TIMING OF SPERM AND OOCYTE NUCLEAR PROGRESSION AFTER INTRACYTOPLASMICSPERM INJECTION

We investigated the time course of human oocyte activation after intra cytoplasmic sperm injection (ICSI) by observing the oocyte chromosome configuration at different times after injection. One day old human oo cytes were injected with spermatozoa and subjected to cytogenetic anal ysis at 2, 3, 4 and 5 h after injection, We found that anaphase is ini tiated in the vast majority of the oocytes between 2 and 3 h after inj ection, and that by 4-5 h after injection most of the oocytes have rea ched the chromatin mass stage, Two distinguishable stages of sperm nuc leus transformation were observed. The first phase - swelling - was re ached within 2 h after the injection and was independent of oocyte act ivation, The second phase - the 'brush'-like stage or decondensed chro matin stage - was found only in activated oocytes, Moreover, this stag e was not reached before the chromatin mass stage (late telophase) of the oocyte. The same proportion of metaphase II oocyte chromosome conf igurations and unchanged sperm nuclei was found at any given time afte r injection, We conclude that: (i) ICSI allows users to obtain an almo st synchronized population of activated oocytes; (ii) anaphase II is i nitiated in the majority of oocytes not later than 2-3 h after injecti on and telophase II is reached similar to 5 h after injection; and (ii i) there are two distinguishable phases of sperm nucleus transformatio n after ICSI: oocyte activation-independent swelling of the sperm head and oocyte activation-dependent chromatin decondensation which is cou pled to the beginning of oocyte chromosome decondensation.