Quantitation of maternal-fetal IgG transport in the chicken.

Quantitative and temporal features of maternal-fetal transport of IgG in the chicken have been determined by means of a sensitive and specific radioimmunoassay. The first step in this two-step process is the transport of maternal IgG from the hen plasma across the oolemma into the maturing oocyte in the ovarian follicle. Oocytes sequester IgG throughout most of their maturation from 10 mg pre-vitellogenic oocytes to fully mature 20 g oocytes ready to be shed into the oviduct. IgG uptake is exactly proportional to mass accumulation at every point. Thus, the concentration of IgG in the yolk is invariant throughout maturation of the oocyte (approximately 8 mg/ml) and the rate of uptake in this single specialized cell can be as great as 45 mg/day during its last 3 days of growth before ovulation. The second step, uptake of yolk IgG across the yolk sac and into the fetal circulation, is detectable at the earliest time points tested (Day 7), but accumulation into the embryonic plasma occurs at a relatively low rate (less than 100 micrograms/day) until a dramatic increase in uptake (greater than 600 micrograms/day) occurs in the last 3 days before hatching. The hatchling has 1-2 mg/ml of maternal IgG in its plasma. Increase in IgG uptake lags significantly (approximately 4 days) behind increase in mass by the developing embryo, suggesting that IgG uptake occurs predominantly in the last few days before hatching. This ensures that the hatching is endowed with those maternal IgGs which will be its planopy when it enters the hostile world.