ABSTRACT
Fluorescent protein tracing, involving F.I.T.C.-conjugated proteins and the fluorescent antibody technique, was employed to study the sites and mechanism of transport of a variety of proteins across the rabbit foetal membranes.
No evidence was found for an intercellular transmission of proteins across the yolk sac splanchnopleure to the exocoel, but all proteins were shown to become localized in absorptive vesicles in the yolk-sac endoderm.
Different proteins became similarly localized in absorptive vesicles having different sizes and profiles. Characteristic broken vesicles were present, and more than one protein was demonstrated in each absorptive vesicle.
The yolk-sac endoderm was confirmed as the selective site for transmission of proteins to the foetal circulation, since only proteins readily detected in the foetal serum were present in, and below, the basement membrane.
The paraplacental chorion was shown to be the site for transmission of proteins to the exocoel and the process to be one of diffusion.
Unlike normal proteins, F.LT.C. conjugates readily became localized within macrophages present in the paraplacental chorion and yolk-sac vascular mesenchyme.
These findings are discussed in the light of differences previously shown to occur between transmission of proteins to the foetal fluids and to the foetal blood, and in the light of a current hypothesis to account for selection of proteins by the yolk-sac endoderm.
