The mucosa of the small intestine is com- posed of the muscularis mucosae, the lam- ina propria, and the lamina epithelialis. Roughly, villi and crypts of Lieberkiihn can be recognized. The lamina epithelialis consists of a continuous layer of columnar epithelial cells and is separated from the lamina propria by a basal lamina. The epithelium in the crypts is formed by en- teroblasts, goblet cells, various kinds of entero-endocrine cells, a few tuft cells and, in most mammaliah species, Paneth cclls (1). The epithelium covering the villi is mainly composed of enterocytes and goblet cells, and a few entero-endocrine cells, tuft cells, and the recently discovered ‘cup’ cell (I, 2). At the apical border, the epithelial cells are attached to each other by the tight junctions (3). Between the epithelial cells lymphocytes are situated (4). Solitary and aggregated lymphatic nodules arc scatter- ed in the lamina propria and in thesubmu- cosa throughout the small intestine. With- in the epithelium overlaying this lymphatic tissue, specialized follicle associated epi- thelial (F AE-) or membranous (M-)cells are present (5-7).