The giant yolk sac: a model for studying early placental transport.

Nine and a half day rat embryos can be cultured for 48 hours in whole heat-inactivated rat serum using the roller culture method described by New, Coppola & Terry (1973). We have prolonged the culture period, usually by seven days. Although the embryo dies almost immediately during this extended culture period, the yolk sac continues to grow and reaches a diameter of approximately 2 cm; we have called this the giant yolk sac. The morphology of the giant yolk sac is very similar to that of control yolk sacs (17 1/2 or 18 1/2 days in vivo), the main difference being the greatly enlarged vacuolar volume in the endodermal cells of the giant yolk sac, which have been studied morphometrically. The pinocytic nature of the giant yolk sac has been demonstrated by its ability to take up colloidal gold. Its rate of uptake of 125I-polyvinylpyrrolidone in whole serum gassed with 95% O2; 5% CO2 has been shown to be similar to the rate of uptake found in control yolk sacs under the same incubation conditions. Acid phosphatase activity was found to be similar in the giant yolk sac and control yolk sacs using both histochemical and biochemical methods. Giant yolk sacs without a contained dead embryo can be produced by removing the embryonic pole of the egg cylinder prior to incubation. They exhibit all the features detailed above. Finally it is shown that the fluid from within the extra-embryonic coelom of the giant yolk sac has some capacity to support the growth and development of 9 1/2 day rat embryos when a source of bulk protein is also provided. This model, therefore, seems to be very useful for the study of transport in a placental system. Its full potential requires further study.