Structure and function of mesothelial cells.

The mesothelium was first described about 180 years ago but only in the last twenty years have we begun to appreciate the roles that mesothelial cells play in maintaining normal serosal membrane integrity and function. Mesothelial cells are sentinel cells that can sense and respond to signals within their microenvironment. They secrete glycosaminoglycans and surfactant to allow the parietal and visceral serosa to slide over each other. They actively transport fluids, cells and particulates across the serosal membrane and between serosal cavities. They synthesise and secrete a diverse array of mediators in response to external signals which play important roles in regulating inflammatory, immune and tissue repair responses. In addition, they are likely to protect from peritoneal dissemination of tumours until the integrity of the mesothelium is breached. Although the importance of the mesothelial cell is being realised, we still do not understand the mechanisms regulating many of their functions. How the cells communicate with each other and surrounding cells, whether mesothelial cells differentiate into different cell types or if a mesothelial stem cell exists, the mechanisms regulating mesothelial repair and the role mesothelial cells play in serosal pathologies, all need further study. Although it has long been accepted that mesothelial cells are similar irrespective of site or species, apart from morphology, few studies have truly compared biochemical and functional characteristics of these cells between species and within different anatomical sites. In a recent study examining the effect of aging on human peritoneal mesothelial cells, there was a positive correlation between the age of the donor's cells and the proinflammatory profile. Although mesothelial cells share many similarities, it is likely that functional and physiological adaptation will alter these cells. Addressing these questions are paramount if we hope to find better ways to protect serosal integrity and prevent peritoneal dissemination of tumours.