In vivo uptake of heterologous alphafetoprotein and serum albumin by ependymal cells of developing chick embryos

The in vivo uptake by neuroepithelial cells of rat serum proteins injected into the mesencephalic cavity of developing chicken was demonstrated by immunohistochemical methods. It was found that 48 h after inoculation, rat alphafetoprotein (AFP) and serum albumin (SA) localize in the same places as do chicken AFP and SA. No labelling was present for rat or chicken IgG. We also present evidence showing that chicken ovalbumin, which is not normally found inside ependymal cells is not internalized even when injected into the cerebrospinal fluid at 30-fold its physiological concentration. The existence of transient specific receptors which could be responsible for this protein uptake is proposed. The possible implications in relation to developmental biology are discussed.