A genetic variant of esterase in mice is used to demonstrate for the first time that an enzyme of maternal origin is taken up intracellularly by two mid-gestation tissues derived from the embryo: trophoblast and yolk sac (visceral endoderm). Although other alternatives are still possible, it is likely that the esterase is transported from the mother to the trophoblast via the serum. If so, the phenomenon is selective, since at least one other prominent serum esterase is not found in trophoblast or yolk sac cells. The enzyme has not been detected in the embryo proper, even at very. late stages of pregnancy.Trophoblast cellsrdeveloping from blastocysts implanted under the kidney capsule of a male mouse also appear to have the ability to take up host esterase. Efforts to demonstrate uptake of the enzyme by trophoblast cells in blastocyst cultures have been unsuccessful.A mammalian embryo develops in the reproductive tract, and as such is constantly exposed to, and protected by, the maternal environment. Preimplantation embryos removed from the reproductive tract and placed in ectopic sites can implant in the foreign tissue and proceed to develop normally for several days (1-4). Blastocysts maintained in culture medium are able to give rise to some cell types that are at least partially differentiated: trophoblast (5-7), yolk sac (7, 8), and part of the embryo proper (8). It would thus appear that signals for early differentiation need not come from the mother. There is, however, a great advantage to be gained from development in the reproductive tract, namely the selective passage of molecules and macromolecules from mother to fetus. By this phenomenon, for example, the fetus is provided with maternal antibodies that protect it after birth until its own immune system is functional (9). Studies on the passage in mice of injected serum proteins from the maternal bloodstream into embryonic tissues late in pregnancy have revealed that some fractions (e.g., human IgG) can cross the placental barrier more easily than others (e.g., human albumin) (10, 11). The studies described here are the first to illustrate that an enzyme naturally present in the maternal blood penetrates the embryonic tissues of the placenta in the first half of gestation.
METHODS AND MATERIALSThe following strains of mice were used in this study: Swiss PO, C57BI/6, SWR/J, SJL/J, C57B1/6J, and C57B1/6J x DBA/2J hybrids. The latter four strains were obtained from the Jackson Laboratory, Bar Harbor, Me. Mice were randommated when postimplantation embryos were required. Preimplantation blastocysts were collected from females superovulated (12) before mating. The day on which vaginal plugs, indicating successful matings, were detected is referred to as the first day of pregnancy. The following tissues were dissected (6) on the 10th or 11th days of gestation: the outer or trophoblast layer of the fetal placenta, containing mainly trophoblast cells, but also Reichert's membrane, parietal endoderm, and, at later stages, part of the allantois; the vis...