Electron micrograph and diagram of a continuous capillary. The endothelial cells that make up the wall of a continuous capillary contain numerous pinocytotic vesicles. The cell junctions are frequently marked by cytoplasmic (marginal) folds that protrude into the lumen. The endothelial cell nuclei are not included within the plane of section in the micrograph, but an endothelial cell with its nucleus is shown in the diagram. Similarly, the electron micrograph shows only a small amount of pericyte cytoplasm; a nucleus is not seen but is shown in the diagram (see the upper right and lower left of the micrograph). Note that the pericyte cytoplasm is enclosed by basal lamina, x30,000.
described: continuous capillaries, fenestrated capillaries, and discontinuous capillaries.
Continuous capillaries are typically found in muscle, lung, and the central nervous system (CNS). With the TEM, they appear in cross sections as two plasma membranes enclosing a ribbon of cytoplasm that may include the nucleus (Fig. 12.11). Occluding junctions can be seen in a typical cross section of a continuous capillary. Numerous pinocytotic vesicles underlie both the luminal and basal plasma membrane surfaces. The vesicles are approximately 70 nm in diameter and function in transport of materials between the lumen and the connective tissue and vice versa.
In some continuous capillaries and postcapillary venules, pericytes (Rouget cells) may be associated with the endothelium (Fig. 12.11). The pericyte, when present, intimately surrounds the capillary, with branching cytoplasmic processes, and is enclosed by a basal lamina that is continuous with that of the endothelium. The pericyte displays features of a relatively unspecialized cell with a large nucleus rich in heterochromatin. It is derived from the same precursor cell that forms endothelial cells in new vessels. It can also give rise to smooth muscle cells during vessel growth (as in development and wound healing).
Fenestrated capillaries are typically found in endocrine glands and sites of fluid and metabolite absorption, such as the gallbladder and intestinal tract. They are characterized by fenestrations, 80 to 100 nm in diameter, which provide channels across the capillary wall (Fig. 12.12). Fenestrated capillaries also have pinocytotic vesicles. Some research suggests that fenestrations are formed when a forming pinocytotic vesicle spans the narrow cytoplasmic layer and simultaneously opens on the opposite surface. A fenestration may have a thin, nonmembranous diaphragm across its opening. This diaphragm has a central thickening and may be the remnant of the glycocalyx formerly enclosed in the pinocytotic vesicle from which the fenestration may have formed.
Fenestrated capillaries in the gastrointestinal tract and gallbladder have fewer fenestrations and a thicker wall
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