Distribution of 3H in rat conceptus cultured in vitro following brief administration of [3H]thymidine.

Rat embryos with intact visceral yolk sacs, explanted at 12 1/2 days of gestation, were cultured in vitro for up to 60 min in medium consisting of fetal calf serum, Eagle's MEM, and [3H]thymidine (1.2 kBq ml-1), using the roller bottle method. The total amount of 3H incorporated into the conceptus during the 60-min incubation was 79.2 Bq, and approximately 33, 23, and 44% of this activity was distributed to the embryo, the yolk sac, and the fluid in the exocoelom and amniotic cavity, respectively. The rate of 3H accumulation in conceptuses decreased with time in culture. It appeared that the decrease in the viability of the conceptus was not responsible for this phenomenon. The concentration of 3H in the yolk sac, i.e., 3H activity per gram wet weight, was 2.1 times that in the medium at the end of culture. In contrast, the 3H concentration in the embryo was significantly lower than that in the medium. These findings suggest that the visceral yolk sac of rat conceptuses may act as a barrier to the transport of tritiated thymidine between the medium and embryo.