Expression of c-MYC in nuclear speckles during mouse oocyte growth and preimplantation development

Myelocytomatosis oncogene (c-myc) is a major transcriptional regulator that controls various biological processes, and its deregulated expression causes carcinogenesis. To investigate the involvement of c-myc in oogenesis and preimplantation development, the expression of c-MYC during these stages was examined by immunocytochemistry. A strong c-MYC signal was detected in the nucleus of growing and fully grown oocytes as well as in preimplantation embryos before the morula stage. The signal intensity decreased slightly at the morula stage, and no signal was detected in blastocysts. Close observation of the nucleus revealed that c-MYC was localized in small granules that appeared to be nuclear speckles controlling pre-mRNA splicing. Although the number of granules decreased during oocyte growth, their size increased. After fertilization, the granules of c-MYC disappeared from the pronuclei, and c-MYC was evenly distributed in the nucleoplasm at the 1-cell stage, but the granules reappeared at the 2-cell stage. These results suggest that c-myc is involved in oocyte growth and preimplantation development and that its role changes during these stages.