Selected examples of epithelia characteristic of different organs are presented here and on the next several pages. For each example, note the shape and arrangement of the epithelial cells, the number of layers of cells, the location of their free surfaces, the shape of the cells at the free surface, and the location of the underlying or adjacent connective tissue. Remember that the morphologic characteristics of an epithelium are directly related to its functions in protection, secretion, absorption, or transport.
Figure 1, intestine, monkey, H&E X640.
This shows the simple squamous epithelium (mesothe-lium) covering the outer surface of the intestine. The epithelium overlies a well-defined layer of connective tissue (CT) containing several small blood vessels (BV); deeper is a layer of smooth muscle (SM). The epithelial cells are very flat, as judged by the shape of their nuclei (TV). Note that cell bound-
Figure 2, mesentery, monkey, silver X640.
The mesentery is lying on the slide, and the microscope is focused on its upper surface to reveal the surface epithelial cells. Cell boundaries are revealed by a deposition of reduced silver along the intercellular spaces. The nuclei (TV) appear oval or round when viewed from the surface, as opposed to flat or elongate when seen on edge, as in a section. If one were to draw a line representing a knife cut across the cells aries are not evident and the nuclei are unevenly spaced. The uneven spacing is because the sectioning knife passes through some cells without including the nucleus. This phenomenon can be understood better and visualized more easily in a nonsectioned (whole mount), silver-impregnated preparation of a piece of a very thin mesentery (the structure that holds the intestines in place), as in Figure 2.
as revealed in this figure, it would be possible to envision why the epithelial nuclei are unevenly spaced in sectioned material; the knife would pass through the cytoplasm of each cell but would not necessarily cut across the nucleus of each cell. Some of the more ovoid nuclei in the preparation belong to fibroblasts (F) in the underlying connective tissue. Because of the thinness of the mesentery, they are in the same focal plane as the epithelial cells and are thus superimposed.
Figure 3, kidney, human, H&E X640.
This micrograph, another example of a simple squamous epithelium, reveals a sectioned renal corpuscle and adjacent kidney tubules. The renal corpuscle consists of a special capillary bed, the glomerulus, that is enclosed by Bowman's capsule, part of which (the visceral layer) is directly adjacent to the capillaries and part of which (the parietal layer) forms a thin-walled spherical structure composed of simple squamous epithelium. The nuclei (N) of the cells forming the parietal layer appear as flattened bodies that show the same uneven spacing as in Figure 1. The free surface of the epithelium faces Bowman's space (B). The cross sections of tubules, marked with asterisks (lower left), provide a good example of a simple cuhoidal epithelium. Although the cell boundaries are not evident, one can judge that the height of each cell approximates its width by the spacing of the nuclei. Thus, it is a cuboidal epithelium.
Figure 4, ovary, monkey, H&E X640.
This example of a simple cuboidal epithelium (Ep) shows the cells that cover the surface of the ovary. The epithelium rests on a highly cellular connective tissue (CT). The surface epithelial cells are approximately square or cuboidal in three dimensions. The free surfaces of these cells face the abdominal cavity and are a modification of the simple squamous epithelium or mesothelium shown in Figures 1 and 2.
Figure 5, liver, human, H&E x400.
These cells also approximate a cube, but they are arranged in sheets separated by blood vessels (BV) called sinusoids. The epithelium is unusual, in that several surfaces of the cell possess a groove that represents the free surface. Where a grooved surface is present on one cell, the adjoining cell possesses a mirror-imaged grooved surface. The opposing grooves form a small lumen or canaliculus through which the bile produced by the cells reaches a bile duct. The canaliculi are not visible at this magnification but are located at the points of the arrows (see bile canaliculus, Plate 62).
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