Pawlina Epithelium PDF

5 Epithelial Tissue OVERVIEW OF EPITHELIAL STRUCTURE AND THE BASAL DOMAIN AND ITS SPECIALIZATIONS FUNCTION / 105 IN CELL-TO-EXTRACELLULAR MATRIX C...

5 Epithelial Tissue OVERVIEW OF EPITHELIAL STRUCTURE AND THE BASAL DOMAIN AND ITS SPECIALIZATIONS FUNCTION / 105 IN CELL-TO-EXTRACELLULAR MATRIX CLASSIFICATION OF EPITHELIUM / 106 ADHESION / 133 CELL POLARITY / 107 Basement Membrane Structure and Function / 133 Cell-to-Extracellular Matrix Junctions / 141 THE APICAL DOMAIN AND ITS Morphologic Modifications of the MODIFICATIONS / 107 Basal Cell Surface / 143 Microvilli / 109 Stereocilia / 110 GLANDS / 143 Cilia / 111 EPITHELIAL CELL RENEWAL / 146 THE LATERAL DOMAIN AND ITS Folder 5.1 Clinical Correlation: Epithelial Metaplasia / 109 SPECIALIZATIONS IN CELL-TO-CELL Folder 5.2 Clinical Correlation: Primary Ciliary ADHESION / 120 Dyskinesia (Immotile Cilia Syndrome) / 118 Occluding Junctions / 120 Folder 5.3 Clinical Correlation: Junctional Complexes as Anchoring Junctions / 126 a Target of Pathogenic Agents / 126 Communicating Junctions / 129 Folder 5.4 Functional Considerations: Basement Morphologic Specializations of the Lateral Membrane and Basal Lamina Terminology / 135 Cell Surface / 133 Folder 5.5 Functional Considerations: Mucous and Serous Membranes / 147 HISTOLOGY 101 / 148 O V E R V I E W O F E P I TH ELIA L They exhibit functional and morphologic polarity. In other words, different functions are associated with S T R U C T U R E A N D F U NC TIO N three distinct morphologic surface domains: a free Epithelium covers body surfaces, lines body cavities, and surface or apical domain, a lateral domain, and constitutes glands. a basal domain. The properties of each domain are determined by specific lipids and integral membrane Epithelium is an avascular tissue composed of cells that cover proteins. the exterior body surfaces and line internal closed cav- ities (including the vascular system) and body tubes that Their basal surface is attached to an underlying basement membrane, a noncellular, protein–polysaccharide-rich communicate with the exterior (the alimentary, respiratory, layer demonstrable at the light microscopic level by histo- and genitourinary tracts). Epithelium also forms the secre- chemical methods (see Fig. 1.2, page 6). tory portion (parenchyma) of glands and their ducts. In addition, specialized epithelial cells function as receptors In special situations, epithelial cells lack a free surface for the special senses (smell, taste, hearing, and vision). (epithelioid tissues). The cells that make up epithelium have three principal In some locations, cells are closely apposed to one another characteristics: but lack a free surface. Although the close apposition of They are closely apposed and adhere to one another by these cells and the presence of a basement membrane means of specific cell-to-cell adhesion molecules that form would classify them as epithelium, the absence of a free specialized cell junctions (Fig. 5.1). surface more appropriately classifies such cell aggregates 105 Pawlina_CH05.indd 105 9/29/14 6:43 PM  LUMEN junctional complex apical domain 106 C L A S S I F I C AT I O N O F E P I T H E L I U M lateral domain intercellular space basal domain a b basement basement Epithelial Tissue membrane membrane 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, limit- ing 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. as epithelioid tissues. The epithelioid cells are derived CLASSIFICATION OF EPITHELIUM CHAPTER 5 from progenitor mesenchymal cells (nondifferentiated cells of embryonic origin found in connective tissue). Although The traditional classification of epithelium is descriptive and the progenitor cells of these epithelioid tissues may have based on two factors: the number of cell layers and the shape arisen from a free surface or the immature cells may have of the surface cells. The terminology, therefore, reflects only had a free surface at some time during development, the structure, not function. Thus, epithelium is described as: mature cells lack a surface location or surface connec- tion. Epithelioid organization is typical of most endocrine simple when it is one cell layer thick and glands; examples of such tissue include the interstitial cells stratified when it has two or more cell layers. of Leydig in the testis (Plate 3, page 154), the lutein cells The individual cells that compose an epithelium are of the ovary, the islets of Langerhans in the pancreas, described as: the parenchyma of the adrenal gland, and the anterior lobe of the pituitary gland. Epithelioreticular cells of the squamous when the width of the cell is greater than its thymus also may be included in this category. Epithelioid height; patterns