Mesothelium secretes lamellar bodies in a similar manner to type II pneumocyte secretion of surfactant.
In the early 1970s it was found that a specific lipid-fixing technique of tissue preparation for electron microscopy (tannic acid-glutaraldehyde) preserved and thus unmasked distinctive inclusions in Type II pneumocytes which were called lamellar bodies. This discovery was the first crucial step in demonstrating that lamellar bodies were the storage granules from which alveolar surfactant was secreted. In a previous study a comparison between mesothelium and Type II pneumocytes showed close ultrastructural similarities. In the present investigation of normal peritoneal tissue from man, monkey, rabbit and mouse, following primary tannic acid-glutaraldehyde fixation and modified embedding procedures similar to those used for lung, examination by transmission electron microscopy demonstrated in all mesothelial cells examined, characteristic lamellar structures similar to those described in Type II pneumocytes. Exocytotic extrusion of lamellar bodies from the apical portion of the mesothelial cell, and the presence of lamellar bodies on the cell surface in a manner identical to that found in Type II pneumocytes was also observed. These findings provide compelling evidence that a process of specialized biosynthesis and secretion of phospholipids similar to that established for Type II pneumocytes also occurs in mesothelial cells.