Initial lymphatics--morphology and function of the endothelial cells.

In considering lymph formation, the function of the initial lymph sinus is usually considered a passive process. The cells, however, of the initial lymphatics hold a key position in absorbing fluid from the interstitial space. The present study using rats and guinea pigs in different days of the estrus cycle or pregnancy examined by light, scanning electron, and transmission electron microscopy suggests that the forming and closing of the open-junction formations is an active component of the lymphatic endothelium. Open-interface structures represent a further entry into initial lymphatic pathways. The existence of "endothelial cellular buds" probably act structurally to build elements of the initial lymph sinus. In short, the endothelium of the initial lymphatic sinus appears to be a structure of utmost flexibility able to respond promptly to increased amounts of lymph fluid transported from the interstitial space.