are also formed by accumulations of connective cuboidal when the width, depth, and height are approxi- tissue macrophages in response to certain types of injury mately the same; and and infections as well as by many tumors derived from columnar when the height of the cell appreciably exceeds epithelium. the width (the term low columnar is often used when Epithelium creates a selective barrier between the external a cell’s height only slightly exceeds its other dimensions). environment and the underlying connective tissue. Thus, by describing the number of cell layers (i.e., simple Covering and lining epithelium forms a sheet-like cellular or stratified) and the surface cell shape, the various config- investment that separates underlying or adjacent connec- urations of epithelia are easily classified. The cells in some tive tissue from the external environment, internal cavities, exocrine glands are more or less pyramidal, with their apices or fluid connective tissue such as the blood and lymph. directed toward a lumen. However, these cells are still clas- Among other roles, this epithelial investment functions as a sified as either cuboidal or columnar, depending on their selective barrier that facilitates or inhibits the passage of height relative to their width at the base of the cell. specific substances between the exterior (including the body In a stratified epithelium, the shape and height of cavities) environment and the underlying connective tissue the cells usually vary from layer to layer, but only the shape compartment. of the cells that form the surface layer is used in classifying the Pawlina_CH05.indd 106 9/29/14 6:43 PM  epithelium. For example, stratified squamous epithelium con- absorption, as in the columnar epithelium of the intes- sists of more than one layer of cells, and the surface layer tines and proximal convoluted tubules in the kidney; consists of flat or squamous cells. transportation, as in the transport of materials or cells 107 In some instances, a third factor—specialization of the along the surface of an epithelium propelled by motile apical cell surface domain—can be added to this classi- cilia (transport of dust particles in the bronchial tree) or CHAPTER 5 fication system. For example, some simple columnar epithe- in the transport of materials across an epithelium (pino- lia are classified as simple columnar ciliated when the apical cytosis or endocytosis) to and from the connective tissue; surface domain possesses cilia. The same principle applies to mechanical protection, as in the stratified squamous stratified squamous epithelium, in which the surface cells epithelium of the skin (epidermis) and the transitional may be keratinized or nonkeratinized. Thus, epidermis would epithelium of the urinary bladder; and be designated as stratified squamous keratinized epithelium receptor function to receive and transduce exter- because of the keratinized cells at the surface. nal stimuli, as in the taste buds of the tongue, olfactory Epithelial Tissue epithelium of the nasal mucosa, and the retina of the eye. Pseudostratified epithelium and transitional epithelium are special classifications of epithelium. Epithelia involved in secretion or absorption are typically Two special categories of epithelium are pseudostratified and simple or, in a few cases, pseudostratified. The height of the transitional. cells often reflects the level of secretory or absorptive activity. Simple squamous epithelia are compatible with a high rate of Pseudostratified epithelium appears stratified, although transepithelial transport. Stratification of the epithelium usu- some of the cells do not reach the free surface; all rest on the ally correlates with transepithelial impermeability. Finally, in basement membrane (Plate 2, page 152). Thus, it is actually a some pseudostratified epithelia, basal cells are the stem cells simple epithelium. The distribution of pseudostratified ep- that give rise to the mature functional cells of the epithelium, ithelium is limited in the body. Also, it is often difficult to dis- thus balancing cell turnover. cern whether all of the cells contact the basement membrane. T H E A P I C A L D O MA I N A N D I T S MO DI FI C ATI ON S For these reasons, identification of pseudostratified epithelium usually depends on knowing where it is normally found. C ELL P O LA R ITY Transitional epithelium (urothelium) is a term Epithelial cells exhibit distinct polarity. They have an apical applied to the epithelium lining the lower urinary tract, domain, a lateral domain, and a basal domain. Specific extending from the minor calyces of the kidney down to biochemical characteristics are associated with each cell sur- the proximal part of the