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Epithelium and Exocrine Glands
2.1
Overview
Epithelium is one of the four basic tissues, with a wide distribution and many functions. It consists of continuous sheets of cells that cover exposed body surfaces. It also lines internal cavities, such as those of the digestive, respiratory, cardiovascular, and genitourinary systems. During embryonic development, epithelium invaginates into underlying tissues to proliferate and form secretory glands. Its two classes are thus covering and lining and glandular. A selective barrier that protects other tissues, it transports material along its surface uni- or bidirectionally. Other functions include synthesis, secretion, absorption, and, because cells are exposed on free surfaces, sensory reception. Epithelium is made almost entirely of contiguous and adhesive cells bound together by intercellular junctions and a small amount of extracellular matrix. Epithelium has tissue polarity and an apical (or free) surface, lateral surfaces between adjacent cells, and a basal surface in contact with an underlying basement membrane. Epithelia lack a direct blood supply and are fed via diffusion from underlying tissues. Unlike other basic tissues, epithelia have a high mitotic index with constant cell renewal—an advantage because cells undergo mechanical stress and trauma. However, they are susceptible to formation of malignant tumors called carcinomas. Epithelia have diverse embryonic origins and may come from ectoderm, mesoderm, or endoderm. Covering and lining epithelium is classed histologically according to the shape of the surface cells— squamous, cuboidal, columnar, pseudostratified, or transitional —and the number of cell layers— simple or stratified.
Clinical Point
Epithelial cells have a high mitotic index and are exposed to the surface, which gives pathogens and carcinogens free access to them. The most common types of cancerous (or malignant ) tumors (or neoplasms ) in adults originate from epithelial cells; these tumors invade or metastasize to distant tissues and organs. Neoplasms that grow slowly are benign tumors and include papillomas, which arise from surface epithelium, and adenomas, which originate from glandular epithelium. Malignant neoplasms of surface epithelium are carcinomas, and those originating from glandular epithelium are adenocarcinomas. Knowing a tumor's histologic characteristics often helps determine diagnosis, staging, and treatment.
2.2
Structure and Function of Simple Squamous Epithelium
Simple squamous epithelium consists of a single layer of flattened cells usually joined by intercellular junctions and resting on a basement membrane whose thickness depends on location. The cells, shaped like scales (Latin squama ), are best seen in a surface view and have irregular, serrated outlines that fit together like pieces of a jigsaw puzzle. One nucleus is in the widest part of each cell, so a local bulge protrudes into the free surface. In a plane perpendicular to the surface, cells look like spindles with tapering ends on both sides of the nucleus. Cell borders are hard to see in hematoxylin and eosin (H&E) sections, but special techniques and electron microscopy can elucidate them. This type of epithelium is typical at sites that make up blood-tissue barriers. Thinness of the epithelium also permits diffusion and bidirectional movement of gases, fluids, and nutrients from the free surface to underlying tissues. Names of this epithelium depend on location: Simple squamous epithelium of the lining of the heart, blood vessels, and lymphatic channels is an endothelium; mesothelium consists of simple squamous cells forming serous membranes lining internal body cavities. This distinction is important to pathologists because cells behave differently in inflammation and tumor formation. Simple squamous epithelium in the kidney constitutes the parietal layer of Bowman capsule and thin loops of Henle; it is also found in the middle and inner ear, and in lungs where it lines pulmonary alveoli.
Clinical Point
Rare, aggressive tumors called malignant mesotheliomas may arise from parietal and visceral serous membranes of pleural, peritoneal, and pericardial cavities. Pleural mesothelioma —the most common—is usually caused by occupational exposure to asbestos. It has a long latency time (25-40 years) from first contact to onset of symptoms, which include shortness of breath, chest pain, and pleural fluid accumulation. Magnetic resonance imaging, positron emission tomography, needle biopsy, and electron microscopy are useful for diagnosis. The often poor prognosis is due to a tendency to metastasize to lymph nodes and other organs. Surgery, radiation, and chemotherapy can help in some cases.
2.3
Ultrastructure and Function of Simple Squamous Epithelium
Ultrastructure of simple squamous epithelial cells reflects their functional diversity. The rich, varied organelle content of the cytoplasm indicates high metabolic activity, active synthesis and secretion, and selective permeability. Cells usually have a complex cytoskeleton to maintain shape and provide internal scaffolding to resist pressure changes and wear and tear. The cytoskeleton consists of a network of intermediate filaments (tonofilaments), which are interwoven in each cell. Prominent actin-containing thin filaments (microfilaments) and motor proteins allow changes in cell shape and provide pliability. This epithelium is a metabolically active diffusion barrier at many sites, with striking features: smooth-surfaced transcytotic vesicles (first termed pinocytotic vesicles) and clathrin-coated endocytotic vesicles, which participate in transepithelial transport. The many intercellular junctions include desmosomes and intermediate junctions, which anchor cells together, and tight junctions, which act as a permeability barrier to indiscriminate passage of material. Gap junctions also allow ionic and metabolic communication between cells.
