Studies on Blood Capillaries

Abstract

The pathway by which intravenously injected ferritin molecules move from the blood plasma across the capillary wall has been investigated in the muscle of the rat diaphragm. At 2 min after administration, the ferritin molecules are evenly distributed in high concentration in the blood plasma of capillaries and occur within vesicles along the blood front of the endothelium. At the 10-min time point, a small number of molecules appear in the adventitia, and by 60 min they are relatively numerous in the adventitia and in phagocytic vesicles and vacuoles of adventitial macrophages. Thereafter, the amount of ferritin in the adventitia and pericapillary regions gradually increases so that at 1 day the concentration in the extracellular spaces approaches that in the blood plasma. Macrophages and, to a lesser extent, fibroblasts contain large amounts of ferritin. 4 days after administration, ferritin appears to be cleared from the blood and from the capillary walls, but it still persists in the adventitial macrophages and fibroblasts. At all time points examined, ferritin molecules within the endothelial tunic were restricted to vesicles or to occasional multivesicular or dense bodies; they were not found in intercellular junctions or within the cytoplasmic matrix. Ferritin molecules did not accumulate within or against the basement membranes. Over the time period studied, the concentration of ferritin in the blood decreased, first rapidly, then slowly, in two apparently exponential phases. Liver and spleen removed large amounts of ferritin from the blocd. Diaphragms fixed at time points from 10 min to 1 day, stained for iron by the Prussian Blue method, and prepared as cleared whole mounts, showed a progressive and even accumulation of ferritin in adventitial macrophages along the entire capillary network. These findings indicate: (1) that endothelial cell vesicles are the structural equivalent of the large pore system postulated in the pore theory of capillary permeability; (2) that the basement membrane is not a structural restraint in the movement of ferritin molecules across the capillary wall; (3) that transport of ferritin occurs uniformly along the entire length of the capillary; and (4) that the adventitial macrophages monitor the capillary filtrate and partially clear it of the tracer.

Morphometric Data on the Endothelium of Blood Capillaries

“…FIGURE 4 Myocardium . Small sector of a cross fracture through the endothelium (e) of a blood capillary.…”

“…Among the latter, the plasmalemmal vesicles in the endothelium of muscular capillaries and the fenestrae in that of visceral capillaries are of particular importance, since they have been implicated in the movement of water and water-soluble molecules across the endothelial layer (2,(3)(4)(5)(6)(7)(8) . A number of incidental observations have already mentioned an uneven distribution of fenestrae and vesicles in a variety of capillaries (6,(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) .…”

Exocytosis of pinocytic contents by Chinese hamster ovary cells.

“…Cells were exposed to 1 mg/ml HRP for (a) 2 upon cell lysis. /3-Hexosaminidase and acid phosphatase are lysosomal matrix enzymes (7) .…”

“…Exocytosis of pinocytic contents may occur as part of endocytic membrane cycling. In certain specialized cell types such as capillary endothelium (2,19), fetal yolk sac (11), and neonatal intestinal epithelial cells (14), exocytosis of pinocytic contents has previously been described. In these cell types pinocytic content exocytosis constitutes a physiological transport process for the movement of macromolecules from one side of a tissue layer to another.…”

“…The fate of pinocytic vesicle contents is generally thought to be delivery to and ultimate degradation within a lysosomal compartment except in certain specialized cell types (capillary endothelium [2,19], fetal yolk sac [1l], and neonatal intestinal epithelial cells [14]). Based on time-lapse photomicrography studies, Gey et al (4) speculated that pinocytized fluid whose fate could not be accounted for by transfer to other organelles was discharged from the cell .…”