urethra. Urothelium is a strati- face. These characteristics and the geometric arrangements of fied epithelium with specific morphologic character- the cells in the epithelium determine the functional polarity istics that allow it to distend (Plate 3, page 154). This of all three cell domains. epithelium is described in Chapter 20. The free or apical domain is always directed toward the The cellular configurations of the various types of epithelia exterior surface or the lumen of an enclosed cavity or tube. and their appropriate nomenclature are illustrated in Table 5.1. The lateral domain communicates with adjacent cells and is characterized by specialized attachment areas. The basal Endothelium and mesothelium are the simple squamous domain rests on the basal lamina, anchoring the cell to epithelia lining the vascular system and body cavities. underlying connective tissue. Specific names are given to epithelium in certain locations: The molecular mechanism responsible for establishing polarity in epithelial cells is required to first create a fully Endothelium is the epithelial lining of the blood and functional barrier between adjacent cells. Junctional com- lymphatic vessels. plexes (which will be discussed later in this chapter) are being Endocardium is the epithelial lining of ventricles and formed in the apical parts of the epithelial cells. These special- atria of the heart. ized attachment sites not only are responsible for tight cell Mesothelium is the epithelium that lines the walls and cov- adhesions but also allow epithelium to regulate paracellular ers the contents of the closed cavities of the body (i.e., the ab- movements of solutes down their electroosmotic gradients. dominal, pericardial, and pleural cavities; Plate 1, page 150). In addition, junctional complexes separate the apical plasma Both endothelium and endocardium, as well as mesothe- membrane domain from basal and lateral domains and allow lium, are almost always simple squamous epithelia. An excep- them to specialize and recognize different molecular signals. tion 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 TH E A P IC A L D O M AIN AN D ITS is found in the spleen in which endothelial cells of the venous M O D IFIC ATIO NS sinuses are rod-shaped and arranged like the staves of a barrel. In many epithelial cells, the apical domain exhibits special Diverse epithelial functions can be found in different structural surface modifications to carry out specific functions. organs of the body. In addition, the apical domain may contain specific enzymes A given epithelium may serve one or more functions, depend- (e.g., hydrolases), ion channels, and carrier proteins (e.g., glu- ing on the activity of the cell types that are present: cose transporters). The structural surface modifications include: secretion, as in the columnar epithelium of the stomach microvilli, cytoplasmic processes containing a core of and the gastric glands; actin filaments; Pawlina_CH05.indd 107 9/29/14 6:43 PM  TA B LE 5. 1 Types of Epithelium 108 Classification Some Typical Locations Major Function T H E A P I C A L D O M A I N A N D I T S M O D I F I C AT I O N S Simple squamous Vascular system (endothelium) Exchange, barrier in central Body cavities (mesothelium) nervous system Bowman’s capsule (kidney) Exchange and lubrication Respiratory spaces in lung Simple cuboidal Small ducts of exocrine glands Absorption and conduit Surface of ovary (germinal epithelium) Barrier Kidney tubules Absorption and secretion Thyroid follicles Simple columnar Small intestine and colon Absorption and secretion Stomach lining and gastric glands Secretion Gallbladder Absorption Pseudostratified Trachea and bronchial tree Secretion and conduit Ductus deferens Absorption and conduit Efferent ductules of epididymis Epithelial Tissue Stratified squamous Epidermis Barrier and protection Oral cavity and esophagus Vagina Stratified cuboidal Sweat gland ducts Barrier and conduit CHAPTER 5 Large ducts of exocrine glands Anorectal junction Stratified columnar Largest ducts of exocrine glands Barrier and conduit Anorectal junction Transitional Renal calyces Barrier, distensible property (urothelium) Ureters Bladder Urethra Pawlina_CH05.indd 108 9/29/14 6:43 PM  FOLDER 5.1 Clinical Correlation: Epithelial Metaplasia 109 Epithelial metaplasia is a reversible conversion of one return to their normal pattern of differentiation. If abnor- CHAPTER 5 mature epithelial cell type to another mature epithelial mal stimuli persist for a long time, squamous metaplas- cell type. Metaplasia is generally an adaptive response to tic cells may transform into squamous cell carcinoma. stress, chronic inflammation, or other abnormal stimuli. Cancers of the lung, cervix, and bladder