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a Centre de recherche en reproduction animale, Faculté de médecine vétérinaire, Université de Montréal, Saint-Hyacinthe, Canada J2S 7C6
b Department of Obstetrics and Gynecology, McGill University, Montreal, Canada H3A 1A1
One difference between chromatin of bovine oocytes and blastomeres is that somatic subtypes of histone H1 are undetectable in oocytes and are assembled onto embryonic chromatin during the fourth cell cycle. We investigated whether this chromatin modification is reversed when nuclei containing somatic H1 are transplanted into ooplasts. Donor nuclei obtained from morula-stage bovine embryos were fused to ooplasts at different times before and after parthenogenetic activation of the ooplasts. After fusion, immunoreactive H1 became undetectable, and the loss occurred more rapidly when fusion was performed near the time of ooplast activation compared with several hours after activation, when the host oocytes were at a stage corresponding to interphase. Although the loss of immunoreactive H1 occurred independently of DNA replication and transcription, exposure of reconstructed oocytes to cycloheximide or 6-dymethylaminopurine (6-DMAP) delayed the loss of immunoreactive H1 from transplanted nuclei. During further development of nuclear-transplant 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 previously observed in unmanipulated embryos. We conclude that factors in oocyte cytoplasm are able to modify morula chromatin so that somatic H1 becomes undetectable, and that the amount or activity of these factors declines over time in activated ooplasts.
2 Correspondence: Lawrence C. Smith, Centre de recherche en reproduction animale (CRRA), Faculté de médecine vétérinaire, Université de Montréal, C.P. 5000, Saint-Hyacinthe, Canada J2S 7C6. FAX: 450 778 8103; smithl{at}medvet.umontreal.ca
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