium are usually based on the shape of the cells and the number of cell layers rather than on function. Cell shapes include squamous (flattened), cuboidal, and columnar. Layers are described as simple (single layer) or stratified (multiple layers). Figure 4.1 shows epithelia from two sites. Both are simple epithelia (i.e., one cell layer thick). The major distinction between the two examples is the shape of the cells, cuboidal versus columnar. In both epithelia, however, the cells occupy a surface position. Connective tissue is characterized on the basis of its extracellular matrix.


Histology_Ross

Histology_Ross. Connective tissue is characterized on the basis of its extracellular matrix. Unlike epithelial cells, connective tissue cells are conspicuously separated from one another. The intervening spaces are occupied by material produced by the cells. This extracellular material is called the extracellular matrix. The nature of the cells and matrix varies according to the function of the tissue. Thus, subclassification of connective tissue takes into account not only the cells but also the composition and organization of the extracellular matrix.


Histology_Ross

Histology_Ross. A type of connective tissue found in close association with most epithelia is loose connective tissue (Fig. 4.2a). In fact, it is the connective tissue that most epithelia rest on. The extracellular matrix of loose connective tissue contains loosely arranged collagen fibers and numerous cells. Some of these cells, the fibroblasts, form and maintain the extracellular matrix. However, most of the cells are migrants from the vascular system and have roles associated with the immune system. In contrast, where only strength is required, collagen fibers are more numerous and densely packed. Also, the cells are relatively sparse and limited to the fiber-forming cell, the fibroblast (Fig. 4.2b). This type of connective tissue is described as dense connective tissue.


Histology_Ross

Histology_Ross. Bone and cartilage are two other types of connective tissue characterized by the material associated with collagen (i.e., calcium [bones] and hyaluronan [cartilage]). Again, in both of these tissues, it is the extracellular material that characterizes the tissue, not the cells. FIGURE 4.1  Simple epithelia. a. An H&E–stained section showing a pancreatic duct lined by a single layer of contiguous cuboidal epithelial cells. The free surface of the cells faces the lumen; the basal surface is in apposition to the connective tissue. 540. b. An H&E–stained section showing a single layer of tall columnar epithelial cells lining the gallbladder. Note that the cells are much taller than the lining cells of the pancreatic duct. The free surface of the epithelial cells is exposed to the lumen of the gallbladder, and the basal surface is in apposition to the adjacent connective tissue. 540.


Histology_Ross

Histology_Ross. FIGURE 4.2  Loose and dense connective tissue. a. Mallory-Azan–stained specimen of a section through the epiglottis, showing the lower part of its stratified epithelium (Ep), subjacent loose connective tissue (LCT ), and dense connective tissue (DCT ) below. Loose connective tissue typically contains many cells of several types. Their nuclei vary in size and shape. The elongated nuclei most likely belong to fibroblasts. Because dense connective tissue contains thick collagen bundles, it stains more intensely with the blue dye. Also, note the relatively fewer nuclei. 540. b. A Mallory-stained specimen of dense connective tissue, showing a region composed of numerous, densely packed collagen fibers. The few nuclei (N) that are present belong to fibroblasts. The combination of densely packed fibers and the paucity of cells characterizes dense connective tissue. Relatively few small blood vessels (BV ) are shown on this section. 540.


Histology_Ross

Histology_Ross. To function efficiently to effect movement, most muscle cells are aggregated into distinct bundles that are easily Muscle tissue is categorized on the basis of a functional distinguished from the surrounding tissue. Muscle cells property, the ability of its cells to contract. are typically elongated and oriented with their long axes Muscle cells are characterized by large amounts of the in the same direction (Fig. 4.3). The arrangement of contractile proteins actin and myosin in their cytoplasm nuclei is also consistent with the parallel orientation of and by their particular cellular arrangement in the tissue. muscle cells.


Histology_Ross

Histology_Ross. FIGURE 4.3  Muscle tissue. a. An H&E–stained specimen showing a portion of three longitudinally sectioned skeletal muscle fibers (cells). Two striking features of these large, long cells are their characteristic cross-striations and the many nuclei located along the periphery of the cell. 420. b. A Mallory-stained specimen showing cardiac muscle fibers that also exhibit striations. These fibers are composed of individual cells that are much smaller than those of skeletal muscle and are arranged end to end to form long fibers. Most of the fibers are seen in longitudinal array. The organized aggregation–that is, the parallel array of the fibers in the case of muscle tissue, allows for collective effort in performing their function. Intercalated disks (arrows) mark the junction of adjoining cells. 420.


Histology_Ross

Histology_Ross. Although the shape and arrangement of cells in specific muscle types (e.g., smooth muscle, skeletal muscle, and cardiac muscle) are quite different, all muscle types share a common characteristic: The bulk of the cytoplasm consists of the contractile proteins actin and myosin. Although these proteins are ubiquitous in all cells, only in muscle cells are they present in such large amounts and organized in such highly ordered arrays that their contractile activity can produce movement in an entire organ or organism. Nerve tissue consists of nerve cells (neurons) and associated supporting cells of several types.


Histology_Ross

Histology_Ross. Although all cells exhibit electrical properties, nerve cells or neurons are highly specialized to transmit electrical impulses from one site in the body to another; they are also specialized to integrate those impulses. Nerve cells receive and process information from the external and internal environment and may have specific sensory receptors and sensory organs to accomplish this function. Neurons are characterized by two different types of processes through which they interact with other nerve cells and with cells of epithelia and muscle. A single, long axon (sometimes longer than a meter) carries impulses away from the cell body, which contains the neuron’s nucleus. Multiple dendrites receive impulses and carry them toward the cell body. (In histologic sections, it is usually impossible to differentiate axons and dendrites because they have the same structural appearance.) The axon terminates at a neuronal junction called a synapse at which electrical impulses are transferred


Histology_Ross

Histology_Ross. to differentiate axons and dendrites because they have the same structural appearance.) The axon terminates at a neuronal junction called a synapse at which electrical impulses are transferred from one cell to the next by secretion of neuromediators. These chemical substances are released at synapses to generate electrical impulses in the adjacent communicating neuron.


Histology_Ross

Histology_Ross. In the central nervous system (CNS), which comprises the brain and spinal cord, the supporting cells are called neuroglial cells. In the peripheral nervous system (PNS), which comprises the nerves in all other parts of the body, the supporting cells are called Schwann (neurilemmal) cells and satellite cells. Supporting cells are responsible for several important functions. They separate neurons from one another, produce the myelin sheath that insulates and speeds conduction in certain types of neurons, provide active phagocytosis to remove cellular debris, and contribute to the blood–brain barrier in the CNS.


Histology_Ross

Histology_Ross. In an ordinary hematoxylin and eosin (H&E)–stained section, nerve tissue may be observed in the form of a nerve, which consists of varying numbers of neuronal processes along with their supporting cells (Fig. 4.4a). Nerves are most commonly seen in longitudinal or cross sections in loose connective tissue. Nerve cell bodies in the PNS, including the autonomic nervous system (ANS), are seen in aggregations called ganglia, where they are surrounded by satellite cells (Fig. 4.4b). Neurons and supporting cells are derived from neuroectoderm, which forms the neural tube in the embryo. Neuroectoderm originates by invagination of an epithelial layer, the dorsal ectoderm of the embryo. Some nervous system cells, such as ependymal cells and cells of the choroid plexus in the CNS, retain the absorptive and secretory functions characteristic of epithelial cells.


Histology_Ross

Histology_Ross. FIGURE 4.4  Nerve tissue. a. A Mallory-stained section of a peripheral nerve. Nerve tissue consists of a vast number of threadlike myelinated axons held together by connective tissue. The axons have been cross-sectioned and appear as small, red, dotlike structures. The clear space surrounding the axons previously contained myelin that was dissolved and lost during preparation of the specimen. The connective tissue is stained blue. It forms a delicate network around the myelinated axons and ensheathes the bundle, thus forming a structural unit, the nerve. 270. b. An Azan-stained section of a nerve ganglion, showing the large, spherical nerve cell bodies and the nuclei of the small satellite cells that surround the nerve cell bodies. The axons associated with the nerve cell bodies are unmyelinated. They are seen as nerve fiber bundles (NFB) between clusters of the cell bodies. 270.


Histology_Ross

Histology_Ross. In the early developing embryo during the gastrulation phase, a trilaminar embryo (trilaminar germ disk) is being formed. The three germ layers include the ectoderm, mesoderm, and endoderm, which give rise to all the tissues and organs. The ectoderm is the outermost of the three germ layers. The derivatives of the ectoderm may be divided into two major classes: surface ectoderm and neuroectoderm. Surface ectoderm gives rise to:  epidermis and its derivatives (hair, nails, sweat glands, sebaceous glands, and the parenchyma and ducts of the mammary glands),  cornea and lens epithelia of the eye,  enamel organ and enamel of the teeth,  components of the internal ear,  adenohypophysis (anterior lobe of pituitary gland), and  mucosa of the oral cavity and lower part of the anal canal.


Histology_Ross

Histology_Ross. Neuroectoderm gives rise to:  the neural tube and its derivatives, including components of the central nervous system, ependyma (epithelium lining the cavities of the brain and spinal cord), pineal body, posterior lobe of pituitary gland (neurohypophysis), and the sensory epithelium of the eye, ear, and nose;  the neural crest and its derivatives, including components of the peripheral nervous system (cranial, spinal, and autonomic ganglia, peripheral nerves, and Schwann cells); glial cells (oligodendrocytes and astrocytes); chromaffin (medullary) cells of the adrenal gland; enteroendocrine (APUD) cells of the diffuse neuroendocrine system; melanoblasts, the precursors of melanocytes; the mesenchyme of the head and its derivatives (such as pharyngeal arches that contain muscles, connective tissue, nerves, and vessels); odontoblasts; and corneal and vascular endothelium.


