ULTRASTRUCTURE OF FETAL ALIMENTARY ORGANS - STOMACH AND SPIRAL INTESTINE IN THE SOUTHERN STINGRAY, DASYATIS-AMERICANA

In the fetal southern stingray, Dasyatis americana, both the stomach a nd spiral intestine function early in development to digest and absorb nutrient histotroph elaborated by uterine villi termed trophonemata. The gastric mucosa consists of a surface columnar mucous epithelium th at is confluent with gastric pits or foveolae. Gastric glands are popu lated by oxynticopeptic and enteroendocrine cells. The surface mucous cells are pyramidal with apical microvilli. Oxynticopeptic cells are l ow columnar with a distinct and elaborate tubulovesicular system in th e apical cytoplasm. Microvilli line the lumen of the gastric glands an d cells have elaborate interdigitating lateral folds. Enteroendocrine cells are characterized by basal granules and a prominent rough endopl asmic reticulum. The fetal intestine is filled with bile-tinged viscou s fluid. A core of submucosa supports spiral intestinal plicae that fo rm the spiral valve from which villi project. The most prominent chara cteristic of the cells are enormous supranuclear vesicles formed by co alescence of smaller endocytotic vesicles. The apical cytoplasm has a profusion of smooth tubules, endoplasmic reticulum, and lysosomes. The large vesicles are interpreted as storage depots for continually inge sted histotroph. Small vesicles may then bud off to be digested via th e lysosomal system.