2.4
Structure and Function of Simple Cuboidal Epithelium
Simple cuboidal epithelium consists of one layer of cells whose height roughly equals their width, so in sections perpendicular to the surface, cells resemble small box-like cubes. Cells in horizontal section appear to be a mosaic of polygonal tiles. As in other epithelia, cells rest on a basement membrane that firmly attaches to underlying connective tissue. Each cell has one spherical, centrally placed nucleus. This epithelium provides protection, forms conduits for gland ducts, and may be specialized for active secretion and absorption. On the surface of the ovary, it is ovarian surface epithelium. It also lines renal tubules and small collecting ducts of the kidney, which engage in ion transport. The thyroid—an endocrine gland—contains spherical follicles of these cuboidal cells. The parenchyma of most exocrine glands, such as salivary glands and pancreas, consists of cuboidal to columnar epithelial cells in grape-like clusters called acini. In the eye, cells of pigmented epithelium of the retina and epithelium of the ciliary body are simple cuboidal and specialized for ion transport and secretion. Free surfaces of these cuboidal cells often have microvilli, which are best seen by electron microscopy. Their cytoplasm has more organelles than that of simple squamous epithelial cells, with more mitochondria and endoplasmic reticulum, which are evidence of high metabolic and functional activities.
2.5
Structure and Function of Simple Columnar Epithelium
Simple columnar epithelium consists of one layer of cells that are taller than they are wide and look like closely packed, slender columns. Bases of cells rest on a basement membrane; apical surfaces contact a lumen. The ovoid nucleus is centrally or basally placed. This epithelium, widely distributed in the body, is mainly found in sites engaged in protection of wet surfaces, nutrient absorption, and secretion. It forms major ducts of glands, convoluted tubules of the kidney, and inner lining of the stomach, small and large intestines, gallbladder, small bronchi of the lungs, and parts of the male and female (oviducts and uterus) reproductive tracts. Free surfaces of cells often bear microvilli —thin, finger-like cellular projections—for increased surface area. When microvilli are large (1-2 µm high), uniform in size, and closely packed, they form a striated border. Lateral cell borders have junctional complexes, which include an apical tight junction, interme-diate (adherens) junction, and desmosome. The cytoplasm is packed with many organelles. At certain sites, the epithelium may consist of more than one type of cell, with mucus-secreting goblet cells being common. In some areas, the epithelium may have cilia.
2.6
Structure and Function of Pseudostratified Epithelium
Pseudostratified epithelium consists of more than one type of epithelial cell, of varied size and shape. In sections perpendicular to the surface, the nuclei usually appear at different levels, so two or three layers of crowded nuclei are seen. A basal layer belongs to replacement (stem) cells with mitotic potential for regeneration. More apical layers contain elongated nuclei of tall columnar cells, many of which may have cilia on their free surfaces. All cells contact an underlying basement membrane, but only some reach the free surface and do not penetrate the whole thickness of the epithelium. These features give the epithelium a false impression of stratification—thus, its name. More aptly a type of simple epithelium, it lines many parts of the upper respiratory tract (nasal cavities, auditory tube, nasopharynx, larynx, trachea, and large bronchi). Mucous goblet cells usually occur in this epithelium, and where they mingle with ciliated columnar cells, the tissue is called respiratory epithelium. It acts as a mucociliary escalator to entrap and rid airways of foreign particles by sweeping, coordinated ciliary motion. Pseudostratified epithelium lacking goblet cells is also found in parts of the male reproductive tract, where some cells have apical nonmotile stereocilia and mainly lining, secretory, and absorptive roles are performed.
2.7
Structure and Function of Stratified Squamous Epithelium
Stratified squamous epithelium is a tough, resilient multilayered epithelium that mainly protects against abrasion and dehydration. It also prevents invasion of pathogens, bacteria, and other infectious agents. Its name derives from the shape of the outer layer of flattened cells. Two types exist— keratinized and nonkeratinized. In areas exposed to air and subject to abrasion, such as epidermis of skin, the surface layer consists of dead cells lacking nuclei and containing plates of the protein keratin, which strengthens and waterproofs the tissue. This keratinized stratified squamous epithelium, with a dry, scale-like surface, also lines the outer surface of the tympanic membrane, parts of the oral cavity (gingiva and hard palate), and some mucocutaneous junctions (lips and distal anal canal). In other areas covered with fluid and with a moist surface, superficial squamous cells retain nuclei and lack keratin. This nonkeratinized stratified squamous epithelium lines most of the oral cavity, pharynx, epiglottis, vocal cords, esophagus, anal canal, vagina, parts of the male and female urethra, and cornea. Secretions from closely associated glands lubricate the surface of this epithelium.
Clinical Point
After injury by trauma or infection to skin and other soft tissues, the capacity of epithelial cells to undergo mitosis and regenerate is clinically important. Complex reparative events known as wound healing include an inflammatory phase followed by proliferative and remodeling stages. Epithelial cells from nearby areas replicate, change shape, and migrate across the defect to cover the wound. Cells secrete various growth factors and activators that enhance repair. Angiogenesis, whereby new capillaries grow from endothelial cells, also occurs. The basement membrane is critical for rapid recovery; if it is destroyed, healing is relatively slow.
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