Human sperm nuclei can transform into condensed chromosomes in Xenopus egg extracts

The conditions necessary for inducing pronucleus formation and chromosome condensation of human sperm nuclei were studied using Xenopus egg extracts. When lysolecithin (LC)-and dithiothreitol (DTT)-treated spermatozoa were incubated with egg extract prepared with an extraction medium containing 10 mM EGTA, sperm nuclei transformed into chromosomes, bypassing the pronuclear stage. On the contrary, LC-DTT-treated spermatozoa incubated in egg extract without EGTA mimicked LC-permeabilized Xenopus spermatozoa in that they underwent pronucleus formation accompanied by DNA synthesis and subsequent chromosome condensation in the correct chronological order. These observations suggest the importance of intracellular calcium for regulating the states of nuclear chromatin. LC-permeabilized human spermatozoa were not responsive to the egg extract without any additives, but formed pronuclei when incubated with the egg extract supplemented with 5-10 mM DTT. However, subsequent chromosome condensation of sperm nucleus did not occur in the DTT extract. This suggests that disulfide-reducing agent blocks transformation of sperm pronuclei into condensed chromosomes.