Uptake and digestion of 125I-labelled bovine serum albumin by the rat visceral yolk sac cultured in vitro as a closed vesicle.

A system for culturing the rat visceral yolk sac in vitro as a closed vesicle--the 'giant' yolk sac--has been employed to investigate the vectorial nature of the uptake and digestion of exogenous protein substrates. Uptake of 125I-labelled formaldehyde-denatured bovine serum albumin by such yolk sacs was found to be similar to that observed in yolk sacs removed directly from the mother at 17.5 days' gestation, provided that homologous serum was used as a culture medium. However, unlike the control yolk sacs, giant yolk sacs tended to accumulate substrate within the tissue with increasing culture time. The concentration of digestion products released to the inside of the closed vesicle was found to be greater than that released to the surrounding culture medium at time intervals up to five hours. Giant yolk sacs preloaded with 125I-labelled bovine serum albumin were found to release material to the culture medium or the inside of the vesicle almost entirely in the acid-soluble (digested) form. This system is a useful model for studying the polar nature of epithelia, particularly those involved in the uptake and transport of nutritional and/or informational macromolecules.