Subcellular organization of the placenta in the Atlantic sharpnose shark, Rhizoprionodon terraenovae

The Atlantic sharpnose shark, Rhizoprionodon terraenovae, is a viviparous anamniote that develops a yolk sac placenta composed of: a) uterine mucosa, b) egg envelope and c) fetal yolk sac mucosa. The transporting uterine mucosa is a squamous epithelial bilayer with prominent lateral and basal infoldings between contiguous cells. The surface cells have prominent secretion vesicles that empty their contents to the exterior. Immediately beneath the epithelium is a basal lamina and a profuse vascular supply with a continuous endothelium. The epithelium of paraplacental uterine sites is mucous. The tertiary egg envelope is retained throughout gestation and separates the distal part of the yolk sac from the maternal uterine mucosa. The egg envelope is compact on the yolk sac surface but displays delaminations on the uterine surface. The fetal yolk sac is composed of two portions, viz., a proximal, saccular region and a heavily vascularized, rugose, distal portion. The proximal portion has ultrastructural characteristics of a steroid hormone producing tissue, including massive smooth endoplasmic reticulum frequently forming whorled arrays. However, definitive evidence that the yolk sac is an endocrine organ is lacking. The distal portion of the fetal yolk sac is composed of a squamous epithelial bilayer that is separated from the underlying vascular network by a continuous basal lamina. The endothelium of the vessels is fenestrated. Cytoplasmic characteristics of these cells include an extensive Golgi complex, smooth walled caveolae, vesicles with electron-dense contents that are presumably endocytotic in nature and dense bodies that are suggested to be lysosomes that are involved in the digestion of material that may be yolk metabolites.