["2-cell block" and the viability of BALB/c embryos when explanted at the second cleavage cell cycle]

The embryos of BALB/c mice were explanted from hormonally stimulated females in a standard culture during the second cleavage division corresponding to the stage of the embryonic genome activation. Observations over the cultivated embryos have shown that explantation of the embryos both at the two-cell stage and during the first half of the second cleavage division brings most embryos (more than 80%) to the state of "two-cell block." Thereafter, the number of blocked embryos progressively decreases to less than 20% after explantation in the end of the cleavage division, which corresponds to the proportion of the embryos in the reproductive tract which were delayed at the 2-cell stage by the beginning of the third cleavage division. After explantation at the time close to the middle of the cleavage division, the survival of the blocked embryos is reduced and in the end of the two-cell stage again increases. The BALB/c embryos should stay in the maternal reproductive tract during the entire second cleavage division before explantation in order to provide for viability and developmental potencies in vitro close to those in vivo. The results obtained are discussed with reference to the controlling mechanisms of early embryogenesis and cell cycle.