The preformed stomas connecting the pleural cavity and the lymphatics in the parietal pleura.

Lympatics in the parietal pleura and diaphragm of rabbits and mice formed lacunas, terminal dilatations of lymphatic channels, with cribriform laminas bordering the pleural space at one end and one-way valves on the other end. The laminas consisted of a coarse mesh of connective tissue fibers covered by mesothelial cells at one side and endothelial cells at the other. At the fenestrated portions of the meshwork the basal surfaces of the 2 lining cells were closely apposed and some preformed stomas were found bridging the pleural and lymphatic spaces. No such structures were found in the visceral pleura, but similar cribriform laminas were also present in human parietal pleura at autopsy. Intrapleurally injected carbon particles reached the lacunas faster than similarly injected chicken red blood cells in living rabbits. Carbon particles appeared to pass through the closely apposed lining cells as well as the stomas but not into the intact interstitial structures. These structural and functional findings supported the observation that large particles and cells in the pleural cavity are removed only through the lymphatics and only at specific sites in the parietal pleura.