Fenestrated capillaries are typically found in endocrine glands and sites of fluid and metabolite absorption such as the gallbladder, kidney, and intestinal tract. They are characterized by fenestrations, 80 to 100 nm in diameter, that provide channels across the capillary wall (Fig. 13.22). 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. When viewed from the luminal surface, this diaphragm has a cartwheel-like shape with a central thickening and 14 wedge-shaped gaps. It is derived from 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 when no absorption is occurring. When absorption takes place, the walls thin, and the number of pinocytotic vesicles and fenestrations increases rapidly. The ionic changes in the perivascular connective tissue, caused by the absorbed solutes, stimulate pinocytosis. These observations support the suggested mode of formation of the fenestrations described above.
Discontinuous capillaries (also called sinusoidal capillaries or sinusoids) are typically found in the liver, spleen, and bone marrow. They are larger in diameter and more irregularly shaped than other capillaries. Structural features of these capillaries vary from organ to organ and include specialized cells. Kupffer cells (stellate sinusoidal macrophages) and vitamin A–storing Ito cells (hepatic stellate cells) in the liver occur in association with the endothelial cells. In the spleen, endothelial cells exhibit a unique spindle shape with gaps between the neighboring cells; the basal lamina underlying the endothelium may be partially or even completely absent.
Functional Aspects of Capillaries
To understand capillary function, two important points— vasomotion (i.e., capillary blood flow) and the extent or density of the capillary network—should be considered.
Blood flow is controlled through local and systemic signals. In response to vasodilating agents (e.g., NO, low O2 tension), the smooth muscle in the walls of the arterioles relaxes, resulting in vasodilation and increased blood ﬂow through the capillary system. Pressure within the capillaries increases, and much of the plasma ﬂuid is driven into the tissue. This process occurs in peripheral edema. Local endothelial-derived factors, systemic signals carried by the autonomic nervous system, and norepinephrine released by the adrenal gland cause the smooth muscle of the arterioles to contract (vasoconstriction), resulting in decreased blood ﬂow through the capillary bed. In this condition, capillary pressure can decrease and greatly increase absorption of tissue ﬂuid. This situation occurs during loss of blood volume and can add considerable amount of ﬂuid into the blood, preventing hypovolemic shock.
FIGURE 13.21 • 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. 30,000.