PERITONEAL LYMPHATIC STOMATA OF THE DIAPHRAGM IN THE MOUSE - PROCESS OF THEIR FORMATION

Background: Lymphatic stomata are channels connecting the peritoneal c avity with the lymphatics in the diaphragm, The process of sequential formation of the stomata has not been studied. The objective of this s tudy was to examine the morphogenesis of the lymphatic stomata in mice . Methods: Ultrathin sections of diaphragms from ddY mice obtained on embryonic day 18 and postnatal days 0, 4, and 10 were observed with a transmission electron microscope. Results: By embryonic day 18 and pos tnatal day 0, lymphatics were already observed in the submesothelial c onnective tissue on the peritoneal side of the fetal diaphragm, The ly mphatic endothelial cells, but not the mesothelial cells covering the diaphragm, protruded short cytoplasmic processes into the submesotheli al connective tissue, and these almost reached the basal surfaces of i ndividual mesothelial cells, By postnatal days 4 and 10, the lymphatic endothelial cells frequently protruded cytoplasmic processes into the submesothelial connective tissue, and the endothelial cell processes broke the continuity of both the basal lamina beneath the mesothelial cells and the submesothelial connective tissue, Neighboring endothelia l processes formed a pair of U-shaped folds that were connected with e ach other via intercellular junctions at the apexes of the U-shaped fo lds, The disassembly of the intercellular junctions between the U-shap ed folds was observed, and the basal surface of the mesothelial cell f aced the lymphatic lumen, Dehiscence of the intercellular junctions be tween the mesothelial cells overlaying the lymphatics was observed, an d lymphatic stomata were present, On the pleural side of the diaphragm , lymphatics were already present on embryonic day 18, but it was not observed that the endothelial process spanned the submesothelial conne ctive tissue to the basal surface of the mesothelial cell. Conclusions : These results suggest the following process of the formation of the lymphatic stomata, (1) Neighboring lymphatic endothelial cells span th e submesothelial connective tissue to the basal surfaces of mesothelia l cells, (2) The lymphatic stomata are formed by the disassembly of th e intercellular junctions between the neighboring endothelial cells an d between the mesothelial cells overlying the endothelial cells. (C) 1 997 Wiley-Liss, Inc.