Developmentally regulated loss and reappearance of immunoreactive somatic histone H1 on chromatin of bovine morula-stage nuclei following transplantation into oocytes

One difference between chromatin of bovine oocytes and blastomeres is that somatic subtypes of histone H1 are undetectable in oocytes and are assemble d onto embryonic chromatin during the fourth cell cycle. We investigated wh ether this chromatin modification is reversed when nuclei containing somati c H1 are transplanted into ooplasts. Donor nuclei obtained from morula-stag e bovine embryos were fused to ooplasts at different times before and after parthenogenetic activation of the ooplasts. After fusion, immunoreactive H 1 became undetectable, and the loss occurred more rapidly when fusion was p erformed near the time of ooplast activation compared with several hours af ter activation, when the host oocytes were at a stage corresponding to inte rphase. Although the loss of immunoreactive H1 occurred independently of DN A replication and transcription, exposure of reconstructed oocytes to cyclo heximide or 6-dymethylaminopurine (6-DMAP) delayed the loss of immunoreacti ve H1 from transplanted nuclei. During further development of nuclear-trans plant embryos, somatic H1 remained undetectable at the 2- and 4-cell stages , and it reappeared on the chromatin at the 8- to 16-cell stage, as previou sly observed in unmanipulated embryos. We conclude that factors in oocyte c ytoplasm are able to modify morula chromatin so that somatic H1 becomes und etectable, and that the amount or activity of these factors declines over t ime in activated ooplasts.