Histology_Ross

Histology_Ross. Mesoderm is the middle of the three primary germ layers of an embryo. It gives rise to:  connective tissue, including embryonic connective tissue (mesenchyme), connective tissue proper (loose and dense connective tissue), and specialized connective tissues (cartilage, bone, adipose tissue, blood and hemopoietic tissue), and lymphatic tissue;  heart, blood vessels, and lymphatic vessels, including their endothelial lining;  kidneys and the gonads (ovaries and testes) with genital ducts and their derivatives (ureters, uterine tubes, uterus, ductus deferens);  mesothelium, the epithelium lining the pericardial, pleural, and peritoneal cavities; and the adrenal cortex.


Histology_Ross

Histology_Ross. Endoderm is the innermost layer of the three germ layers. In the early embryo it forms the wall of the primitive gut and gives rise to epithelial portions or linings of the organs arising from the primitive gut tube. Derivatives of the endoderm include:  alimentary canal epithelium (excluding the epithelium of the oral cavity and lower part of the anal canal, which are of ectodermal origin);  extramural digestive gland epithelium (e.g., the liver, pancreas, and gallbladder);  lining epithelium of the urinary bladder and most of the urethra; respiratory system epithelium; thyroid, parathyroid, and thymus gland epithelial components; parenchyma of the tonsils; lining epithelium of the tympanic cavity and auditory (Eustachian) tubes.


Histology_Ross

Histology_Ross. Thyroid and parathyroid glands develop as epithelial outgrowths from the floor and walls of the pharynx; they then lose their attachments from these sites of original outgrowth. As an epithelial outgrowth of the pharyngeal wall, the thymus grows into the mediastinum and also loses its original connection. Figure 4.5 summarizes the derivatives of the three germ layers. Recognition of tissues is based on the presence of specific components within cells and on specific cellular relationships. Keeping these few basic facts and concepts about the fundamental four tissues in mind can facilitate the task of examining and interpreting histologic slide material. The first goal is to recognize aggregates of cells as tissues and determine the special characteristics that they present. Are the cells present at a surface? Are they in contact with their neighbors, or are they separated by definable intervening material? Do they belong to a group with special properties such as muscle or nerve?


Histology_Ross

Histology_Ross. The structure and the function of each fundamental tissue are examined in subsequent chapters. In focusing on a single specific tissue we are, in a sense, artificially separating the constituent tissues of organs. However, this separation is necessary to understand and appreciate the histology of the various organs of the body and the means by which they operate as functional units and integrated systems.


Histology_Ross

Histology_Ross. chapter 4 Tissues: Concept and Classific ation I DE NTI FYI N G TISSU ES 103 FIGURE 4.5  Derivatives of the three germ layers. Schematic drawing illustrates the derivatives of the three germ layers: ectoderm, endoderm, and mesoderm. GI, gastrointestinal. (Based on Moore KL, Persaud TVN. The Developing Human, Clinically Oriented Embryology. Philadelphia: WB Saunders, 1998.) cranial and sensory ganglia and nerves adrenal medulla melanocytes pharyngeal arch cartilages head mesenchyme and connective tissue Schwann cells odontoblasts surface ectoderm epidermis, hair, nails, cutaneous, and mammary glands anterior pituitary gland enamel of theeth internal ear corneal epithelium and lens of eye neuroectoderm (neural tube) central nervous system retina pineal body posterior pituitary gland head mesoderm cranium (skull) connective tissue of head dentin paraxial mesoderm skeletal muscle of trunk and limbs except cranium muscles of head dermis of skin connective tissue intermediate mesoderm


Histology_Ross

Histology_Ross. mesoderm cranium (skull) connective tissue of head dentin paraxial mesoderm skeletal muscle of trunk and limbs except cranium muscles of head dermis of skin connective tissue intermediate mesoderm urogenital system including gonads, ducts and accessory glands lateral mesoderm connective tissue and muscle of viscera serous membranes of pleura, pericardium, and peritoneum blood and lymph cells cardiovascular and lymphatic systems spleen adrenal cortex endoderm respiratory tract (trachea, bronchi, lungs) GI tract (pharynx, esophagus, stomach, small and large intestines) urinary bladder and urachus thyroid gland tympanic cavity auditory tube tonsils parathyroid glands liver pancreas epithelial lining of: epithelial parts of: It is of clinical interest that, under certain conditions, abnor-mal differentiation may occur. Most of the tumors derive from the cells that originate from a single germ cell layer. However, if the tumor cells arise from the pluripotential stem cells, their mass may


Histology_Ross

Histology_Ross. differentiation may occur. Most of the tumors derive from the cells that originate from a single germ cell layer. However, if the tumor cells arise from the pluripotential stem cells, their mass may contain cells that differentiate and re-semble cells originating from all three germ layers. The result is formation of a tumor that contains a variety of mature tis-sues arranged in an unorganized fashion. Such masses are referred to as teratomas. Since pluripotential stem cells are primarily encountered in gonads, teratomas almost al-ways occur in the gonads. In the ovary, these tumors usually develop into solid masses that contain characteristics of the


Histology_Ross

Histology_Ross. FOLDER 4.1 Clinical Correlation: Ovarian Teratomas


Histology_Ross

Histology_Ross. FOLDER 4.1 Clinical Correlation: Ovarian Teratomas (Cont.) mature basic tissues. Although the tissues fail to form functional structures, frequently organlike structures may be seen (i.e., teeth, hair, epidermis, bowel segments, and so forth). Teratomas may also develop in the testis, but they are rare. Moreover, ovarian teratomas are usually benign, whereas teratomas in the testis are composed of less-differentiated tissues that usually lead to malignancy. An example of a solid-mass ovarian teratoma containing fully differentiated tissue is shown in the center micrograph of Figure F4.1.1. The low power reveals the lack of organized structures but does not allow identification of the specific tissues present. However, with higher magnification, as shown in the insets (a–f) mature differentiated tissues are evident. This tumor represents a mature teratoma of the ovary often called a demoid cyst. This benign tumor has a normal female kariotype 46XX; based on genetic studies, these


Histology_Ross

Histology_Ross. tissues are evident. This tumor represents a mature teratoma of the ovary often called a demoid cyst. This benign tumor has a normal female kariotype 46XX; based on genetic studies, these tissues are thought to arise through parthenogenic oocyte development. Mature teratomas are common ovarian tumors in childhood and in early reproductive age.


Histology_Ross

Histology_Ross. The example given in Figure F4.1.1 shows that one can readily identify tissue characteristics, even in an unorganized structure. Again, the important point is the ability to recognize aggregates of cells and to determine the special characteristics that they exhibit. FIGURE F4.1.1  Ovarian teratoma. In the center is an H&E–stained section of an ovarian teratoma seen at low magnification. This mass is composed of various basic tissues that are well differentiated and easy to identify at higher magnification. The abnormal feature is the lack of organization of the tissues to form functional organs. The tissues within the boxed areas are seen at higher magnification in micrographs a–f. The higher magnification allows identification of some of the basic tissues that are present within this tumor. 10. a.


Histology_Ross

Histology_Ross. a. Simple columnar epithelium lining a cavity of a small cyst. 170. Inset. Higher magnification of the epithelium and the underlying connective tissue. 320. b. Dense regular connective tissue forming a tendon like structure. 170. c. Area showing hyaline cartilage (C) and developing bone spicules (B). 170. d. Brain tissue with glial cells. 170. e. Cardiac muscle fibers. 220. Inset. Higher magnification showing intercalated disks (arrows). 320. f. Skeletal muscle fibers cut in cross section. 220.


Histology_Ross

Histology_Ross. OVERVIEW OF EPITHELIAL STRUCTURE AND FUNCTION / 105 CLASSIFICATION OF EPITHELIUM / 106 CELL POLARITY / 107 THE APICAL DOMAIN AND ITS MODIFICATIONS / 109 Microvilli / 109 Stereocilia / 110 Cilia / 113 THE LATERAL DOMAIN AND ITS SPECIALIZATIONS IN CELL-TO-CELL ADHESION / 121 Occluding Junctions / 124 Anchoring Junctions / 127 Communicating Junctions / 131 Morphologic Specializations of the Lateral Cell Surface / 133 THE BASAL DOMAIN AND ITS SPECIALIZATIONS IN CELL-TO– EXTRACELLULAR MATRIX ADHESION / 134 Basement Membrane Structure and Function / 134 Cell-to–Extracellular Matrix Junctions / 144 Morphologic Modifications of the Basal Cell Surface / 146 GLANDS / 146 EPITHELIAL CELL RENEWAL / 150 Folder 5.1 Clinical Correlation: Epithelial Metaplasia / 109 Folder 5.2 Clinical Correlation: Primary Ciliary Dyskinesia / 120 Folder 5.3 Clinical Correlation: Junctional Complexes as a Target of Pathogenic Agents / 128 Folder 5.4 Functional Considerations: Basement Membrane and Basal Lamina


Histology_Ross

Histology_Ross. Primary Ciliary Dyskinesia / 120 Folder 5.3 Clinical Correlation: Junctional Complexes as a Target of Pathogenic Agents / 128 Folder 5.4 Functional Considerations: Basement Membrane and Basal Lamina Terminology / 138 Folder 5.5 Functional Considerations: Mucus and Serous Membranes / 150


Histology_Ross

Histology_Ross. Epithelium covers body surfaces, lines body cavities, and constitutes glands. Epithelium is an avascular tissue composed of cells that cover the exterior body surfaces and line internal closed cavities (including the vascular system) and body tubes that communicate with the exterior (the alimentary, respiratory, and genitourinary tracts). Epithelium also forms the secretory portion (parenchyma) of glands and their ducts. In addition, specialized epithelial cells function as receptors for the special senses (smell, taste, hearing, and vision). The cells that make up epithelium have three principal characteristics:  They are closely apposed and adhere to one another by means of specific cell-to-cell adhesion molecules that form specialized cell junctions (Fig. 5.1).


