The sheep trophoblast and placental function: an ultrastructural study.

Both the uninucleate cubiodal epithelial cells and the binucleate giant cells of the sheep trophoblast undergo marked structural modifications during pregnancy. Uninucleate epithelial cells show ultrastructural features which may be correlated in temporal order, with the secretion of steroid hormones, the production of ovine placental lactogen, and the performance of normal non-endocrine placental activities. The binucleate cells appear to be of two main types, one concerned with glycogen storage, and the other with the synthesis of a glycoprotein secretory substance. The binucleate cells are always enclosed by a thin continuous lamina of cuboidal cell cytoplasm which separates them completely from the maternal syncytium and the fetal connective tissue. The cuboidal epithelial and binucleate giant cells may either be closely apposed or else separated by canals or spaces of varying complexity. The presence of these interfacial canals indicates that the ovine trophoblast is another example of an epithelium in which the local movement of water and solutes is mediated by standing gradient osmotic flow systems. Such flow systems may well be an integral part of eutherian placental transport mechanisms.