UPTAKE OF MICROPARTICLES BY RAT VISCERAL YOLK-SAC

The visceral yolk sac (VYS) is responsible for a major part of the ami no acid nutrition of the early post-implantation rat embryo and possib ly also at the fetal stage of gestation. The mechanism involves endocy tic uptake of proteins by the tissue's epithelial cells followed by in tralysosomal digestion to amino acids. The amino acids so generated ar e used for protein synthesis in both the embryo and the VYS. Previous reports had indicated that the endocytic capacity of the VYS might be limited to exclude larger macromolecules. This study demonstrates that Percoll, which comprises 30-nm silica particles coated with polyvinyl pyrrolidone (PVP), is as effectively captured by the 17.5-day rat VYS cultured in vitro as PVP itself. Uptake of I-125-labelled Percoll was progressive with time over 5 h and was inhibited by a low incubation t emperature, 2,4-dinitrophenol (50 mu g/ml), EGTA (5 mM), colchicine (1 0 mu g/ml) or cytochalasin B (10 mu g/ml). After uptake of I-125-label led Percoll, VYSs released only 20 per cent of their radioactivity whe n re-incubated in fresh medium for 3 h. These data, and electron micro graphs showing Percoll in intracellular vacuoles, are all consistent w ith uptake by endocytosis. Percoll's rate of uptake by the VYS indicat es that, like I-125-labelled PVP, it enters the cell chiefly by fluid- phase pinocytosis. It is concluded that endocytosis bu the VYS will ef ficiently capture even the largest globular proteins, and that previou s indications of a relatively low size exclusion reflected the loosely coiled configuration of the synthetic polymers used in the earlier st udies. (C) 1997 W. B. Saunders Company Ltd.