often originate The original cells are substituted by cells that are better from squamous metaplastic epithelium. Squamous suited to the new environment and more resistant to columnar epithelium may give rise to glandular the effects of abnormal stimuli. Metaplasia results from adenocarcinomas. reprogramming of epithelial stem cells that changes the When metaplasia is diagnosed, all efforts should Epithelial Tissue patterns of their gene expression. be directed toward removing the pathogenic stimulus The most common epithelial metaplasia is columnar- (i.e., cessation of smoking, eradication of infectious to-squamous and occurs in the glandular epithelium, agents, etc.) and monitoring the metaplastic site to ensure where the columnar cells become replaced by the stratified that cancerous changes do not begin to develop. squamous epithelium. For example, squamous metapla- sia frequently occurs in the pseudostratified respiratory epi- thelium 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 example, simple columnar epithelium of the cervical canal is replaced by the stratified squamous nonkeratinized epithelium (Fig. F5.1.1). In addition, squamous metaplasia is noticeable in the urothelium (transitional epithelium) and is associated T H E A P I C A L D O MA I N A N D I T S MO DI FI C ATI ON S with chronic parasitic infections such as schistosomiasis. Squamous-to-columnar epithelial metaplasia may also occur. For example, as a result of gastroesophageal FIGURE F5.1.1 ▲ Squamous metaplasia of the uterine cervix. Photomicrograph of a cervical canal lined by simple columnar reflux (Barrett’s esophagus), the stratified squamous non- epithelium. Note that the center of the image is occupied by an island keratinized epithelium of the lower part of the esophagus containing squamous stratified epithelium. This metaplastic epithe- can undergo metaplastic transformation into an intestinal- lium is surrounded on both sides by simple columnar epithelium. like simple columnar epithelium containing goblet cells. Since metaplasia is triggered by reprogramming of stem cells, meta- Metaplasia is usually a reversible phenomenon, and if plastic squamous cells have the same characteristics as normal strati- the stimulus that caused metaplasia is removed, tissues fied squamous epithelium. ⫻240. (Courtesy of Dr. Fabiola Medeiros.) stereocilia (stereovilli), microvilli of unusual length; and based on light microscope observations, any microvilli pres- cilia, cytoplasmic processes containing bundle of micro- ent are usually short and not numerous, which explains why tubules. they may escape detection in the light microscope. The varia- tions seen in microvilli of various types of epithelia are shown Microvilli in Figure 5.2. The microvilli of the intestinal epithelium Microvilli are finger-like cytoplasmic projections on the (striated border) are the most highly ordered and are even apical surface of most epithelial cells. more uniform in appearance than those that constitute the brush border of kidney cells. As observed with the electron microscope (EM), microvilli vary widely in appearance. In some cell types, microvilli are The internal structure of microvilli contains a core of actin short, irregular, bleb-like projections. In other cell types, filaments that are cross-linked by several actin-bundling they are tall, closely packed, uniform projections that greatly proteins. increase the free cell surface area. In general, the number and Microvilli contain a conspicuous core of about 20 to 30 shape of the microvilli of a given cell type correlate with the actin filaments. Their barbed (plus) ends are anchored to cell’s absorptive capacity. Thus, cells that principally trans- villin, a 95 kDa actin-bundling protein located at the tip of port fluid and absorb metabolites have many closely packed, the microvillus. The actin bundle extends down into the api- tall microvilli. Cells in which transepithelial transport is less cal cytoplasm. Here it interacts with a horizontal network of active have smaller, more irregularly shaped microvilli. actin filaments, the terminal web, which lies just below the Among the fluid-transporting epithelia (e.g., those of the base of the microvilli (Fig. 5.3a). The actin filaments inside intestine and kidney tubules), a distinctive border of verti- the microvillus are cross-linked at 10-nm intervals by other cal striations at the apical surface of the cell, representing actin-bundling proteins such as fascin (57 kDa), espin an astonishing number of 15,000 close-packed microvilli, is (30 kDa), and fimbrin (68 kDa). This cross-linkage provides easily seen in the light microscope. In intestinal absorptive support and gives rigidity to the microvilli. In addition, the cells, this