Timing of the stages of the maturation divisions, ovulation, fertilization and the first cleavage of eggs of adult mice treated with gonadotrophins.

Abstract

SUMMARY The stages of the maturation divisions, ovulation, fertilization and the first cleavage in the eggs of two strains of adult mice have been timed following superovulation treatment. Superovulation was induced by a priming injection of pregnant mares' serum followed after a 40 hr interval by human chorionic gonadotrophin (HCG). The oocytes were in the dictyate (germinal vesicle) stage until 2 hr after the injection of HCG. They then completed the prophase of the first maturation division, and the first metaphase plates were found 30 min later. From approx. 4½ to 8 hr all oocytes were in metaphase. The first maturation division and extrusion of the first polar body were then rapidly completed just before ovulation. Ovulation began, as judged by the presence of eggs with adherent cumulus in the uterine tubes, 11 hr after the injection of HCG and was virtually complete by 14 hr. One strain of mice ovulated slightly later than another strain. The time at which ovulation occurred was very similar in mated and unmated females. Fertilization and the first cleavage were studied in females which mated before ovulation began, and the mean intervals between various stages were as follows. Spermatozoa penetrated through the cumulus and zona pellucida in approx. 1 hr, and remained in the perivitelline space for 50 min. Pronuclei were formed 4¼ hr later, and the first cleavage division occurred at approx. 25 hr after penetration of spermatozoa into the eggs. These estimates are similar to those reported by other workers. The formation of oocytes in the adult mammalian ovary, the timing of early stages in the prophase of the first maturation division, and the relation between natural and induced ovulation are discussed.