Pinocytosis in the rat visceral yolk sac: potential role in amino acid nutrition during the fetal period.

At 9.5 to 10.5 days post-conception (p.c.) the rat embryo receives amino acids primarily by pinocytosis of maternal proteins by the visceral yolk sac (VYS) endodermal cells and their subsequent digestion in lysosomes. As this mechanism may also contribute to the nutritional support of the rapidly growing fetus later in gestation, this study determines the potential of the VYS to contribute to the amino acid needs of the rat fetus on 13.5, 15.5 and 17.5 days p.c. Wistar rats were killed on 11.5, 13.5, 15.5, 17.5 and 19.5 days p.c. and the protein content of fetuses, VYSs and serum was determined. Additional VYSs were isolated on the same days p.c. and the uptake of [14C]sucrose, a measure of fluid-phase pinocytosis, was determined. The rate of protein accretion by the fetus and the capacity of the VYS to pinocytose and digest proteins were calculated on 13.5, 15.5 and 17.5 days p.c. There was a decrease in pinocytic capacity, per mg protein, over the period of development studied but, owing to the growth of the tissue, the total capacity of the yolk sac to pinocytose and digest proteins increased from 13.5 to 17.5 days p.c. Comparison of the VYSs capacity to pinocytose and digest proteins and the rate of protein accretion by the fetus suggests that the VYS could support a significant proportion of the increase in protein content of the growing fetus. This conclusion is reinforced by the limited capacity of the maternal plasma to supply free amino acids to the fetus.(ABSTRACT TRUNCATED AT 250 WORDS)