surface structure was originally called the striated core of actin filaments is associated with myosin I, a mol- border; in the kidney tubule cells, it is called the brush ecule that binds the actin filaments to the plasma membrane border. Where there is no apparent surface modification of the microvillus. The addition of villin to epithelial cells Pawlina_CH05.indd 109 9/29/14 6:43 PM  presence of myosin II and tropomyosin in the terminal web explains its contractile ability; these proteins decrease the 110 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 intermicrovillus space. T H E A P I C A L D O M A I N A N D I T S M O D I F I C AT I O N S The functional and structural features of microvilli are summarized in Table 5.2. Stereocilia 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 sys- tem, and the sensory (hair) cells of the inner ear. They a are included in this section because this unusual surface modification is traditionally treated as a separate structural entity. Stereocilia of the genital ducts are extremely long pro- cesses 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 elec- tron 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 paintbrush because of the way they aggregate into pointed bundles. Epithelial Tissue Like microvilli, stereocilia are supported by internal bundles of actin filaments that are cross-linked by fimbrin. The actin filaments’ barbed (plus) ends are oriented toward the tips of the stereocilia and the pointed (minus) ends at the base. This b organization of actin core shares many construction principles with the microvilli, yet it can be as long as 120 ␮m. Stereocilia develop from microvilli by the lateral addition of actin filaments to the actin bundle as well as by elongation of the actin filaments. Unlike microvilli, an 80 kDa actin- binding protein, ezrin, closely associated with the plasma membrane of stereocilia, anchors the actin filaments to the CHAPTER 5 plasma membrane. The stem portion of the stereocilium and the apical cell protrusion contain the cross-bridge–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. Stereocilia of the sensory epithelium of the ear also derive from microvilli. They are exquisitely sensitive to c mechanical vibration and serve as sensory mechanore- ceptors rather than absorptive structures. They are uniform FIGURE 5.2 ▲ Electron micrographs showing variation in microvilli of different cell types. a. Epithelial cell of uterine gland; in diameter and organized into ridged bundles of increasing small projections. b. Syncytiotrophoblast of placenta; irregular, branching heights, forming characteristic staircase patterns (Fig. 5.5a). microvilli. c. Intestinal absorptive cell; uniform, numerous, and regularly Their internal structure is characterized by the high density of arranged microvilli. All figures ⫻20,000. actin filaments extensively cross-linked by espin, which is critical to normal structure and function of stereocilia. growing in cultures induces formation of microvilli on the Stereocilia of sensory epithelia lack both ezrin and ␣-actinin. free apical surface. Since stereocilia can be easily damaged by overstimulation, The terminal web is composed of actin filaments they have a molecular mechanism to continuously renew stabilized by spectrin (468 kDa), which also anchors the their structure, which needs to be maintained in proper terminal web to the apical cell membrane (Fig. 5.3b). The working condition for a lifetime. Using fluorescent-labeled Pawlina_CH05.indd 110 9/29/14 6:43 PM  111 villin espin CHAPTER 5 fimbrin actin filament fascin Epithelial Tissue myosin I spectrin terminal web myosin II T H E A P I C A L D O MA I N A N D I T S MO DI FI C ATI ON S intermediate a filaments b b 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. actin molecules, researchers found that actin monomers are domain of many epithelial cells. Motile cilia and their being constantly added at the tips and removed at the base counterparts, flagella, possess a typical 9 ⫹ 2 axone- of the stereocilia while the entire bundle of actin filaments mal organization with microtubule-associated motor moves toward the base of the stereocilium (Fig. 5.5b and c). proteins that are necessary for the generation of forces This treadmilling effect of the actin core structure is highly needed to induce motility. regulated and depends on the length of the stereocilium. Primary cilia (monocilia) are solitary projections The functional and structural features of stereocilia in com- found on almost all eukaryotic cells. The term monocilia parison to microvilli and cilia are summarized in Table 5.2. implies that only a single cilium per cell is usually pres- ent. Primary cilia are immotile because of different