Histology_Ross

Histology_Ross.  They exhibit functional and morphologic polarity. In other words, different functions are associated with three distinct morphologic surface domains: a free surface or apical domain, a lateral domain, and a basal domain. The properties of each domain are determined by specific lipids and integral membrane proteins.  Their basal surface is attached to an underlying basement membrane, a noncellular, protein–polysaccharide-rich layer demonstrable at the light microscopic level by histochemical methods (see Fig. 1.2, page 6).


Histology_Ross

Histology_Ross. FIGURE 5.1 • Diagram of small intestine absorptive epithelial cells. a. All three cellular domains of a typical epithelial cell are indicated on the diagram. The junctional complex provides adhesion between adjoining cells and separates the luminal space from the intercellular space, limiting the movement of fluid between the lumen and the underlying connective tissue. The intracellular pathway of fluid movement during absorption (arrows) is from the intestinal lumen into the cell, then across the lateral cell membrane into the intercellular space, and, finally, across the basement membrane to the connective tissue. b. This photomicrograph of a plastic-embedded, thin section of intestinal epithelium, stained with toluidine blue, shows cells actively engaged in fluid transport. Like the adjacent diagram, the intercellular spaces are prominent, reflecting fluid passing into this space before entering the underlying connective tissue. 1,250.


Histology_Ross

Histology_Ross. In special situations, epithelial cells lack a free surface (epithelioid tissues).


Histology_Ross

Histology_Ross. In some locations, cells are closely apposed to one another but lack a free surface. Although the close apposition of these cells and the presence of a basement membrane would classify them as epithelium, the absence of a free surface more appropriately classifies such cell aggregates as epithelioid tissues. The epithelioid cells are derived from progenitor mesenchymal cells (nondifferentiated cells of embryonic origin found in connective tissue). Although the progenitor cells of these epithelioid tissues may have arisen from a free surface or the immature cells may have had a free surface at some time during development, the mature cells lack a surface location or surface connection. Epithelioid organization is typical of most endocrine glands; examples of such tissue include the interstitial cells of Leydig in the testis (Plate 3, page 156), the lutein cells of the ovary, the islets of Langerhans in the pancreas, the parenchyma of the adrenal gland, and the anterior lobe of the


Histology_Ross

Histology_Ross. cells of Leydig in the testis (Plate 3, page 156), the lutein cells of the ovary, the islets of Langerhans in the pancreas, the parenchyma of the adrenal gland, and the anterior lobe of the pituitary gland. Epithelioreticular cells of the thymus also may be included in this category. Epithelioid patterns are also formed by accumulations of connective tissue macrophages in response to certain types of injury and infections, as well as by many tumors derived from epithelium.


Histology_Ross

Histology_Ross. Epithelium creates a selective barrier between the external environment and the underlying connective tissue. Covering and lining epithelium forms a sheetlike cellular investment that separates underlying or adjacent connective tissue from the external environment, internal cavities, or fluid connective tissue such as the blood and lymph. Among other roles, this epithelial investment functions as a selective barrier that facilitates or inhibits the passage of specific substances between the exterior (including the body cavities) environment and the underlying connective tissue compartment. The traditional classification of epithelium is descriptive and based on two factors: the number of cell layers and the shape of the surface cells. The terminology, therefore, reflects only structure, not function. Thus, epithelium is described as:  simple when it is one cell layer thick and  stratifed when it has two or more cell layers.


Histology_Ross

Histology_Ross. The individual cells that compose an epithelium are described as:  squamous when the width of the cell is greater than its  cuboidal when the width, depth, and height are approximately the same; and  columnar when the height of the cell appreciably exceeds the width (the term low columnar is often used when a cell’s height only slightly exceeds its other dimensions). Thus, by describing the number of cell layers (i.e., simple or stratified) and the surface cell shape, the various configurations of epithelia are easily classified. The cells in some exocrine glands are more or less pyramidal, with their apices directed toward a lumen. However, these cells are still classified as either cuboidal or columnar, depending on their height relative to their width at the base of the cell.


Histology_Ross

Histology_Ross. In a stratifed epithelium, the shape and height of the cells usually vary from layer to layer, but only the shape of the cells that form the surface layer is used in classifying the epithelium. For example, stratified squamous epithelium consists of more than one layer of cells, and the surface layer consists of flat or squamous cells. In some instances, a third factor—specialization of the apical cell surface domain—can be added to this classification system. For example, some simple columnar epithelia are classified as simple columnar ciliated when the apical surface domain possesses cilia. The same principle applies to stratified squamous epithelium, in which the surface cells may be keratinized or nonkeratinized. Thus, epidermis would be designated as stratified squamous keratinized epithelium because of the keratinized cells at the surface. Pseudostratified epithelium and transitional epithelium are special classifications of epithelium.


Histology_Ross

Histology_Ross. Pseudostratified epithelium and transitional epithelium are special classifications of epithelium. Two special categories of epithelium are pseudostratified and transitional.  Pseudostratifed epithelium appears stratified, although some of the cells do not reach the free surface; all rest on the basement membrane (Plate 2, page 154). Thus, it is actually a simple epithelium. The distribution of pseudostratified epithelium is limited in the body. Also, it is often difficult to discern whether all of the cells contact the basement membrane. For these reasons, identification of pseudostratified epithelium usually depends on knowing where it is normally found.


Histology_Ross

Histology_Ross.  Transitional epithelium (urothelium) is a term applied to the epithelium lining the lower urinary tract, extending from the minor calyces of the kidney down to the proximal part of the urethra. Urothelium is a stratifed epithelium with specific morphologic characteristics that allow it to distend (Plate 3, page 156). This epithelium is described in Chapter 20. The cellular configurations of the various types of epithelia and their appropriate nomenclature are illustrated in Table 5.1. Endothelium and mesothelium are the simple squamous epithelia lining the vascular system and body cavities. Specific names are given to epithelium in certain locations:  Endothelium is the epithelial lining of the blood and lymphatic vessels.  Endocardium is the epithelial lining of ventricles and atria of the heart.


Histology_Ross

Histology_Ross.  Endocardium is the epithelial lining of ventricles and atria of the heart.  Mesothelium is the epithelium that lines the walls and covers the contents of the closed cavities of the body (i.e., the abdominal, pericardial, and pleural cavities; Plate 1, page 152). Both endothelium and endocardium, as well as mesothelium, are almost always simple squamous epithelia. An exception is found in postcapillary venules of certain lymphatic tissues in which the endothelium is cuboidal. These venules are called high endothelial venules (HEV). Another exception is found in the spleen in which endothelial cells of the venous sinuses are rod-shaped and arranged like the staves of a barrel. Diverse epithelial functions can be found in different organs of the body.


Histology_Ross

Histology_Ross. Diverse epithelial functions can be found in different organs of the body. A given epithelium may serve one or more functions, depending on the activity of the cell types that are present:  secretion, as in the columnar epithelium of the stomach and the gastric glands; absorption, as in the columnar epithelium of the intestines and proximal convoluted tubules in the kidney;  transportation, as in the transport of materials or cells along the surface of an epithelium by motile cilia or in the transport of materials across an epithelium to and from the connective tissue;  protection, as in the stratified squamous epithelium of the skin (epidermis) and the transitional epithelium of the urinary bladder; and  receptor function to receive and transduce external stimuli, as in the taste buds of the tongue, olfactory epithelium of the nasal mucosa, and the retina of the eye.


Histology_Ross

Histology_Ross. Epithelia involved in secretion or absorption are typically simple or, in a few cases, pseudostratified. The height of the cells often reflects the level of secretory or absorptive activity. Simple squamous epithelia are compatible with a high rate of transepithelial transport. Stratification of the epithelium usually correlates with transepithelial impermeability. Finally, in some pseudostratified epithelia, basal cells are the stem cells that give rise to the mature functional cells of the epithelium, thus balancing cell turnover. Epithelial cells exhibit distinct polarity. They have an apical domain, a lateral domain, and a basal domain. Specific biochemical characteristics are associated with each cell surface. These characteristics and the geometric arrangements of the cells in the epithelium determine the functional polarity of all three cell domains.


Histology_Ross

Histology_Ross. The free or apical domain is always directed toward the exterior surface or the lumen of an enclosed cavity or tube. The lateral domain communicates with adjacent cells and is characterized by specialized attachment areas. The basal domain rests on the basal lamina anchoring the cell to underlying connective tissue.


Histology_Ross

Histology_Ross. TABLE Types of Epithelium 5.1 Classification Some Typical Locations Major Function Simple squamous Vascular system (endothelium) Exchange, barrier in Body cavities (mesothelium) central nervous system Bowman’s capsule (kidney) Exchange and lubrication Respiratory spaces in lung Barrier Exchange Simple cuboidal Small ducts of exocrine glands Absorption, conduit Surface of ovary Barrier (germinal epithelium) Absorption and secretion Kidney tubules Thyroid follicles Simple columnar Small intestine and colon Absorption and secretion Stomach lining and gastric Secretion glands AbsorptionGallbladder Pseudostratified Trachea and bronchial tree Secretion, conduitDuctus deferens Efferent ductules of epididymis Absorption, conduit Stratified squamous Epidermis Barrier, protection Oral cavity and esophagus Vagina Stratified cuboidal Sweat gland ducts Large ducts of exocrine Barrier, conduit glands Anorectal junction Stratified columnar Largest ducts of exocrine Barrier, conduit glands Anorectal


Histology_Ross

Histology_Ross. Vagina Stratified cuboidal Sweat gland ducts Large ducts of exocrine Barrier, conduit glands Anorectal junction Stratified columnar Largest ducts of exocrine Barrier, conduit glands Anorectal junction Transitional Renal calyces (urothelium) Ureters Barrier, distensible property Bladder Urethra tttttttt  FOLDER 5.1


