Fine structure of the term umbilical cord in the Atlantic sharpnose shark, Rhizoprionodon terraenovae

The fine structure of the umbilical cord and appendiculae in the Atlantic sharpnose shark, Rhizoprionodon terraenovae, is examined by light, scanning and transmission electron microscopy. During ontogeny of placental sharks, the yolk sac and stalk become progressively modified as a functional hematrophic placenta and umbilical cord respectively. In most placental sharks the umbilical cord is smooth. In the Atlantic sharpnose shark, the epithelial ectoderm of the somatopleure forms richly vascularized extensions termed appendiculae. Scanning electron microscopy reveals that the base and shaft of appendiculae are flattened while the distal portion may be expanded to form one to three lobes. The surface of appendiculae is composed of two distinct cell types, the most plentiful are microvillar cells. The second cell type contains prominent granules. These cells are much larger than the former and are partially submerged below the surface, except for the cell apex. These cells undergo secretory cycles ending in expulsion of their contents. The possible function of the granulated cells is discussed. The umbilical cord contains an umbilical vein, umbilical artery, ductus vitellointestinalis and extraembryonic coelom. The endodermal ductus initially conveys yolk from the yolk sac to the fetal gut by activity of ciliated cells lining it. The ductus persists in the adult. Microvillar cells, also present in the ductus, may play a role in the absorption of yolk metabolites early in development, prior to yolk depletion. Enteroendocrine cells are wedged between the ciliated and microvillar cells. These cells may exert paracrine regulation of local areas of the ductus.