Studies on Blood Capillaries

Abstract

The wall of the blood capillaries of skeletal muscles (diaphragm, tongue, hind legs) and myocardium of the rat, guinea pig, and hamster consists of three consecutive layers or tunics: the endothelium (inner layer), the basement membrane with its associated pericytes (middle layer), and the adventitia (outer layer). The flattened cells of the endothelium have a characteristic, large population of cytoplasmic vesicles which, within the attenuated periphery of the cells, may attain a maximum frequency of 1 20 /M 2 of cell front and occupy -18% of the cytoplasmic volume; these values decrease as the cells thicken toward the perikaryon. The vesicles are 650-750 A in over-all diameter and are bounded by typical unit membranes. They occur as single units or are fused to form short chains of two to three vesicles. Each configuration may lie entirely within the cytoplasm or open onto the cell surface. In the latter case, the unit membrane of the vesicle is continuous, layer by layer, with the plasmalemma. Chains of vesicles opening simultaneously on both the blood and tissue fronts of the endothelial tunic have not been observed either in sections or in a tridimensional reconstruction of a sector of endothelial cell cytoplasm. Adjacent endothelial cells are closely apposed to one another and appear to be joined over a large part of their margins, possibly over their entire perimeter, by narrow belts of membrane fusion (zonulae occludentes). Except for tongue capillaries, patent intercellular gaps are rare or absent. The middle layer is formed by a continuous basement membrane (500 A thick) and by pericytes which lie in between leaflets of this membrane. The tips of the pericyte pseudopodia penetrate through the inner leaflet of the basement membrane and join the endothelium in maculae occludentes. The adventitia is a discontinuous layer comprising cellular (macrophages, fibroblasts, mast cells) and extracellular (fibrils, amorphous matrix) elements. The same general type of construction appears to be used along the entire length of the capillary.

Temperature dependence of protein transport across lymphatic endothelium in vitro.

“…Horseradish peroxidase (HRP),`for instance, when introduced into the general circulation, appears rapidly in cytoplasmic vesicles of lymphatic endothelium (45). Parallel evidence, both structural (7,27) and functional (28), supports the view that vesicles subserve a transport function across blood vascular endothelium.…”

RETRACTED: The periosteum

“…115 These cells have ability to contract and hence may regulate blood flow in the microvasculature. 23 Pericytes may also function as resting stem cells and differentiate into smooth muscle cells. 81 They may also play a regulatory role in controlling capillary proliferation during wound healing, 31 and support capillaries in maintaining structural rigidity of the micro-vessel wall.…”

Stabilization of breast cancer xenograft tumour neovasculature by angiopoietin-1

“…Endothelial cells are in contact with these supporting cells via functional gap junctions, separated only by a fenestrated basement membrane (Bruns and Palade, 1968;Spitznas and Reale, 1975;Spagnoli et al, 1981;Leiser et al, 1985). The association of these cells is critical for vascular stabilization, and is accompanied by the inhibition of endothelial cell proliferation (Folkman and D'Amore, 1996;Hanahan and Folkman, 1996).…”