INTRODUCTION
Cells, Tissues, and Organs
Epithelium is a tissue. A tissue is an aggregation of cells and intercellular substances specialized to perform particular functions. Despite its structural and functional complexity, the animal body is composed of only four basic types of tissue: epithelium, connective tissue, muscle, and nervous tissue (Table 2-1). Organs consist of various arrangements of the four basic tissues (Fig. 2-1).
Characteristics of Epithelium
Epithelium exists in two major forms: surface epithelium and glandular epithelium. Surface epithelium consists of sheets of aggregated cells of similar type that cover all of the external surfaces and line all of the internal surfaces of the body. Glandular epithelium, the secretory cells of endocrine and exocrine glands, results from the proliferation of surface epithelial cells into underlying connective tissue. Table 2-2 summarizes the main characteristics of epithelium as a tissue.
The characteristics of the cells of epithelium include specialization for a variety of different functions, including protection, absorption, secretion, excretion, and formation of barriers for selective permeability. The location of cellular organelles and variations in morphologic features of the luminal, basal, and lateral portions of the cell membrane indicate a definite polarization of epithelial cells. In mammals, essentially all epithelial cells contain cytoplasmic filaments composed of the protein cytokeratin, an exception being endothelial cells. Cells not of epithelial origin (e.g., mesenchymal cells) lack cytokeratin. This characteristic is used in diagnostic pathology to identify the cell of origin of malignant neoplasms (cancers), which consist of cells that are unrecognizable by routine histologic methods. In addition to functional diversity and specific structural characteristics, epithelial cells continuously proliferate via mitosis to replace cells lost through attrition.
At the basal surface of all epithelial cells that make contact with underlying connective tissue, a thin sheet of extracellular matrix, the basement membrane, is present. The basement membrane usually is not visible in routine light-microscopic sections, but can be demonstrated with the periodic acid-Schiff (PAS) technique or silver stains. As seen with the electron microscope, the basement membrane invariably consists of two distinct layers: the lamina lucida, a low-density layer next to the epithelial cell membrane, and an underlying electron-dense lamina densa (lamina basalis or basal lamina) (Fig. 16-9). These two laminae are synthesized by the epithelial cells and are composed principally of proteoglycans (primarily heparan sulfate) as well as laminin, fibronectin, and type IV collagen. In most basement membranes, a third component, the lamina fibroreticularis (subbasal lamina), is present (Fig. 16-9). The lamina fi-broreticularis is composed principally of reticular fibers (type III collagen) and connects the lamina densa to the subepithelial connective tissue. In addition to providing attachment for the epithelium, the lamina fibroreticularis permits stretching and recoil of the epithelium in distensible organs. The basement membrane is described in greater detail in Chapter 16.
TABLE 2-1 The Basic Tissues
| Epithelium |
| Connective tissue |
| Muscle |
| Nervous tissue |
In addition to underlying all epithelia that contact connective tissue, a basement membrane is found between two epithelial layers in both the renal corpuscle of the kidney and the pulmonary alveolus. In these locations, the lamina fibroreticularis is absent and the structure is referred to simply as the basal lamina. A basal lamina also surrounds many individual cells including smooth muscle cells, skeletal muscle cells, and cardiac muscle cells, as well as adipocytes and neurolemmocytes.
FIGURE 2-1 Structural organization of the animal body.
TABLE 2-2 Characteristics of Epithelium
| Covers body surfaces (surface epithelium) |
| Forms secretory cells of glands (glandular epithelium) |
| Cells are functionally diverse |
| Cells contain cytoplasmic cytokeratin filaments |
| Cells are capable of mitosis (regeneration) |
| Contacts a basement membrane |
| Devoid of blood vessels (avascular) |
| Derived from each of the three embryonic germ layers |
The basement membrane serves in a variety of capacities, for example, as an ultrafilter in capillaries, particularly those of the renal corpuscle; as a selective barrier to exchange of macromole-cules; and as a guide to epithelial cell movements. Since blood and lymph vessels do not penetrate the basement membrane, overlying epithelial cells receive their nutritional support by diffusion through the basement membrane from capillaries located in the underlying connective tissue.
All three embryonic germ layers take part in the formation of epithelium. Ectoderm is the origin of the epithelium of the external body surfaces, such as the epidermis of the skin. Most of the lining epithelium (i.e., that lining the luminal surfaces of the digestive and respiratory systems) originates from endoderm, while mesoderm gives rise to the lining of the vascular system, the lining of the serous membrane body cavities, and parts of the urinary and reproductive systems.
SURFACE EPITHELIUM
Classification
The classification of the various surface epithelial types is based on the number of layers present and the shape of the epithelial cells. A surface epithelium consisting of a single layer of cells resting on the basement membrane is described as a simple epithelium. A stratified epithelium is composed of two or more layers of cells with only the basal layer resting on the basement membrane. The names given to the various types of stratified epithelia are based on the shape of the surface cells without regard to the shape of those within the deeper layers. A surface epithelium is considered pseudostratified if all of its cells contact the basement membrane but not all extend to the free surface. As a result, nuclei are located at different levels within a pseudostratified epithelium, giving the false impression of stratification. Figure 2-2 depicts the classification of epithelial tissue.
Microscopic Structure
Simple Squamous Epithelium
Simple squamous epithelium consists of a single layer of thin, flat, scalelike cells. On surface view (Figs. 2-3A and 2-4), the cells have an irregular shape with a slightly serrated border. They fit together to form a continuous sheet. A spherical to oval nucleus is located near the center of the cell. On cross section, the cell appears thicker in the area of the nucleus and has thin attenuated strands of cytoplasm on either side (Fig. 2-4).