ar- Cilia rangements of microtubules in the axoneme and lack of Cilia are common surface modifications present on nearly microtubule-associated motor proteins. They function as every cell in the body. They are hair-like extensions of the api- chemosensors, osmosensors, and mechanosen- cal plasma membrane containing an axoneme, the microtu- sors, and they mediate light sensation, odorant, and bule-based internal structure. The axoneme extends from the sound perception in multiple organs in the body. It is now basal body, a centriole-derived, microtubule-organizing widely accepted that primary cilia of cells in developing center (MTOC) located in the apical region of a ciliated cell. tissues are essential for normal tissue morphogenesis. The basal bodies are associated with several accessory struc- Nodal cilia are found in the embryo on the bilaminar tures that assist them with anchoring into cell cytoplasm. embryonic disc at the time of gastrulation. They are Cilia, including basal bodies and basal body–associated struc- concentrated in the area that surrounds the primitive tures, form the ciliary apparatus of the cell. node, hence their name nodal cilia. They have a similar axonemal internal architecture as primary cilia; however, In general, cilia are classified as motile, primary, or nodal. they are distinct in their ability to perform rotational Based on their functional characteristics, cilia are classified movement. They play an important role in early embry- into three basic categories: onic development. Motile cilia have been historically the most studied of The functional and structural features of all three types of all cilia. They are found in large numbers on the apical cilia are summarized in Table 5.2. Pawlina_CH05.indd 111 9/29/14 6:43 PM  ezrin 112 fimbrin T H E A P I C A L D O M A I N A N D I T S M O D I F I C AT I O N S actin filament espin cytoplasmic bridge actin filaments Epithelial Tissue ␣-actinin b 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. Motile cilia are capable of moving fluid and particles along Motile cilia contain an axoneme, which represents an orga- CHAPTER 5 epithelial surfaces. nized core of microtubules arranged in a 9 ⫹ 2 pattern. Motile cilia possess an internal structure that allows them to Electron microscopy of a cilium in longitudinal profile re- move. In most ciliated epithelia, such as the trachea, bronchi, veals an internal core of microtubules called axoneme or oviducts, cells may have as many as several hundred cilia (Fig. 5.7a). A cross-sectional view reveals a characteristic con- arranged in orderly rows. In the tracheobronchial tree, the figuration of nine pairs or doublets of circularly arranged mi- cilia sweep mucus and trapped particulate material toward crotubules surrounding two central microtubules (Fig. 5.7b). the oropharynx where it is swallowed with saliva and elimi- The microtubules composing each doublet are constructed nated from the body. In the oviducts, cilia help transport ova so that the wall of one microtubule, designated the B micro- and fluid toward the uterus. tubule, is actually incomplete; it shares a portion of the wall of the other microtubule of the doublet, the A microtubule. Cilia give a “crew-cut” appearance to the epithelial surface. The A microtubule is composed of 13 tubulin protofila- In the light microscope, motile cilia appear as short, fine, ments, arranged in side-by-side configuration, whereas the hair-like structures, approximately 0.25 ␮m in diameter and B microtubule is composed of 10 tubulin protofilaments. 5 to 10 ␮m in length, that emanate from the free surface of Tubulin molecules incorporated into ciliary microtubules the cell (Fig. 5.6). A thin, dark-staining band is usually seen are tightly bound together and posttranslationally modified extending across the cell at the base of the cilia. This dark- in the process of acetylation and polyglutamylation. Such staining band represents structures known as basal bodies. modifications ensure that microtubules of ciliary axoneme These structures take up stain and appear as a continuous are highly stable and resist depolymerization. band when viewed in the light microscope. When viewed When seen in cross-section at high resolution, each dou- with the EM, however, the basal body of each cilium appears blet exhibits a pair of “arms” that contain ciliary dynein, as a distinct individual structure. a microtubule-associated motor protein. This motor protein Pawlina_CH05.indd 112 9/29/14 6:43 PM  (ⴙ end) polymerization and 113 crosslinking CHAPTER 5 Epithelial Tissue treadmilling T H E A P I C A L D O MA I N A N D I T S MO DI FI C ATI ON S disassembly and depolymerization espin-GFP actin-GFP (ⴚ end) a b c FIGURE 5.5 ▲ Dynamic turnover of an internal architecture of stereocilia. 