Yolk sac receptors: Their association with coated vesicles and transport of specific proteins in vitro in the early postimplantation rat conceptus.

This study demonstrates the characteristics of specific receptors for IgG, transferrin and insulin on 11.5 day rat yolk sac endoderm in vitro, using the culture system for the postimplantation rodent embryo as described by New. Coated vesicles are implicated as mediators of a protein transport mechanism, as part of the endocytic process of macromolecular uptake in the yolk sac. Analysis includes immunofluorescent studies, incubation with radiolabelled ligand and immunocyto- chemical localisation of ligand. A peroxidase conjugate was used to localise IgG and colloidal gold was developed as a direct electron microspical membrane receptor probe for IgG and transferrin receptors. Rat IgG and transferrin were found to co-internalise into the same coated vesicle, and were associated with high affinity cell surface receptors. Insulin was found to bind to isolated 9.5 day cells of the egg cylinder and also to isolated 10.5 day, 11.5 day, 12.5 day and full term cells of the yolk sac. The degree of insulin binding to isolated yolk sac cells was comparatively less than the observed binding of insulin to isolated 9.5 day egg cylinder cells, and was found to decrease between days 10.5 and 12.5 of gestation. At 11.5 days the fluid between the yolk sac endoderm and the amnion comprised a mixture of transported serum proteins and synthetic products of the conceptus. Human serum did not support normal growth of rat conceptuses during organogenesis in vitro but human serum proteins were transported by the yolk sac to the yolk sac fluid. The results are discussed in relation to the involvement and formation of coated vesicles and the general importance of protein transport across the inverted yolk sac placenta during organogenesis.