The follicle-associated epithelium of the ileal Peyer's patch in ruminants is distinguished by its shedding of 50 nm particles.

Significant differences were found between the follicle-associated epithelial cells (FAE) covering the domes of the jejunal and ileal Peyer's patches (PP) in calves. Whereas the FAE of the jejunal PP had scattered membraneous (M) cells among absorptive epithelial cells, the FAE of the ileal PP constituted a homogeneous population of cells different from both absorptive cells and M cells. The FAE of the ileal PP had short microvilli or folds, cytoplasmic vesicles and vacuoles containing acid phosphatase. Fifty nanometer membrane-bounded particles were found in lacunae extending from the lateral cell border. These particles appeared to have been shed from the FAE. Finger-like projections, rich in microfilaments, extended into the lacunae. M cells had short, sparse microvilli, many vesicles, few lysosomal structures, and they enfolded groups of mononuclear leucocytes. Transcytosis of macromolecular tracers occurred in both M cells and in the FAE of the ileal PP, though the uptake was greater in the latter. The 50 nm particles were also found between cells in the lymphoid tissue underneath the FAE of the ileal PP. The FAE of PP in pre- and post-natal lambs from 115 days of gestation and onwards, and goat kids, showed similar structural features.