Ultrastructural classification of the endocrine cells of the large intestine of the calf. Cytochemical evidence of the presence of Viallis's pre-EC cells.

GEP (Gastro-Entero-Pancreatic) endocrine cells were very numerous in the mucosal layer of the large intestine of the calf. Their frequence appeared to increase towards the distal portions of the gut. Endocrine cells were dispersed among epithelial cells lining intestinal glands and were frequently grouped together. Cellular shape was pyramidal or elongated; the cytoplasm was electron-lucent and contained highly characteristic secretory granules. Six different types of endocrine cells were identified on the basis of the ultrastructural aspect and cytochemical characteristics (silver-reactivity) of their secretory granules: EC, L, PP, A, D1 and D cells. EC and L cells were the most abundant in all localisations. They were especially numerous in the rectum. A subpopulation of EC cells was negative to Masson-Singh's reaction showing that they lack 5-HT. This observation enabled us to refer this latter cellular type to the "pre-EC" cells, described by Vialli as an earlier evolutive step of the EC cells population. Their presence in the calf gut might be linked to its possible "immaturity", due either to the age or to the alimentary diet.