Histology_Ross

Histology_Ross. Clinical Correlation: Epithelial Metaplasia


Histology_Ross

Histology_Ross. Epithelial metaplasia is a reversible conversion of one mature epithelial cell type to another mature epithelial cell type. Metaplasia is generally an adaptive response to stress, chronic inflammation, or other abnormal stimuli. The original cells are substituted by cells that are better suited to the new environment and more resistant to the effects of abnor-mal stimuli. Metaplasia results from reprogramming of ep-ithelial stem cells that changes the patterns of their gene expression. The most common epithelial metaplasia is columnar-to-squamous and occurs in the glandular ep-ithelium, where the columnar cells become replaced by the stratified squamous epithelium. For example, squamous metaplasia frequently occurs in the pseudostratified respi-ratory epithelium of the trachea and bronchi in response to prolonged exposure to cigarette smoke. It also occurs in the cervical canal in women with chronic infections. In this exam-ple, simple columnar epithelium of the cervical canal is


Histology_Ross

Histology_Ross. in response to prolonged exposure to cigarette smoke. It also occurs in the cervical canal in women with chronic infections. In this exam-ple, simple columnar epithelium of the cervical canal is re-placed by the stratified squamous nonkeratinized epithelium (Fig F5.1.1). In addition, squamous metaplasia is noticeable in the urothelium (transitional epithelium) and is associated with chronic parasitic infections such as schistosomiasis. Squamous-to-columnar epithelial metaplasia may also occur. For example, as a result of gastroesophageal reflux (Barrett’s esophagus), the stratified squamous nonkeratinized epithelium of the lower part of the esopha-gus can undergo metaplastic transformation into an intestinal-like simple columnar epithelium containing gob-let cells. Metaplasia is usually a reversible phenomenon, and if the stimulus that caused metaplasia is removed, tissues re-turn to their normal pattern of differentiation. If abnormal stimuli persist for a long time, squamous


Histology_Ross

Histology_Ross. a reversible phenomenon, and if the stimulus that caused metaplasia is removed, tissues re-turn to their normal pattern of differentiation. If abnormal stimuli persist for a long time, squamous metaplastic cells may transform into squamous cell carcinoma. Cancers of the lung, cervix, and bladder often originate from squamous metaplastic epithelium. Squamous columnar epithelium may give rise to glandular adenocarcinomas. When metaplasia is diagnosed, all efforts should be di-rected toward removing the pathogenic stimulus (i.e., ces-sation of smoking, eradication of infectious agents, etc.) and monitoring the metaplastic site to ensure that cancer-ous changes do not begin to develop. FIGURE F5.1.1 • Squamous metaplasia of the uterine cervix. Photomicrograph of a cervical canal lined by simple columnar epithelium. Note that the center of the image is occupied by an island containing squamous stratified epithelium. This metaplastic epithelium is surrounded on both sides by simple columnar


Histology_Ross

Histology_Ross. columnar epithelium. Note that the center of the image is occupied by an island containing squamous stratified epithelium. This metaplastic epithelium is surrounded on both sides by simple columnar epithelium. Since metaplasia is triggered by reprogramming of stem cells, metaplastic squamous cells have the same characteristics as normal stratified squamous epithelium. 240. (Courtesy of Dr. Fabiola Medeiros.)


Histology_Ross

Histology_Ross. The molecular mechanism responsible for establishing polarity in epithelial cells is required to first create a fully functional barrier between adjucent cells. Junctional complexes (which will be discussed later in this chapter) are being formed in the apical parts of the epithelial cells. These specialized attachment sites not only are responsible for tight cell adhesions but also allow epithelium to regulate paracellular movements of solutes down their electroosmotic gradients. In addition, junctional complexes separate the apical plasma membrane domain from basal and lateral domains and allow them to specialize and recognize different molecular signals.


Histology_Ross

Histology_Ross. In many epithelial cells, the apical domain exhibits special structural surface modifications to carry out specific functions. In addition, the apical domain may contain specific enzymes (e.g., hydrolases), ion channels, and carrier proteins (e.g., glucose transporters). The structural surface modifications include:  microvilli, cytoplasmic processes containing a core of actin filaments;  stereocilia (stereovilli), microvilli of unusual length; and  cilia, cytoplasmic processes containing bundle of micro- tubules. Microvilli are fingerlike cytoplasmic projections on the apical surface of most epithelial cells.


Histology_Ross

Histology_Ross. Microvilli are fingerlike cytoplasmic projections on the apical surface of most epithelial cells. As observed with the electron microscope (EM), microvilli vary widely in appearance. In some cell types, microvilli are short, irregular, bleblike projections. In other cell types, they are tall, closely packed, uniform projections that greatly increase the free cell surface area. In general, the number and shape of the microvilli of a given cell type correlate with the cell’s absorptive capacity. Thus, cells that principally transport fluid and absorb metabolites have many closely packed, tall FIGURE 5.2 • Electron micrographs showing variation in microvilli of different cell types. a. Epithelial cell of uterine gland; small projections. b. Syncytiotrophoblast of placenta; irregular, branching microvilli. c. Intestinal absorptive cell; uniform, numerous, and regularly arranged microvilli. All figures 20,000.


Histology_Ross

Histology_Ross. microvilli. Cells in which transepithelial transport is less active have smaller, more irregularly shaped microvilli.


Histology_Ross

Histology_Ross. Among the fluid-transporting epithelia (e.g., those of the intestine and kidney tubules), a distinctive border of vertical striations at the apical surface of the cell, representing an astonishing number of 15,000 close packed microvilli, is easily seen in the light microscope. In intestinal absorptive cells, this surface structure was originally called the striated border; in the kidney tubule cells, it is called the brush border. Where there is no apparent surface modification based on light microscope observations, any microvilli present are usually short and not numerous, which explains why they may escape detection in the light microscope. The variations seen in microvilli of various types of epithelia are shown in Figure 5.2. The microvilli of the intestinal epithelium (striated border) are the most highly ordered and are even more uniform in appearance than those that constitute the brush border of kidney cells.


Histology_Ross

Histology_Ross. The internal structure of microvilli contains a core of actin filaments that are cross-linked by several actinbundling proteins.


Histology_Ross

Histology_Ross. Microvilli contain a conspicuous core of about 20 to 30 actin flaments. Their barbed (plus) ends are anchored to villin, a 95-kilodalton actin-bundling protein located at the tip of the microvillus. The actin bundle extends down into the apical cytoplasm. Here it interacts with a horizontal network of actin filaments, the terminal web, that lies just below the base of the microvilli (Fig. 5.3a). The actin filaments inside the microvillus are cross-linked at 10-nm intervals by other actin-bundling proteins such as fascin (57 kilodaltons), espin (30 kilodaltons), and fmbrin (68 kilodaltons). This cross-linkage provides support and gives rigidity to the microvilli. In addition, the core of actin filaments is associated with myosin I, a molecule that binds the actin filaments to the plasma membrane of the micro-villus. The addition of villin to epithelial cells growing in cultures induces formation of microvilli on the free apical surface.


Histology_Ross

Histology_Ross. The terminal web is composed of actin flaments stabilized by spectrin (468 kilodaltons), which also anchors the terminal web to the apical cell membrane (Fig. 5.3b). The presence of myosin II and tropomyosin in the terminal web explains its contractile ability; these proteins decrease the diameter of the apex of the cell, causing the microvilli, whose stiff actin cores are anchored into the terminal web, to spread apart and increase the intermicrovillous space. The functional and structural features of microvilli are summarized in Table 5.2. Stereocilia are unusually long, immotile microvilli. Stereocilia are not widely distributed among epithelia. They are, in fact, limited to the epididymis, the proximal part of the ductus deferens of the male reproductive system, and the sensory (hair) cells of the inner ear. They are included in this section because this unusual surface modification is traditionally treated as a separate structural entity.


Histology_Ross

Histology_Ross. FIGURE 5.3 • Molecular structure of microvilli. a. High magnification of microvilli from Figure 5.2c. Note the presence of the actin filaments in the microvilli (arrows), which extend into terminal web in the apical cytoplasm. 80,000. b. Schematic diagram showing molecular structure of microvilli and the location of specific actin filament–bundling proteins (fimbrin, espin, and fascin). Note the distribution of myosin I within the microvilli and myosin II within the terminal web. The spectrin molecules stabilize the actin filaments within the terminal web and anchor them into the apical plasma membrane.


Histology_Ross

Histology_Ross. Stereocilia of the genital ducts are extremely long processes that extend from the apical surface of the cell and facilitate absorption. Unique features include an apical cell protrusion from which they arise and thick stem portions that are interconnected by cytoplasmic bridges. Because electron microscopy reveals their internal structure to be that of unusually long microvilli, some histologists now use the term stereovilli (Fig. 5.4a). Seen in the light microscope, these processes frequently resemble the hairs of a paint brush because of the way they aggregate into pointed bundles. Like microvilli, stereocilia are supported by internal bundles of actin flaments that are cross-linked by fmbrin. The actin filaments’ barbed (plus) ends are oriented toward the tips of the stereocilia and the pointed (minus) ends at the base. This organization of actin core shares many construction principles with the microvilli, yet it can be as long as 120 m.


Histology_Ross

Histology_Ross. Stereocilia develop from microvilli by the lateral addition of actin filaments to the actin bundle as well as elongation of the actin filaments. Unlike microvilli, an 80kilodalton actin-binding protein, ezrin, closely associated with the plasma membrane of stereocilia, anchors the actin filaments to the plasma membrane. The stem portion of the stereocilium and the apical cell protrusion contain the crossbridge–forming molecule -actinin (Fig. 5.4b). A striking difference between microvilli and stereocilia, other than size and the presence of ezrin, is the absence of villin from the tip of the stereocilium. Stereocilia of the sensory epithelium of the ear have some unique characteristics.