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 mi- croscopy 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. 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 filament 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.) uses the energy of adenosine triphosphate (ATP) hydrolysis microtubules at 29-nm intervals. The proteins forming the to move along the surface of the adjacent microtubule (see radial spokes and the nexin connections between the outer Fig. 5.7). The dynein arms occur at 24-nm intervals along doublets make large-amplitude oscillations of the cilia the length of the A microtubule and extend out to form possible. temporary cross-bridges with the B microtubule of the adja- cent doublet. A passive elastic component formed by nexin Basal bodies and basal body–associated structures firmly (165 kDa) permanently links the A microtubule with the B anchor cilia in the apical cell cytoplasm. microtubule of adjacent doublets at 86-nm intervals. The The 9 ⫹ 2 microtubule array courses from the tip of two central microtubules are separate but partially en- the cilium to its base, whereas the outer paired microtu- closed by a central sheath projection at 14-nm intervals bules join the basal body. The basal body is a modified along the length of the cilium (see Fig. 5.7). Radial spokes centriole. It functions as an MTOC consisting of nine short extend from each of the nine doublets toward the two central microtubule triplets arranged in a ring. Each of the paired Pawlina_CH05.indd 113 9/29/14 6:43 PM  Cilia movement originates from the sliding of microtubule BB C BB doublets, which is generated by the ATPase activity of the 114 dynein arms. Ciliary activity is based on the movement of the doublet micro- T H E A P I C A L D O M A I N A N D I T S M O D I F I C AT I O N S tubules 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 ex- hibits 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. However, if all dynein arms along the length of the A mi- FIGURE 5.6 ▲ Ciliated epithelium. Photomicrograph of an H&E– crotubules in all nine doublets attempted to form temporary stained specimen of tracheal pseudostratified ciliated epithelium. The cilia cross-bridges simultaneously, no effective stroke of the cilium (C) appear as hair-like processes extending from the apical surface of the cells. The dark line immediately below the ciliary processes is produced by would result. Thus, regulation of the active shear force is the basal bodies (BB) associated with the cilia. ⫻750. required. Current evidence suggests that the central pair of mi- crotubules in 9 ⫹ 2 cilia undergo rotation with respect to the nine outer doublets. This rotation may be driven by another microtubules of the ciliary axoneme (A and B microtubules) motor protein, kinesin, which is associated with the central is continuous with two of the triplet microtubules of the basal pair of microtubules. The central microtubule pair can act as body. The third incomplete C microtubule in the triplet a “distributor” that progressively regulates the sequence of in- extends from the bottom to the transitional zone at the Epithelial Tissue teractions of the dynein arms to produce the effective stroke. top of the basal body near the transition between the basal body and the axoneme. The two central microtubules of Cilia beat in a synchronous pattern. the cilium originate at the transitional zone and extend to the Motile cilia with a 9 ⫹ 2 pattern display a precise and syn- top of axoneme (see Fig. 5.7b). Therefore, a cross-section of chronous undulating movement. Cilia in successive rows start the basal body would reveal nine circularly arranged micro- their beat so that each row is slightly more advanced in its tubule triplets but not the two central single microtubules of cycle than the following row, thus creating a wave that sweeps the cilium. across the epithelium. As previously discussed, basal feet of Basal bodies are associated with several basal body– basal bodies are most likely responsible for synchronization of associated structures such as alar sheets (transitional fibers), ciliary movement. During the process of cilia formation, all basal feet, and striated rootlets (see Figs. 5.7 and Fig. 5.8). basal feet become oriented in the same direction of effective CHAPTER 5 The alar sheet (transitional fiber) is a collar-like stroke by rotating basal bodies. This orientation allows cilia to extension between the transitional zone of basal body achieve a metachronal rhythm that is responsible for mov- and plasma membrane. It originates near the top end ing mucus over epithelial surfaces or facilitating the flow of of the basal body C microtubule and inserts into the