Shedding of the zona pellucida in normal pregnancy and in various hormonal states in the mouse. A scanning electron microscope study.

Abstract

Blastocysts were rinsed out of the uterus by glutaraldehyde on Day 4 and 5 in normally pregnant and in ovariectomized, hormone-treated mice and subsequently prepared for and examined by scanning electron microscopy (SEM). The zona pellucida surface showed no differences in early and late stages, either in normal pregnancy or after ovariectomy and administration of ovarian hormones. In all types of treatment, hatching of the blastocyst from the zona was observed. The shedding process was oestrogen-dependent and the zona-shedding rate of normal pregnancy could be provoked by 0.01 or 0.1 μg oestradiol-17 β given either late on Day 3 or early on Day 4 but not early on Day 3. A subimplantation dose of oestrogen (0.001 μg) did not seem to influence the shedding process, nor did 0.1 μg if pretreatment with progesterone was omitted after ovariectomy. The zona-free blastocyst in normal pregnancy usually showed signs of abembryonic proliferation and sometimes detachment injuries in the abembryonic pole, indicating incipient implantation. Similar appearances were noted in blastocysts from progesterone-maintained, ovariectomized mice given 0.01 or 0.1 μg oestradiol-17 β early on Day 4 but not on Day 3. In progesterone-maintained, ovariectomized mice given the above oestrogen doses on Day 3, the zona-free blastocysts were similar to those observed in corresponding animals not treated with oestrogen, i. e. without signs of incipient implantation at any stage. A subimplantation dose of oestrogen (0.001 μg) had no visible effect on the surface of the zona-free blastocyst. It is concluded that hatching occurs under normal conditions and can be provoked by various hormonal treatments. Further, the location or maturity of the morulae may account for the lack of oestrogen response in zygotes early on Day 3. Since a zona-shedding curve close to the normal one was obtained after oestrogen treatment that did not induce implantations, it is inferred that the oestrogen surge may not constitute a discrete peak.