Histology_Ross

Histology_Ross. Stereocilia of the sensory epithelium of the ear have some unique characteristics. Stereocilia of the sensory epithelium of the ear also derive from microvilli. They are exquisitely sensitive to mechanical vibration and serve as sensory mechanoreceptors rather than absorptive structures. They are uniform in diameter and organized into ridged bundles of increasing heights, forming characteristic staircase patterns (Fig. 5.5a). Their internal structure is characterized by the high density of actin flaments extensively cross-linked by espin, which is critical to normal structure and function of stereocilia. Stereocilia of sensory epithelia lack both ezrin and -actinin. Since stereocilia can be easily damaged by overstimulation, they have a molecular mechanism to continuously renew their structure, which needs to be maintained in proper working condition for a lifetime. Using fluorescent-labeled actin molecules, researchers found that actin


Histology_Ross

Histology_Ross. TABLE Summary of Apical Domain Modifications in the Epithelial Cells 5.2 FIGURE 5.4 • Molecular structure of stereocilia. a. Electron micrograph of stereocilia from the epididymis. The cytoplasmic projections are similar to microvilli, but they are extremely long. 20,000. b. Schematic diagram showing the molecular structure of stereocilia. They arise from the apical cell protrusions, having thick stem portions that are interconnected by cytoplasmic bridges. Note the distribution of actin filaments within the core of the stereocilium and the actin-associated proteins, fimbrin, and espin, in the elongated portion (enlarged box); and -actinin in the terminal web, apical cell protrusion, and occasional cytoplasmic bridges between neighboring stereocilia.


Histology_Ross

Histology_Ross. monomers are being constantly added at the tips and removed at the base of the stereocilia while the entire bundle of actin filaments moves toward the base of the stereocilium (Fig. 5.5b,c). This treadmilling effect of the actin core structure is highly regulated and depends on the length of the stereocilium. The functional and structural features of stereocilia in comparison to microvilli and cilia are summarized in Table 5.2.


Histology_Ross

Histology_Ross. The functional and structural features of stereocilia in comparison to microvilli and cilia are summarized in Table 5.2. Cilia are common surface modifications present on nearly every cell in the body. They are hairlike extensions of the apical plasma membrane containing an axoneme, the microtubulebased internal structure. The axoneme extends from the basal body, a centriole-derived, microtubuleorganizing center (MTOC) located in the apical region of a ciliated cell. The basal bodies are associated with several accessory structures that assist them with anchoring into cell cytoplasm. Cilia, including basal bodies and basal body–associated structures, form the ciliary apparatus of the cell. In general, cilia are classified as motile, primary, or nodal.


Histology_Ross

Histology_Ross. In general, cilia are classified as motile, primary, or nodal. Based on their functional characteristics cilia are classified into three basic categories:  Motile cilia have been historically the most studied of all cilia. They are found in large numbers on the apical domain of many epithelial cells. Motile cilia and their counterparts, fagella, possess a typical 9 2 axonemal organization with microtubule-associated motor proteins that are necessary for the generation of forces needed to induce motility.  Primary cilia (monocilia) are solitary projections found on almost all eukaryotic cells. The term monocilia imp lies that only a single cilium per cell is usually present. Primary cilia are immotile because of different arrangements


Histology_Ross

Histology_Ross. FIGURE 5.5 • Dynamic turnover of an internal architecture of sterocilia. a. This scanning electron micrograph shows stereocilia of sensory epithelium of the inner ear. They are uniform in diameter and organized into ridged bundles of increasing heights. 47,000. b. Confocal microscopy image shows incorporation of the -actin green fluorescent protein (GFP) and espin-GFP to the tip of the stereocilia (green). Actin filaments in the core of the stereocilia are counterstained with rhodamine/phalloidin (red). 35,000 c.


Histology_Ross

Histology_Ross. Diagram illustrates the mechanism by which the core of actin filaments is remodeled. Actin polymerization and espin cross-linking into the barbed (plus) end of actin filaments occurs at the tip of the stereocilia. Disassembly and actin filaments depolymerization occurs at the pointed (minus) end of actin filament near the base of the stereocilium. When the rate of assembly at the tip is equivalent to the rate of disassembly at the base, the actin molecules undergo an internal rearward flow or treadmilling, thus maintaining the constant length of the stereocilium. (Reprinted with permission from Rzadzinska AK, Schneider ME, Davies C, Riordan GP, Kachar B. An actin molecular treadmill and myosins maintain stereocilia functional architecture and self-renewal. J Cell Biol 2004;164:887–897.) of microtubules in the axoneme and lack of microtubule-concentrated in the area that surrounds the primitive associated motor proteins. They function as chemosen-node, hence their name nodal cilia.


Histology_Ross

Histology_Ross. of microtubules in the axoneme and lack of microtubule-concentrated in the area that surrounds the primitive associated motor proteins. They function as chemosen-node, hence their name nodal cilia. They have a similar axsors, osmosensors, and mechanosensors, and they onemal internal architecture as primary cilia; however, mediate light sensation, odorant, and sound perception in they are distinct in their ability to perform rotational multiple organs in the body. It is now widely accepted movement. They play an important role in early embrythat primary cilia of cells in developing tissues are essen-onic development. tial for normal tissue morphogenesis.


Histology_Ross

Histology_Ross.  Nodal cilia are found in the embryo on the bilaminar The functional and structural features of all three types of embryonic disc at the time of gastrulation. They are cilia are summarized in Table 5.2. Motile cilia are capable of moving fluid and particles along epithelial surfaces. Motile cilia possess an internal structure that allows them to move. In most ciliated epithelia, such as the trachea, bronchi, or oviducts, cells may have as many as several hundred cilia arranged in orderly rows. In the tracheobronchial tree, the cilia sweep mucus and trapped particulate material toward the oropharynx where it is swallowed with saliva and eliminated from the body. In the oviducts, cilia help transport ova and fluid toward the uterus. Cilia give a “crew-cut” appearance to the epithelial surface.


Histology_Ross

Histology_Ross. Cilia give a “crew-cut” appearance to the epithelial surface. In the light microscope, motile cilia appear as short, fine, hairlike structures, approximately 0.25 m in diameter and 5 to10 m in length, that emanate from the free surface of the cell (Fig. 5.6). A thin, dark-staining band is usually seen extending across the cell at the base of the cilia. This dark-staining band represents structures known as basal bodies. These structures take up stain and appear as a continuous band when viewed in the light microscope. When viewed with the EM, however, the basal body of each cilium appears as a distinct individual structure. Motile cilia contain an axoneme, which represents an organized core of microtubules arranged in a 9 2 pattern.


Histology_Ross

Histology_Ross. Motile cilia contain an axoneme, which represents an organized core of microtubules arranged in a 9 2 pattern. Electron microscopy of a cilium in longitudinal profile reveals an internal core of microtubules, called axoneme (Fig. 5.7a). A cross-sectional view reveals a characteristic configuration of nine pairs or doublets of circularly arranged microtubules surrounding two central microtubules (Fig. 5.7b). FIGURE 5.6 • Ciliated epithelium. Photomicrograph of an H&E–stained specimen of tracheal pseudostratified ciliated epithelium. The cilia (C) appear as hairlike processes extending from the apical surface of the cells. The dark line immediately below the ciliary processes is produced by the basal bodies (BB) associated with the cilia. 750.


Histology_Ross

Histology_Ross. The microtubules composing each doublet are constructed so that the wall of one microtubule, designated the B micro-tubule, is actually incomplete; it shares a portion of the wall of the other microtubule of the doublet, the A microtubule. The A microtubule is composed of 13 tubulin protoflaments, arranged in side-by-side configuration, whereas the B micro-tubule is composed of 10 tubulin protoflaments. Tubulin molecules incorporated into ciliary microtubules are tightly bound together and post-translationally modified in the process of acetylation and polyglutamylation. Such modifications ensure that microtubules of ciliary axoneme are highly stable and resist depolymerization.


Histology_Ross

Histology_Ross. When seen in cross-section at high resolution, each doublet exhibits a pair of “arms” that contain ciliary dynein, a microtubule-associated motor protein. This motor protein uses the energy of adenosine triphosphate (ATP) hydrolysis to move along the surface of the adjacent microtubule (see Fig. 5.7). The dynein arms occur at 24-nm intervals along the length of the A microtubule and extend out to form temporary cross-bridges with the B microtubule of the adjacent doublet. A passive elastic component formed by nexin (165 kilodaltons) permanently links the A microtubule with the B microtubule of adjacent doublets at 86-nm intervals. The two central microtubules are separate but partially enclosed by a central sheath projection at 14-nm intervals along the length of the cilium (see Fig. 5.7). Radial spokes extend from each of the nine doublets toward the two central microtubules at 29-nm intervals. The proteins forming the radial spokes and the nexin connections between the outer


Histology_Ross

Histology_Ross. 5.7). Radial spokes extend from each of the nine doublets toward the two central microtubules at 29-nm intervals. The proteins forming the radial spokes and the nexin connections between the outer doublets make large-amplitude oscillations of the cilia possible.


Histology_Ross

Histology_Ross. Basal bodies and basal body–associated structures firmly anchor cilia in the apical cell cytoplasm. The 9 2 microtubule array courses from the tip of the cilium to its base, whereas the outer paired microtubules join the basal body. The basal body is a modified centriole. It functions as an MTOC consisting of nine short microtubule triplets arranged in a ring. Each of the paired microtubules of the ciliary axoneme (A and B microtubules) is continuous with two of the triplet microtubules of the basal body. The third incomplete C microtubule in the triplet extends from the bottom to the transitional zone at the top of the basal body near the transition between the basal body and the axoneme. The two central micro-tubules of the cilium originate at the transitional zone and extend to the top of axoneme (see Fig. 5.7b). Therefore, a cross-section of the basal body would reveal nine circularly arranged microtubule triplets but not the two central single microtubules of the cilium.


