follicle associated epithelium

The follicle-associated epithelium (FAE) is a specialized epithelial layer that overlies the luminal surface of lymphoid follicles in gut-associated lymphoid tissue (GALT). It is found overlying the Peyer's patches of the small intestine and appendix in humans, and in similar structures in mice. The FAE contains specialized M cells and has fewer goblet cells compared to the surrounding villous epithelium.

The follicle-associated epithelium is located on the luminal surface of the intestinal mucosa, specifically covering the dome regions of Peyer's patches in the small intestine (primarily the ileum) and lymphoid follicles in the appendix in humans. It is positioned between the intestinal lumen and the underlying lymphoid follicles.

The follicle-associated epithelium is involved in mucosal immune surveillance. Its M cells are specialized for transcytosis—transporting luminal antigens, microorganisms, and particulate matter from the gut lumen to the underlying lymphoid tissue. This facilitates antigen sampling and presentation to immune cells, contributing to the induction of secretory IgA responses and mucosal immunity.

The follicle-associated epithelium may be associated with the pathogenesis of certain enteric infections, as some pathogens (including Salmonella, Yersinia, and poliovirus) are thought to exploit M cells as entry portals. Inflammatory bowel disease and conditions affecting Peyer's patches may also be associated with changes in follicle-associated epithelium structure and function.

The follicle-associated epithelium is examined using histological sections of intestinal tissue, where it can be identified overlying lymphoid follicle dome regions. Electron microscopy reveals the characteristic morphology of M cells with their irregular apical surface and intraepithelial lymphocyte pockets. Immunohistochemistry using markers such as GP2 identifies M cells within the FAE.