MEIOSIS OF MOUSE EGGS BEFORE AND AFTER SPERM PENETRATION

Mature mice were induced to ovulate by injections of PMS and HCG and their eggs were prepared, using an air‐dry technique, for the examination of chromosomes during the first and second meiotic divisions. Sixty percent of 72 eggs recovered from follicles between 10 and 8.5 hours before ovulation were at prophase, whereas 91%–64% of 139 eggs were at metaphase I from 7 to 3 hours before ovulation. Anaphase I and telophase I were seen in 17 eggs 4.5–0 hours before ovulation. The bivalents were clumped together in a single mass at the earliest stage of diakinesis. Separate, extended bivalents which underwent contraction up to metaphase I were observed later. Counts of bivalents in 82 eggs at diakinesis and metaphase I revealed no deviation from the haploid number (n = 20). The dyads in the first polar body were usually extended and somewhat diffuse in appearance, whereas those in the egg were not. A total of 295 eggs were examined from 24 females inseminated 1.5 hours after ovulation. At 3, 3.5, 4.5 and 5.5 hours after ovulation 3%, 20%, 26% and 43% of recovered eggs respectively were penetrated by sperm, indicating that a certain length of time may be required for capacitation of mouse sperm. Approximately 90% of eggs with an intact sperm head (Type I) were at metaphase II and nearly 100% of eggs with a swollen sperm head (Types III and IV) were at telophase II. Four distinct configurations of metaphase II, “spread,” “clumped,” “compact,” and “granular” were seen in 384 unfertilized eggs recovered from 36 females at various times after induced ovulation. The haploid number of dyads was observed in all of 70 eggs counted at metaphase II in the “spread” configuration. The proportion of unfertilized eggs with a “compact” metaphase II increased from 2% at 5–7 hours to 12% at 24–31.5 hours after ovulation, while the proportion of eggs with a “granular” metaphase II increased from 11% to 47% over the same period. The second meiotic metaphase exhibited these degenerative changes mainly at the end of the fertilizable period of the egg. Copyright © 1969 Wiley‐Liss, Inc., A Wiley Company