The 'giant' yolk sac : an in vitro model for studying early placental transport
In the rat, before the establishment of the chorioallantoic placenta, the nutritional requirements of the post-implantation embryo, are met solely by the visceral yolk sac and therefore a study of its structure and functions is essential to a full understanding of early embryonic nutrition. A method has been developed for maintaining the rat visceral yolk sac in organ culture over a prolonged period, having first removed the embryo by microsurgery at 9.5 days or alternatively allowing it to die within its own amnion. The yolk sac continues to grow as a closed vesicle, and can reach a diameter of 2cm. The system has been called the 'giant' yolk sac. The 'giant' yolk sac and in vivo yolk sac have been compared using various criteria. A detailed morphological study was made, including a quantitative analysis of the vacuolar compartment. The endocytic capacity of both systems was studied using three different substrates; those used were 125I-polyvinylpyrrolidone (PVP), a non-degradable macromolecule, taken up in the fluid phase and accumulated within the yolk sac tissue, 125I-bovine serum albumin (BSA) taken up by adsorptive pinocytosis and digested within the lysosomes and 125I-IgG (and colloidal gold-IgG) taken up with great efficiency by specific receptor mediated endocytosis. Also a preliminary study of 14C-amino acid uptake was made. In many instances the 'giant' yolk sac functioned very similarly to the in vivo yolk sac and therefore seems an ideal model for studying transport across an epithelial sheet. It is particularly useful as its continuous epithelium separates the exocoelom from the external culture medium. The fluid maintained within the exocoelom of the 'giant' yolk sac should be an excellent source of processed histiotroph essential for embryonic nutrition during organogenesis. Experiments carried out indicate that some of the trophic factors necessary for growth are present in this fluid.