Keele Research Repository

An in vitro method, previously described for incubating rat yolk sacs for short periods, was adapted for the incubation of rabbit yolk sacs. It was shown that the endocytic properties of these two tissues are very similar as judged by their rates of uptake of 125 I-labelled PVP, albumin and homologous IgG. By using the in vitro methods for incubating rat and rabbit yolk sacs, it was possible to measure the rates of release of substrate (intact and degraded) from yolk sacs previously "loaded" in vitro with a marker protein. The latter variation of the method was used in an attempt to further investigate the cellular mechanism of prenatal transfer of passive immunity in the rat and rabbit. It was found that in vitro 125 I-labelled IgG internalized by the tissue can be released again into the incubation medium. When the fates of a number of 125 I-labelled homologous and heterologous IgGs were investigated, it was found that definite rank orders existed for the extent of release of intact IgG from the tissues. The rank orders were closely parallel to those reported for transplacental transfer by the same tissues in vivo. Some assumptions underlying the Brambell, Wild and Henmings’ theories of IgG transfer in the rabbit yolk sac were investigated. Evidence for the presence of specific receptors or binding sites on the yolk-sac membrane was obtained both for molecules destined for transfer through the tissue and also for molecules destined for intracellular degradation. No exocytosis from heterolysosomes could be demonstrated,
nor was an enhanced rate of release of 125 I-labelled homologous IgG from the rabbit or rat yolk sac in vitro observed when degradation of these substrates was inhibited by the addition of leupeptin to the incubation medium. These findings are more compatible with the operation of a two-vesicle system with separate vesicles for transcellular transport and degradation, as suggested by Wild.