M cells are damaged and increased in number in inflamed human ileal mucosa.

Ileocolonoscopy and biopsies of patients with spondylarthropathy revealed gut inflammation in 62% of the cases. In order to better understand the pathogenetic mechanisms of spondylarthropathy related gut inflammation the follicle associated epithelium was examined. Biopsies from 9 controls and 18 patients with spondylarthropathy were studied by electron microscopy. Membranous (M) cells were investigated in normal and inflamed ileum. In normal mucosa M cells were scarce whereas in inflamed mucosa their number was increased (up to 24% of follicle associated epithelial cells). They showed a thin rim of cytoplasm covering groups of lymphocytes. In chronic ileitis necrotic M cells, ruptures of M cell cytoplasm and lymphocytes entering the gut lumen were observed. The bursts of M cells at the top of the lymphoid follicles lead to interruption of the gut epithelial lining and give luminal content access to the lymphoid tissue. This pathogenetic mechanism may cause aphthoid ulcers.