ABSTRACT
The mode of zona pellucida denudation and blastocyst proliferation and attachment was investigated in Wistar rats during day 5 of pregnancy and in ovariectomized pregnant animals given 5.0 mg progesterone daily until the 10th day, when 0.2 µg oestradiol was injected together with progesterone.
During day 5 of pregnancy, when only 5 empty zonae were detected from 32 animals, zona denudation occurred by lysis, which took 6 h. Zona denudation occurred by shedding in the ovariectomized pregnant animals; 190 empty zonae were detected from 40 animals. Oestradiol administration to the experimental animals caused lysis of the shed zonae 24-30 h after oestradiol treatment.
Air-dried preparations of the embryos showed that the blastomeres proliferated exponentially from a mean of 27.52 ± 4.0 cells per embryo at 12.00 h on day 5 of pregnancy to 53.96 ± 2.58 cells per embryo at 22.00 h, with a cell doubling time of 10 h in normal animals. In the ovariectomized animals, blastocysts were arrested at about the 100-cell stage on the 10th day. Oestradiol administration to the experimental animals did not induce mitosis; the embryos implanted without further cell division.
The number of free embryos recovered declined from a mean of 12.0 per animal at 12.00 h to 4.25 per animal at 22.00 h on day 5 of pregnancy. The number of embryos recovered from the ovariectomized animals varied between one and 11 per animal .
The results show that cell division continued in embryos within ovariectomized pregnant animals in the absence of oestrogen, until about the 100-cell stage. There was no further cell division before nidation when oestradiol was administered. The empty zonae were, however, lysed between 24 and 30 h after oestradiol treatment. The zona lytic factor appears to be oestradiol dependent and of maternal origin. A causal relationship exists between zona lysis and embryo attachment, which is independent of the number of cells per blastocyst.