Histology_Ross

Histology_Ross. Basal bodies are associated with several basal body– associated structures such as alar sheets (transitional fibers), basal feet, and striated rootlets (see Fig. 5.7 and Fig 5.8).  The alar sheet (transitional fiber) is a collarlike extension between the transitional zone of basal body and plasma membrane. It originates near the top end of the basal body C microtubule and inserts into the cytoplasmic domain of the plasma membrane. It tethers the basal body to the apical plasma membrane (see Fig 5.7).


Histology_Ross

Histology_Ross. FIGURE 5.7 • Molecular structure of cilia. This figure shows a three-dimensional arrangement of microtubules within the cilium and the basal body. Cross section of the cilium (right) illustrates the pair of central microtubules and the nine surrounding microtubule doublets (9 2 configuration). The molecular structure of the microtubule doublet is shown below the cross section. Note that the A microtubule of the doublet is composed of 13 tubulin dimers arranged in a side-by-side configuration (lower right), whereas the B microtubule is composed of 10 tubulin dimers and shares the remaining dimers with those of the A microtubule. The dynein arms extend from the A microtubule and make temporary cross-bridges with the B microtubule of the adjacent doublet. The basal body is anchored by the striated rootlet within the cell cytoplasm. Note the presence of the basal foot in the midsection of the basal body. The cross section of the basal body (lower left) shows the arrangement of nine


Histology_Ross

Histology_Ross. the striated rootlet within the cell cytoplasm. Note the presence of the basal foot in the midsection of the basal body. The cross section of the basal body (lower left) shows the arrangement of nine microtubule triplets. These structures form a ring connected by nexin molecules. Each microtubule doublet of the cilium is an extension of two inner A and B microtubules of the corresponding triplet. The C microtubule is shorter and extends only to the transitional zone. Inset a. Electron micrograph of longitudinally sectioned cilia from the oviduct. The internal structures within the cilia are microtubules. The basal bodies appear empty because of the absence of the central pair of microtubules in this portion of the cilium. 20,000. Inset b. Electron micrograph of cross section of the cilium showing corresponding structures with drawing below. 180,000.


Histology_Ross

Histology_Ross. FIGURE 5.8 • Ciliated surface of the respiratory mucosa. Electron micrograph shows a longitudinally sectioned cilia from a respiratory epithelium of the nasal cavity. At this magnification, most of the basal bodies (BB) appear empty because of the absence of the central pair of microtubules in this portion of the cilium. Structural details of the basal body and basal body–associated structures are well visible on this section as well as on the higher magnification insert. Note that almost all basal bodies on this section possess striated rootlets (SR). They anchor the basal bodies deep within the apical cell cytoplasm. Each basal body has a single asymmetric basal foot (BF) projecting laterally; several are well visible on this section. The transitional zone (TZ) extends from the upper end of the basal body into the axomene (Ax) which is formed by a 9 2 microtubular arrangement. A central pair of microtubules is present on most of these sections. In addition, an alar sheath


Histology_Ross

Histology_Ross. upper end of the basal body into the axomene (Ax) which is formed by a 9 2 microtubular arrangement. A central pair of microtubules is present on most of these sections. In addition, an alar sheath (arrowheads) provides a winglike extension between the transitional zone and plasma membrane. The first and second basal bodies from the right have well preserved alar sheaths. 15,000. Inset 25,000. (Courtesy of Dr. Jeffrey L. Salisbury.)  The basal foot is an accessory structure that is usually Localization of myosin molecules associated with basal feet found in the midregion of the basal body (see Fig 5.8). supports this hypothesis. Since in the typical epithelial ciliated cells all basal feet are  The striated rootlet is composed of longitudinally aligned oriented in the same direction (Fig. 5.9), it has been hy-protofilaments, containing rootletin (a 220-kilodalton pothesized that they function in coordinating ciliary protein). Striated rootlet projects deep into cytoplasm and


Histology_Ross

Histology_Ross. (Fig. 5.9), it has been hy-protofilaments, containing rootletin (a 220-kilodalton pothesized that they function in coordinating ciliary protein). Striated rootlet projects deep into cytoplasm and movement. They are most likely involved in adjusting firmly anchors the basal body within the apical cell cytobasal bodies by rotating them to the desired position. plasm (see Fig 5.8).


Histology_Ross

Histology_Ross. FIGURE 5.9 • Basal bodies and cilia. This diagnostic electron micrograph obtained during biopsy of the nasal mucosa from a child undergoing evaluation for primary ciliary dyskinesia shows a normal appearance of basal bodies (BB) and cilia (C). It is an oblique section through the apical part of ciliated cells. Basal bodies seen in cross section appear as more dense structures than sectioned oblique and longitudinal profiles of the cilia above. Several profiles of microvilli (Mv) are visible at the apical cell surface. 11,000. Inset. Three basal bodies sectioned at the level of basal feet (BF). Note that all basal feet are oriented in the same direction. They most likely rotate the basal body to a desired angle in an effort to coordinate ciliary movement. 24,000. (Courtesy of Patrice C. Abell Aleff.) Cilia movement originates from the sliding of microtubule doublets, which is generated by the ATPase activity of the dynein arms.


Histology_Ross

Histology_Ross. Ciliary activity is based on the movement of the doublet microtubules in relation to one another. Ciliary movement is initiated by the dynein arms (see Fig. 5.7b). The ciliary dynein located in the arms of the A microtubule forms temporary cross-bridges with the B microtubule of the adjacent doublet. Hydrolysis of ATP produces a sliding movement of the bridge along the B microtubule. The dynein molecules produce a continuous shear force during this sliding directed toward the ciliary tip. Because of this ATP-dependent phase, a cilium that remains rigid exhibits a rapid forward movement called the effective stroke. At the same time, the passive elastic connections provided by the protein nexin and the radial spokes accumulate the energy necessary to bring the cilium back to the straight position. Cilia then become flexible and bend toward the lateral side on the slower return movement, the recovery stroke.


Histology_Ross

Histology_Ross. However, if all dynein arms along the length of the A microtubules in all nine doublets attempted to form temporary cross-bridges simultaneously, no effective stroke of the cilium would result. Thus, regulation of the active shear force is required. Current evidence suggests that the central pair of microtubules in 9 2 cilia undergo rotation with respect to the nine outer doublets. This rotation may be driven by another motor protein, kinesin, which is associated with the central pair of microtubules. The central microtubule pair can act as a “distributor” that progressively regulates the sequence of interactions of the dynein arms to produce the effective stroke. Cilia beat in a synchronous pattern.


Histology_Ross

Histology_Ross. Cilia beat in a synchronous pattern. Motile cilia with a 9 2 pattern display a precise and synchronous undulating movement. Cilia in successive rows start their beat so that each row is slightly more advanced in its cycle than the following row, thus creating a wave that sweeps across the epithelium. As previously discussed, basal feet of basal bodies are most likely responsible for synchronization of ciliary movement. During the process of cilia formation, all basal feet became oriented in the same direction of effective stroke by rotating basal bodies. This orientation allows cilia to achieve a metachronal rhythm that is responsible for moving mucus over epithelial surfaces or facilitating the flow of fluid and other substances through tubular organs and ducts. Primary cilia are nonmotile and contain a 9 0 pattern of microtubules.


Histology_Ross

Histology_Ross. Primary cilia are nonmotile and contain a 9 0 pattern of microtubules. Differing from motile cilia with the 9 2 pattern of microtubules is another type of cilia that display a 9 0 microtubule arrangement. Cilia with this pattern have the following characteristics:  They are nonmotile and passively bend by a flow of the fluid. to generate motile force.  The central pair of microtubules is missing.  The axoneme originates from a basal body that resembles a mature centriole positioned orthogonally in relation to its immature counterpart.  Primary cilium formation is synchronized with cell cycle progression and centrosome duplication events.


Histology_Ross

Histology_Ross. These cilia are present in a variety of cells and are called primary cilia or monocilia because each cell usually possesses only one such cilium (Fig. 5.10). They are also found in some epithelial cells (e.g., the epithelial cells of the rete testis in the male reproductive tract, epithelial cells lining the biliary tract, epithelial cells of kidney tubules, epithelial-like ependymal cells lining the fluid-filled cavities of the central nervous system, the connecting stalk of photoreceptor cells in the retina, and the vestibular hair cells of the ear. Primary cilia were formerly classified as nonfunctional vestigial developmental abnormalities of 9 2 motile cilia. Experimental studies of the last decade elevated the status of primary cilia to the level of important cellular-signaling devices functioning comparably to an antenna on a global positioning system (GPS) receiver. Similar to an antenna that takes information from satellites and allows the GPS receiver to calculate the


Histology_Ross

Histology_Ross. devices functioning comparably to an antenna on a global positioning system (GPS) receiver. Similar to an antenna that takes information from satellites and allows the GPS receiver to calculate the user’s exact location, primary cilia receive chemical, osmotic, light, and mechanical stimuli from the extracellular environment. In response to these stimuli, primary cilia generate signals that are transmitted into the cell to modify cellular processes in response to changes in the external environment. In many mammalian cells, signaling through the primary cilia seems to be essential for controlled cell division and subsequent gene expression.


Histology_Ross

Histology_Ross. Primary cilia containing a 9 0 pattern of microtubules function as signal receptors sensing a flow of fluid in developing organs.


