Analysis of relationships between pericytes and gas exchange capillaries in neonatal and mature bovine lungs.

Neonatal and mature bovine lungs were examined ultrastructurally to quantitatively assess pericyte envelopment of gas exchange capillaries and proximities of pericyte margins to endothelial cell junctions. Pericytes were observed on 91% of the cross-sectioned capillary profiles examined, with 18 and 26% coverage in neonatal and mature lungs, respectively. Chi-square analysis indicated a random occurrence of endothelial cell junctions under pericytes; however, pericyte processes tended to end near endothelial cell junctions. In the neonatal and mature lungs, 38 and 40%, respectively, of all endothelial junctions were within 0.5 microns of the margins of pericyte processes; such distances covered only 16 and 17% of the circumferences of the capillary profiles examined. Thus, endothelial cell junctions were located near pericyte margins over twice as often as would occur in a random distribution. If pericytes are contractile cells, as recent research indicates, they may function as regulators of lymph formation by influencing the permeability of endothelial cell junctions in the gas exchange capillaries.