Ultrastructure of fetal alimentary organs: stomach and spiral intestine in the southern stingray, Dasyatis americana

Abstract

In the fetal southern stingray, Dasyatis americana, both the stomach and 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 that is confluent with gastric pits or foveolae. Gastric glands are populated by oxynticopeptic and enteroendocrine cells. The surface mucous cells are pyramidal with apical microvilli. Oxynticopeptic cells are low columnar with a distinct and elaborate tubulovesicular system in the apical cytoplasm. Microvilli line the lumen of the gastric glands and cells have elaborate interdigitating lateral folds. Enteroendocrine cells are characterized by basal granules and a prominent rough endoplasmic reticulum. The fetal intestine is filled with bile-tinged viscous fluid. A core of submucosa supports spiral intestinal plicae that form the spiral valve from which villi project. The most prominent characteristic of the cells are enormous supranuclear vesicles formed by coalescence 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 ingested histotroph. Small vesicles may then bud off to be digested via the lysosomal system.