An Ultrastructural Study of the 'Extraneous Coat' of Human Colonic Mucosa

Summary A study of the ultrastructure of the human colonic mucosa revealed a remarkably well developed, hairy extraneous coat on the luminal surface of the epithelial cells. Mucosal biopsies for this study were obtained from the rectosigmoids of eight patients. The surface of the microvilli and the intermicrovillous plasmalemma is covered with hairlike fibrils extending from the outer leaflet of the plasmalemma into the lumen. This layer measures up to 1.5 p, in thickness and is best developed on the surface epithelium and less well developed on the crypt epithelium. The extraneous coat appears to be an integral part of the epithelial cell, but its site of origin and its precise function remain speculative.