fluid and other substances through tubular organs and ducts. cytoplasmic domain of the plasma membrane. It teth- Primary cilia are nonmotile and contain a 9 ⫹ 0 pattern of ers the basal body to the apical plasma membrane (see microtubules. Fig. 5.7). Differing from motile cilia with the 9 ⫹ 2 pattern of The basal foot is an accessory structure that is usually microtubules is another type of cilia that display a 9 ⫹ 0 found in the midregion of the basal body (see Fig. 5.8). microtubule arrangement. Cilia with this pattern have Since in the typical epithelial ciliated cells all basal feet the following characteristics: are oriented in the same direction (Fig. 5.9), it has been hypothesized that they function in coordinating ciliary They are nonmotile and passively bend by the flow of the movement. They are most likely involved in adjusting fluid. basal bodies by rotating them to the desired position. Lo- They lack microtubule-associated motor proteins needed calization of myosin molecules associated with basal feet to generate motile force. supports this hypothesis. The central pair of microtubules is missing. The striated rootlet is composed of longitudinally The axoneme originates from a basal body that resembles aligned protofilaments containing rootletin (a 220 kDa a mature centriole positioned orthogonally in relation to protein). Striated rootlet projects deep into cytoplasm its immature counterpart. and firmly anchors the basal body within the apical cell Primary cilium formation is synchronized with cell cycle cytoplasm (see Fig. 5.8). progression and centrosome duplication events. Pawlina_CH05.indd 114 9/29/14 6:43 PM  115 CHAPTER 5 b microtubule doublet Epithelial Tissue central sheath projections central pair of microtubules radial spoke T H E A P I C A L D O MA I N A N D I T S MO DI FI C ATI ON S 9 8 7 6 10 5 B 4 nexin 11 12 13 3 10 1 1 2 dynein “arms” 9 8 A 3 2 7 6 54 transitional tubulin zone subunits alar sheet a basal foot basal body microtubule triplet 13 10 10 A B C microtubule striated rootlet protofilaments 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 configu- ration). 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 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. Pawlina_CH05.indd 115 9/29/14 6:43 PM 116 BB T H E A P I C A L D O M A I N A N D I T S M O D I F I C AT I O N S BF BF Ax SR TZ BB BF SR FIGURE 5.8 ▲ Ciliated surface of the respiratory mucosa. Electron micrograph shows a longitudinally sectioned cilia from a respiratory epi- thelium 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 axoneme (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 wing-like 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.) Epithelial Tissue 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 C 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 CHAPTER 5 vestibular hair cells of the ear. Primary cilia were formerly clas- sified as nonfunctional vestigial developmental abnormalities of 9 ⫹ 2 motile cilia. Experimental studies of the last decade Mv elevated the status of primary cilia to the level of important cel- lular-signaling devices functioning comparably to an antenna BF 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 BB receive chemical, osmotic, light, and mechanical stimuli from the extracellular environment. In response to these stimuli, pri- mary cilia generate signals that are transmitted into the cell to FIGURE 5.9 ▲ Basal bodies and cilia. This diagnostic electron micrograph obtained during biopsy of the nasal mucosa from a child modify cellular processes in response to changes in the external undergoing evaluation for primary ciliary dyskinesia shows a normal environment. In many mammalian cells, signaling through the appearance of basal bodies (BB) and cilia (C ). It is an oblique section primary cilia seems to be essential for controlled cell division through the apical part of ciliated cells. Basal bodies seen in cross sec- and subsequent gene expression. tion appear as more dense structures than sectioned oblique and lon- gitudinal profiles of the cilia above. Several profiles of microvilli (Mv) are Primary cilia containing a 9 ⫹ 0 pattern of microtubules visible at the apical cell surface. ⫻11,000. Inset. Three basal bodies sec- function as signal receptors sensing a flow of fluid in tioned at the level of basal feet (BF ). Note that all basal feet are oriented developing organs. 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 Primary cilia function in secretory organs such as the kidneys, Patrice C. Abell Aleff.) liver, or pancreas as sensors of fluid flow. They extend from the Pawlina_CH05.indd 116 9/29/14 6:43 PM  117 CHAPTER 5 Epithelial Tissue a b 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 fr