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Morphometric analysis of electron micrographs of the eel rete mirabile revealed that pericytes occupied nearly one-third of the cellular volume of this organ with over 75% of the pericyte volume associated with arterially derived capillaries. These pericytes were highly arborized, extending processes which encircled the capillaries and covering, on average, more than 85% the ablumenal surface of arterially derived capillaries. Pericytes and endothelial cells interacted in a manner suggesting that pericyte activity modulates both capillary blood flow and permeability.

Micropinocytosis was quantitated in a suspension of capillary endothelium isolated from epididymal fat. A rhodamine-ferritin conjugate (RhF) was utilized as a tracer molecule for micropinocytic ingestion. RhF is electron dense, which permits ultrastructural determination of its mode of incorporation into endothelium, and its fluorescence allows quantitation of its uptake by micropinocytosis. The amount of RhF incorporated with time was standardized against endothelial cell DNA content as measured by fluorescence enhancement of mithramycin when bound to DNA. RhF was incorporated into isolated endothelium for periods exceeding 90 min.

In adult mammalian lungs pinocytotic vesicles (PV) play a role in macromolecular transport across endothelium. The time course of their development in fetal lungs was examined. Morphometric techniques were used to measure the relative capillary volume as percentage of lung volume. Mean diameter ( D ), numerical density (Nv), and endothelial fractional volume (Vv) represented by PV were determined in randomly selected capillaries, measuring 5.3–8.4 μm in diameter, from right upper (RUL) and left lower (LLL) lobes of lungs from fetal lambs ranging in gestational ages from 39 to 144 days. Three newborn lambs, 1 hr after birth, were also examined. There was a 20-fold increase in the volume density of capillaries during the period studied. The vesicular D ± SD was 739.6 ± 4.1 and 735.1 ± 2.8 A in RUL and LLL, respectively, and remained unchanged throughout gestation. By contrast, following a gradual increase in Nv from 24.7 to 65.1/μm3 in the glandular stage (39–85 days), there was a rapid rise in Nv during both canalicular (95–121 days) and alveolar (130–144 days) stages of development, reaching a final Nv of 254.8/μm3. In newborn lambs, 1 hr after birth, there was no significant change in Nv. There was a significant correlation between Nv and fetal age, but there was no difference in Nv between RUL and LLL at any time. Thus, a more than 10-fold increase in Nv of PV occurs in ovine pulmonary capillary endothelium during gestation, the biochemical and/or physiological basis of which remains to be determined. By contrast, observations made on coated vesicles indicate that Nv of these structures does not change with gestation.

Tufts of capillary endothelium isolated from rat epididymal fat ingest [ 14 C]sucrose by the process of micropinocytosis. Ingestion of sucrose for periods in excess of 30 min was quantitated using the concentration of cellular DNA as a standard. Capillaries incubated in the metabolic inhibitors 2,4-dinitrophenol, NaN 3 , or iodoacetate exhibit a dose-dependent reduction of cellular ATP. Micropinocytic ingestion of [ 14 C]sucrose is not affected by concentrations of these inhibitors which markedly reduce or deplete ATP pools. Pure oxygen or nitrogen atmospheres also have no effect on micropinocytosis. Ingestion of [ 14 C]sucrose is reduced, however, at temperatures lower than 37° or in the presence of Zn 2+ .

It was found that ferritin molecules are transported across inflamed blood capillary endothelium in large vacuoles as well as in small vesicles. Ferritin could be seen leaving such organelles, attached to the ablumenal plasma membranes, 30 to 60 sec after it had been injected into the blood, and well before there was any buildup of the tracer in the tissues adjacent to the endothelium.