Histology_Ross

Histology_Ross. Primary cilia function in secretory organs such as the kidneys, liver, or pancreas as sensors of fluid flow. They extend from the surface of epithelial cells lining secretory ducts into the extracellular lumen (Fig. 5.11). For instance, primary cilia found in the glomerulus and tubular cells of the kidney function as mechanoreceptors; fluid flow through the renal corpuscle and tubules causes them to bend, which initiates an influx of calcium into the cell (Fig. 5.11). In humans, mutations in two genes, ADPKD1 and ADPKD2, appear to affect development of these primary cilia, leading to polycystic kidney disease (PKD). The proteins encoded by these genes, polycystin-1 and polycystin-2, respectively, are essential in the formation of the calcium channels associated with primary cilia (see Fig 5.11b). This autosomal recessive disorder is characterized by multiple expanding cysts in both kidneys, which ultimately destroy the renal cortex and lead to renal failure. However, individuals with


Histology_Ross

Histology_Ross. 5.11b). This autosomal recessive disorder is characterized by multiple expanding cysts in both kidneys, which ultimately destroy the renal cortex and lead to renal failure. However, individuals with PKD often exhibit other pathologies not associated with the kidney that are now attributed to ciliary


Histology_Ross

Histology_Ross. FIGURE 5.10 • Primary cilia in the connective tissue and the kidney tubule. a. Electron micrograph shows a fibroblast surrounded by the extracellular matrix from the uterine connective tissue containing a primary cilium. The primary cilium is characterized by a (9 0) pattern of the microtubule arrangement. 45,000. Inset shows higher magnification of the cilium. Note the visible basal body and doublets of microtubules emerging from the basal body. 90,000. b. This scanning electron micrograph shows a single primary cilium projecting into the lumen of the collecting tubule of the kidney. Primary cilia are prominent on the free surface of the collecting tubule cells and function as mechanoreceptors that become activated by fluid flow through the tubules. Passive bending of cilia opens calcium channels and initiates signaling cascades by the influx of calcium into the cell cytoplasm. 65,000. (Courtesy of Dr. Tetyana V. Masyuk.)


Histology_Ross

Histology_Ross. FIGURE 5.11 • Primary cilium in the kidney tubule is a primary sensor for the fuid fow. Primary cilia in kidney function as sensors for the flow of fluid through the tubules. Deflection of the primary cilium opens the mechanoreceptor calcium channels, which are formed by cystic kidney disease– associated proteins (polycystin-1 and policystin-2). This subsequently initiates the influx of calcium into the cell, releasing additional intracellular calcium from the endoplasmic reticulum. Scanning electron micrograph inset shows primary cilia projecting into the lumen of abnormalities. These include cysts in the pancreas and liver that are accompanied by an enlargement and dilatation of the biliary tree system. Other changes include retinitis pigmentosa (abnormalities of the photoreceptors cells of the retina that cause progressive vision loss), sensorineural hearing loss, diabetes, and learning disabilities. The knowledge of the distribution of primary cilia in the body may help to explain


Histology_Ross

Histology_Ross. of the retina that cause progressive vision loss), sensorineural hearing loss, diabetes, and learning disabilities. The knowledge of the distribution of primary cilia in the body may help to explain the crucial role of these once-forgotten cellular projections in the normal function of many vital internal organs.


Histology_Ross

Histology_Ross. During early embryonic development, nodal cilia containing a 9 0 pattern of microtubules establish the left–right asymmetry of internal organs. Recent studies suggest that specific primary cilia observed in embryos, despite their 9 0 architectural pattern, are motile and play an important role in early embryonic development by generating the left–right asymmetry of internal organs. During gastrulation, a clockwise rotation of these cilia was observed on the ventral surface of the bilaminar embryonic disc in the area near the primitive node, hence the name nodal cilia. These cilia contain motor proteins (dyneins or kinesins) and are capable of rotational movement in a counterclockwise direction as previously described. Most likely, the absence of the central pairs of microtubules is responsible for such movement, the trajectory of which resembles a full cone in contrast to a half-cone trajectory traceable in the motile 9 2 cilia (Table 5.2).


Histology_Ross

Histology_Ross. Movement of nodal cilia in the region known as the the collecting tubule. 27,000. (Courtesy of Dr. C. Craig Tisher.) primitive node generates a leftward, or “nodal” flow. This flow is detected by sensory receptors on the left side of the body, which then initiate signaling mechanisms that differ from those on the right side of the embryo. When nodal cilia are immotile or absent, nodal ﬂow does not occur, leading to random placement of internal body organs. Therefore, primary ciliary dyskinesia (immotile cilia syndrome) often results in situs inversus, a condition in which the position of the heart and abdominal organs are reversed. The first stage of ciliogenesis includes generation of centrioles.


Histology_Ross

Histology_Ross. The first stage of ciliary apparatus formation (ciliogenesis) in differentiating cells involves a generation of multiple centrioles. This process occurs either in the centriolar pathway (by duplication of pairs of existing centrioles, see page 69 in Chapter 2) or more commonly in the acentriolar pathway in which centrioles are formed de novo without involvement of existing centrioles. Both pathways give rise to multiple procentrioles, the immediate precursors of centrioles. Procentrioles mature (elongate) to form centrioles, one for each cilium, and migrate to the apical surface of the cell. After perpendicularly aligning themselves and securing to the apical cell membrane by alar sheets (transitional fibers), centrioles assume the function of basal bodies. The next stage of ciliary apparatus formation involves formation of the remaining basal body– associated structures that include basal feet and striated rootlets. From each of the nine triplets that make up the basal body, a


Histology_Ross

Histology_Ross. apparatus formation involves formation of the remaining basal body– associated structures that include basal feet and striated rootlets. From each of the nine triplets that make up the basal body, a microtubule doublet grows upward by poly


Histology_Ross

Histology_Ross. FOLDER 5.2 Clinical Correlation: Primary Ciliary Dyskinesia (Immotile Cilia Syndrome)


Histology_Ross

Histology_Ross. Cilia are present in almost all organs and play a significant role in the human body. There is increasing evidence that cilial dysfunction is involved in many human disorders. Sev-eral hereditary disorders grouped under the general name of primary ciliary dyskinesia (PCD), also known as immotile cilia syndrome, affect the function of cilia. PCD represents a group of autosomal recessive hereditary disorders affecting 1 in 20,000 individuals at birth. The clinical features of PCD reflect the distribution of motile cilia. For instance, the mucociliary transport that occurs in the respiratory epithelium is one of the important mechanisms protecting the body against invading bacteria and other pathogens. Motile cilia covering the epithelium of the respiratory tract are responsible for the clearance of the airway. Failure of the mucociliary transport system occurs in Kartagener’s syndrome, which is caused by a structural abnormality that results in absence of dynein arms (Fig. F5.2.1). In


Histology_Ross

Histology_Ross. of the airway. Failure of the mucociliary transport system occurs in Kartagener’s syndrome, which is caused by a structural abnormality that results in absence of dynein arms (Fig. F5.2.1). In addition, EM examination of basal bodies from individuals with the Kartagener’s syndrome often reveals misoriented basal feet pointing in different di-rections. Young’s syndrome, which is characterized by malformation of the radial spokes and dynein arms, also af-fects ciliary function in the respiratory tract. The most prominent symptoms of PCD are chronic respiratory dis -tress (including bronchitis and sinusitis), otitis media (in-flammation of the middle ear cavity), persistent cough, and asthma. Respiratory problems are caused by severely impaired or absent ciliary motility that results in reduced or absent mucociliary transport in the tracheobronchial tree. Flagellum of the sperm, cilia of the efferent ductules in the testis, and cilia of the female reproductive system share the same


Histology_Ross

Histology_Ross. reduced or absent mucociliary transport in the tracheobronchial tree. Flagellum of the sperm, cilia of the efferent ductules in the testis, and cilia of the female reproductive system share the same organization (9 2) pattern with the cilia of the respiratory tract. Therefore, males with PCD are sterile be-cause of immotile flagella. In contrast, some females with the syndrome may be fertile; however, there is an increased incidence of ectopic pregnancy. In such individuals, the FIGURE F5.2.1 • Electron micrograph of the cilium from an individual with primary cilary dyskinesia (PCD). Note the absence of dynein arms on microtubule doublets. 180,000. (Courtesy of Patrice Abell-Aleff.) ciliary movement may be sufficient, though impaired, to permit transport of the ovum through the oviduct to the uterus. Some individuals with PCD may also develop symp-toms of hydrocephalus internus (accumulation of fluid in the brain) or transient dilatation of inner brain ventricles. The ependymal cells


Histology_Ross

Histology_Ross. the uterus. Some individuals with PCD may also develop symp-toms of hydrocephalus internus (accumulation of fluid in the brain) or transient dilatation of inner brain ventricles. The ependymal cells lining the cerebrospinal fluid–filled spaced in the brain possess motile cilia with a 9 2 pat-tern. These cilia may be important for the circulation of cerebrospinal fluid through the narrow spaces between the brain ventricles. About 50% of patients with diagnosed PCD have situs inversus (a condition in which the organs of the viscera are transposed through the sagittal plane), providing a link be-tween left–right asymmetry and nodal cilia. Diagnosis of PCD in individuals with clinical syn-dromes compatible with PCD can be established by EM (see Fig. F5.2.1).


Histology_Ross

Histology_Ross. merization of and -tubulin molecules. A growing projection of the apical cell membrane become visible and contains the nine doublets found in the mature cilium. During the elongation stage of motile cilia, the assembly of two single, central microtubules starts in the transitional zone from -tubulin rings. The subsequent polymerization of tubulin molecules occurs within the ring of doublet microtubules, thus yielding the characteristic axonemal 9 2 arrangement. Subsequently, the axoneme grows upward from the basal body, pushing the cell membrane outward to form the mature cilium. Ciliogenesis dependents on the bidirectional intraflagellar transport mechanism that supplies precursor molecules to the growing cilium.


