Abstract
This chapter describes the morphology and structure of the mesothelium and discusses some of the many roles mesothelial cells play under normal and pathological conditions. Mesothelial cells form a monolayer of cobblestone-like cells that line the peritoneal, pleural, and pericardial cavities and most internal organs. Under normal conditions, mesothelial cells form a monolayer, and play important roles in maintaining normal serosal membrane integrity and function. Mesothelial cells are unique in that they function as an epithelium but express both epithelial and mesenchymal markers and have an inherent ability to undergo Epithelial to mesenchymal transition (EMT), a process that has recently been termed mesothelial to mesenchymal transition (MMT). MMT is proposed to play a role in the establishment of endometriosis where retrograde menstrual tissue embeds on the peritoneal surface and forms an active lesion. The processes that occur during pathological EMT are thought to be comparable to physiological EMT as they are controlled by similar signaling pathway regulators and effector molecules.