Plasmalemmal vesicles in pulmonary capillary endothelium of developing fetal lamb lungs

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 microns 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/microns3 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/microns3. 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.