Timing of sperm penetration, pronuclear formation, pronuclear DNA synthesis, and first cleavage in naturally ovulated mouse eggs.

Timing of development of naturally ovulated mouse eggs from sperm penetration to first cleavage, including that of DNA synthesis, was established. In an attempt to limit variability, partial synchronization of ovulation was accomplished by shortening the length of the dark period to five hours, and partial synchronization of sperm entry was attempted by mating the females soon after the ovulated eggs reached the ampulla and by limiting the period at which mating could occur to only 20 minutes. Evidence of sperm penetration (presence of one or more sperm in perivitelline space or inside the vitellus) was found beginning 1.75 hours after the end of the mating period. Pronuclei were formed three to four hours after sperm entry. Pronuclear DNA, synthesis began about eight hours postmating, 3.25 to 4.5 hours after pronuclear formation, or about 6.25 to 8.5 hours after sperm entry; it was completed in almost all zygotes by 16 hours postmating. The first completed cleavage division was found 17 to 18 hours postmating, and almost all eggs had cleaved by 20 hours.