Fertilization of the mouse oocyte: Sequence and timing of nuclear progression to the two‐cell stage

From 88 superovulated, mated CFI (Carworth) females, 2643 eggs were collected at two hourly intervals between 12 and 36 hours after administration of human chorionic gonadotrophin and then fixed, stained and examined as whole‐mounts by light microscopy. Mating and ovulation were essentially complete at 10 and 14 hours respectively. Metaphase II oocytes continued to decrease in number until 20 hours. Telophase II, a maximum at 16 hours (22% of all oocytes), was not seen after 20 hours. Pronuclei first appeared at 16 hours and at 24 hours, 87% of the ova were pronuclear. Metaphase of first cleavage, first detected at 28 hours, represented 17% of the ova at 32 hours. The first cleavage division occurred between 30 and 34 hours. The pronuclear chromatin network, which is always visible, is more diffuse in the larger, male pronucleus. During first cleavage prophase, the chromatin strands shorten and distinct chromosomes appear, those of male origin being less condensed. The two chromosome groups then combine to form first cleavage metaphase. Pronuclear fusion was not observed. Twenty‐three per cent of the ova had not undergone first cleavage by 36 hours: 6% were at earlier stages and presumably arrested while 14% were fragmented or had degenerated metaphase II chromosomes. Anomalies of oocyte maturation and fertilization accounted for 1.6% of all ova.