FIGURE 2-2 Classification of the various forms of epithelial tissue.
Simple squamous epithelium lines moist internal surfaces, such as the serous membrane body cavities, the internal surface of the heart, and the luminal surface of blood and lymph vessels. The simple squamous epithelium lining the serous membrane body cavities (pleural, pericardial, and peritoneal cavities) is referred to as mesothelium; that lining the heart, blood vessels, and lymph vessels is referred to as endothelium. Simple squamous epithelium is also found lining the pulmonary alveoli, the anterior chamber of the eye, the internal surface of the tympanic membrane, the membranous labyrinth of the internal ear, the glomerular capsule, and a portion of the loop of the nephron.
Simple Cuboidal Epithelium
Simple cuboidal epithelium is a single layer of cells whose width and height are approximately equal. These cells appear as squares in cross sections but are more hexagonal when seen from the surface (Figs. 2-3B and 2-5). When the height is slightly less than the width of a cell, it is known as low cuboidal epithelium, and when the height is slightly greater than the width, the epithelium is called tall cuboidal epithelium. The classification of epithelia is not always clear-cut, however, and many times intermediate forms require some subjective judgment regarding classification.
Simple cuboidal epithelium can be found lining the ducts of many glands and the collecting ducts of the kidney, as a component of the choroid plexus in the brain and ciliary body of the eye, and lining the follicles of the thyroid gland. The epithelium of the lens of the eye and the retinal pigment epithelium are also examples of simple cuboidal epithelium.
Simple Columnar Epithelium
Simple columnar epithelium consists of tall, narrow cells with considerably greater height than width (Figs. 2-3C and 2-6). Usually, the nuclei are oval and are located near the base of the cell. Generally, simple columnar epithelium lines the luminal surface of organs that perform absorptive or secretory functions, for example, the glandular stomach, the small and large intestines, and the gallbladder in the digestive system; the bulbourethral gland in the male reproductive system; and the uterus and uterine tube in the female reproductive system.
Pseudostratified Epithelium
Pseudostratified epithelium exists in two forms: pseudostratified columnar epithelium and transitional epithelium. In both forms, all of the cells contact the underlying basement membrane, but not all of them reach the free surface.
FIGURE 2-3 Schematic drawings illustrating the major types of surface epithelia. A. Simple squamous epithelium. B. Simple cuboidal epithelium. C. Simple columnar epithelium bearing microvilli. D. Pseudostratified columnar epithelium with cilia and goblet cells. E. Transitional epithelium. F. Stratified squamous epithelium, nonkeratinized. G. Stratified cuboidal epithelium. H. Stratified columnar epithelium.
Pseudostratified columnar epithelium
Pseudostratified columnar epithelium is composed of a single layer of cells, but because the cells are irregular in size and shape, their nuclei are located at various levels. Therefore, the epithelium appears to have several layers (Figs. 2-3D and 2-7). The cells that extend from the basement membrane to the surface of pseudostratified columnar epithelium are ciliated or nonciliated epithelial cells and goblet cells (unicellular mucous glands). Basal cells are attached to the basement membrane but do not reach the surface of the epithelium. By division and differentiation, basal cells replace the other epithelial cell types lost by attrition.
Ciliated, pseudostratified columnar epithelium with goblet cells lines the greater part of the nasal cavity, paranasal sinuses, and nasopharynx, as well as the auditory tubes, the trachea, and the larger bronchi. In the respiratory system, the goblet cells contribute to a thin mucous film over the epithelium. Dust particles in the inhaled air become trapped in this mucus, and the current created by the ciliated cells moves the dust-laden mucus to the body openings. A pseudostratified columnar epithelium with stereocilia but lacking goblet cells is found lining the duct of the epididymis and the ductus deferens.
Transitional epithelium
Transitional epithelium, a pseudostratified type with a wide variety of appearances, lines hollow organs capable of considerable distention, such as the renal pelvis and calices, ureter, urinary bladder, and urethra. Transitional epithelium may also be seen in the palpebral conjunctiva, larynx, and nasopharynx. The cells increase in size from the basal “layers” to the superficial “layers” of transitional epithelium. The shape of the epithelial cells depends on the degree of organ distention at the time of fixation. When the epithelium is under little tension, the surface cells are large and “pillow-shaped,” whereas the deeper cells are smaller and irregularly shaped (Figs. 2-3E and 2-8). When the epithelium is stretched, the cells become flattened and elongated, and the total height of the epithelium decreases.
FIGURE 2-4 Simple squamous epithelium. A. Surface view of silver-impregnated mesothelium, mesentery. Silver stain (×500). B. Cross section of mesothelium, peritoneum on the surface of the urinary bladder. Hematoxylin and eosin (×1200). (B: from Stinson AW, Brown EM. Veterinary Histology Slide Sets. East Lansing, MI: Michigan State University, Instructional Media Center, 1970.)
The luminal surface of transitional epithelial cells appears relatively smooth with the light microscope. In electron micrographs, however, areas of thickened plasmalemma (plaques) anchored by numerous cytoplasmic filaments are seen on the luminal surface of the cell. The region between the membrane plaques is a normal cell membrane, and when the bladder contracts, the plaques fold together much like a hinge, producing typical transitional epithelial surface ridges. Upon distention, the plaques unfold, allowing expansion of the luminal surface.
FIGURE 2-5 Simple cuboidal epithelium, collecting duct, kidney (horse). Hematoxylin and eosin (×900).
FIGURE 2-6 Simple columnar epithelium, lining of gallbladder (pig). Hematoxylin and eosin (×900).
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