Changes in the peritoneal membrane with dialysis treatment. (a) Normal peritoneal membrane consists of an intact mesothelium overlying a thin submesothelial compact zone containing extracellular matrix, blood vessels, and a few scattered cells—fibroblasts and peritoneal macrophages. (b) After time on dialysis, activated fibroblasts or myofibroblasts appear along with increased submesothelial extracellular matrix and angiogenesis. Mesothelial cells are injured and sometimes denuded from the peritoneal surface.