Histology_Ross

Histology_Ross. During growth and elongation of the cilium, precursor molecules are delivered from the cell body to the most distal end of the elongating axoneme by intrafagellar transport (IFT). Since cilia have no molecular machinery for protein synthesis, the IFT is the only mechanism for delivering proteins required for cilia assembly and growth. In some ways, the IFT can be compared to the vertical lift assembly used at a construction site to move building materials and tools up and down a building. As the building increases in height, the track of the lift extends as well. Similarly, the IFT utilizes raftlike platforms assembled from about 17 different intraflagellar transport proteins that move up and down the growing axoneme between the outer doublets of microtubules and plasma membrane of the elongating cilium (Fig. 5.12). Cargo molecules (including inactive cytoplasmic dynein molecules) are loaded onto the IFT platform while it is docked near the base of the cilium. Utilizing kinesin II as


Histology_Ross

Histology_Ross. cilium (Fig. 5.12). Cargo molecules (including inactive cytoplasmic dynein molecules) are loaded onto the IFT platform while it is docked near the base of the cilium. Utilizing kinesin II as a motor protein, the fully loaded platform is moved upward toward the tip of the cilium (anterograde transport). The “building materials” are then unloaded at the tip of the cilium (the site of axoneme assembly). Here particles turn around, and the platform heads back to the base of the cilium (retrograde transport) after picking up turnover products (including inactivated kinesin II). During this process, cytoplasmic dynein is activated and utilized as a motor protein to bring the platform back to the base of the cilium (see Fig. 5.12). Several proteins, including IFT raft proteins (kinesis, cytoplasmic dynein, polaris, IFT20, etc.), are important to ciliogenesis and subsequent maintenance of the functional cilium. Mutations in genes encoding these proteins result in loss of cilia or ciliary


Histology_Ross

Histology_Ross. dynein, polaris, IFT20, etc.), are important to ciliogenesis and subsequent maintenance of the functional cilium. Mutations in genes encoding these proteins result in loss of cilia or ciliary dysfunctions.


Histology_Ross

Histology_Ross. The lateral domain of epithelial cells is in close contact with the opposed lateral domains of neighboring cells. Like the other domains, the lateral domain is characterized by the presence of unique proteins, in this case the cell adhesion molecules (CAMs) that are part of junctional specializations. The molecular composition of the lipids and proteins that form the lateral cell membrane differ significantly from the composition of those that form the apical cell membrane. In addition, the lateral cell surface membrane in some epithelia may form folds and processes, invaginations and evaginations that create interdigitating and interleaving tongue-and-groove margins between neighboring cells. Viewed with the light microscope, terminal bars represent epithelial cell-to-cell attachment sites.


Histology_Ross

Histology_Ross. Viewed with the light microscope, terminal bars represent epithelial cell-to-cell attachment sites. Before the advent of EM, the close apposition of epithelial cells was attributed to the presence of a viscous adhesive substance referred to as intercellular cement. This cement stained deeply at the apicolateral margin of most cuboidal and columnar epithelial cells. When viewed in a plane perpendi cular to the epithelial surface, the stained material appears as a dotlike structure. When the plane of section passes parallel to and includes the epithelial surface, however, the dotlike component is seen as a dense bar or line between the apposing cells (Fig. 5.13). The bars, in fact, form a polygonal structure (or band) that encircles each cell to bind them together. Arrangement of this band can be compared to the plastic rings that hold together a six-pack of canned beverages.


Histology_Ross

Histology_Ross. Because of its location in the terminal or apical portion of the cell and its barlike configuration, the stainable material visible in light microscopy was called the terminal bar. It is now evident that intercellular cement as such does not exist. The terminal bar, however, does represent a significant structural complex. Electron microscopy has shown that it includes a specialized site that joins epithelial cells (Fig. 5.14a). It is also the site of a considerable barrier to the passage (diffusion) of substances between adjacent epithelial cells. The specific structural components that make up the barrier and the attachment device are readily identified with the EM and are collectively referred to as a junctional complex (see Table 5.4, page 135). These complexes are responsible for joining individual cells together. There are three types of junctional complexes (Fig. 5.14b):  Occluding junctions are impermeable and allow epithelial cells to function as a barrier. Also called tight


Histology_Ross

Histology_Ross. individual cells together. There are three types of junctional complexes (Fig. 5.14b):  Occluding junctions are impermeable and allow epithelial cells to function as a barrier. Also called tight junctions, occluding junctions form the primary intercellular diffusion barrier between adjacent cells.


Histology_Ross

Histology_Ross. By limiting the movement of water and other molecules through the intercellular space, they maintain physico chemical separation of tissue compartments. Because they are located at the most apical point between adjoin ing epithelial cells, occluding junctions prevent the mi gration of lipids and specialized membrane proteins between the apical and lateral surfaces, thus maintaining the integrity of these two domains. In addition, occlud ing junctions recruit various signaling molecules to the cell surface and link them to the actin flaments of the cell cytoskeleton.


Histology_Ross

Histology_Ross. FIGURE 5.12 • Intrafagellar transport mechanism within the cilium. Assembly and maintenance of cilia depends on the intraflagellar transport mechanism (IFT ) that utilizes raftlike platforms. They move up and down between the outer doublets of microtubules and plasma membrane of the elongating cilium. Cargo molecules (including inactive cytoplasmic dynein) are loaded onto the IFT platform while it is docked near the base of the cilium. Using kinesin II as a motor protein, the fully loaded platform is moved upward toward the plus end of microtubules at the tip of the cilium (anterograde transport). The cargo is then unloaded at the tip of the cilium (the site of axoneme assembly). Here particles turn around, and the platform powered by cytoplasmic dynein heads back to the base of the cilium (retrograde transport) after picking up turnover products (including inactivated kinesin II). Inset. Electron micrograph of a longitudinal section of a Chlamydomonas flagellum with two groups of IFT


Histology_Ross

Histology_Ross. (retrograde transport) after picking up turnover products (including inactivated kinesin II). Inset. Electron micrograph of a longitudinal section of a Chlamydomonas flagellum with two groups of IFT platforms. 55,000. (Reprinted with permission from Pedersen LB, Veland IR, Schrer JM, and Christensen ST. Assembly of primary cilia. Dev Dyn. 2008; 237:1993–2006.)


Histology_Ross

Histology_Ross. FIGURE 5.13 • Terminal bars in pseudostratifed epithelium. Photomicrograph of an H&E–stained specimen showing the terminal bars in a pseudostratified epithelium. The bar appears as a dot (arrowheads) when seen on its cut edge. When the bar is coursing parallel to the cut surface and lying within the thickness of the section, it is seen as a linear or barlike profile (arrows). 550. FIGURE 5.14 • Junctional complex. a. Electron micrograph of the apical portion of two adjoining epithelial cells of the gastric mucosa, showing the junctional complex. It consists of the zonula occludens (ZO), zonula adherens (ZA), and macula adherens (MA). 30,000. b. Diagram showing the distribution of cell junctions in the three cellular domains of columnar epithelial cells. The apical domain with microvilli has been lifted to better illustrate spatial arrangements of junctional complexes within the cell.


Histology_Ross

Histology_Ross.  Anchoring junctions provide mechanical stability to epithelial cells by linking the cytoskeleton of one cell to the cytoskeleton of an adjacent cell. These junctions are important in creating and maintaining the structural unity of the epithelium. Anchoring junctions interact with both actin and intermediate flaments and can be found not only on the lateral cell surface but also on the basal domain of the epithelial cell. Through their signal transduction capability, anchoring junctions also play important roles in cell-to-cell recognition, morphogenesis, and differentiation.  Communicating junctions allow direct communication between adjacent cells by diffusion of small (1,200 daltons) molecules (e.g., ions, amino acids, sugars, nucleotides, second messengers, metabolites). This type of intercellular communication permits the coordinated cellular activity that is important for maintaining organ homeostasis.


Histology_Ross

Histology_Ross. The zonula occludens (pl., zonulae occludentes) represents the most apical component in the junctional complex between epithelial cells. The zonula occludens is created by localized sealing of the plasma membrane of adjacent cells.


Histology_Ross

Histology_Ross. Examination of the zonula occludens or tight junction with the transmission electron microscope (TEM) reveals a narrow region in which the plasma membranes of adjoining cells come in close contact to seal off the intercellular space (Fig. 5.15a). At high resolution, the zonula occludens appears not as a continuous seal but as a series of focal fusions between the cells. These focal fusions are created by transmembrane proteins of adjoining cells that join in the intercellular space (Fig. 5.15b). The arrangement of these proteins in forming the seal is best visualized by the freeze fracture technique (Fig. 5.15c). When the plasma membrane is fractured at the site of the zonula occludens, the junctional proteins are observed on the P-face of the membrane, where they appear as ridgelike structures. The opposing surface of the fractured membrane, the E-face, reveals complementary grooves resulting from detachment of the protein particles from the opposing surface. The ridges and grooves


Histology_Ross

Histology_Ross. The opposing surface of the fractured membrane, the E-face, reveals complementary grooves resulting from detachment of the protein particles from the opposing surface. The ridges and grooves are arranged as a network of anastomosing particle strands, thus creating a functional seal within the intercellular space. The number of strands as well as the degree of anastomosing varies in different cells.


Histology_Ross

Histology_Ross. FIGURE 5.15 • Zonula occludens. a. Electron micrograph of the zonula occludens showing the close approximation of the outer lamellae of adjoining plasma membranes. The extracellular domains of proteins involved in the formation of this junction (occludins) appear as single, electron-dense lines (arrows). 100,000. b. Diagram showing the organization and pattern of distribution of the transmembrane protein occludin within the occluding junction. Compare the linear pattern of grooves with the ridges detected in the freeze fracture preparation on the right side. c. Freeze fracture preparation of zonula occludens shown here reveals an anastomosing network of ridges (arrows) located on the fracture membrane surface near the apical part of the cell [note microvilli (Mv) present at the cell surface]. This is the P-face of the membrane. (The E-face of the fractured membrane would show a complementary pattern of grooves.) The ridges represent linear arrays of transmembrane proteins (most likely


Histology_Ross

Histology_Ross. This is the P-face of the membrane. (The E-face of the fractured membrane would show a complementary pattern of grooves.) The ridges represent linear arrays of transmembrane proteins (most likely occludins) involved in the formation of the zonula occludens. The membrane of the opposing cell contains a similar network of proteins, which is in register with the first cell. The actual sites of protein interaction between the cells form the anastomosing network. 100,000. (Reprinted with permission from Hull BE, Staehelin LA. Functional significance of the variations in the geometrical organization of tight junction networks. J Cell Biol 1976;68:688–704.)