SUBCELLULAR ORGANIZATION OF THE PLACENTA IN THE ATLANTIC SHARPNOSE SHARK, RHIZOPRIONODON-TERRAENOVAE

The Atlantic sharpnose shark, Rhizoprionodon terraenovae, is a vivipar ous anamniote that develops a yolk sac placenta composed of: a) uterin e mucosa, b) egg envelope and c) fetal yolk sac mucosa. The transporti ng uterine mucosa is a squamous epithelial bilayer with prominent late ral and basal infoldings between contiguous cells, The surface cells h ave prominent secretion vesicles that empty their contents to the exte rior. Immediately beneath the epithelium is a basal lamina and a profu se vascular supply with a continuous endothelium. The epithelium of pa raplacental uterine sites is mucous. The tertiary egg envelope is reta ined throughout gestation and separates the distal part of the yolk sa c from the maternal uterine mucosa. The egg envelope is compact on the yolk sac surface but displays delaminations on the uterine surface. T he fetal yolk sac is composed of two portions, viz., a proximal, saccu lar region and a heavily vascularized, rugose, distal portion. The pro ximal portion has ultrastructural characteristics of a steroid hormone producing tissue, including massive smooth endoplasmic reticulum freq uently forming whorled arrays. However, definitive evidence that the y olk sac is an endocrine organ is lacking. The distal portion of the fe tal yolk sac is composed of a squamous epithelial bilayer that is sepa rated from the underlying vascular network by a continuous basal lamin a. The endothelium of the vessels is fenestrated. Cytoplasmic characte ristics of these cells include an extensive Golgi complex, smooth wall ed caveolae, vesicles with electron-dense contents that are presumably endocytotic in nature and dense bodies that are suggested to be lysos omes that are involved in the digestion of material that may be yolk m etabolites,