Immunocytochemical Localization of Methylated DNA in Human Pachytene Oocytes and Questions of Parental Imprinting

SUMMARY

A study of immunoreactive sites utilizing an antibody to 5-methylcytidine on human mid-late pachytene oocyte bivalents was made. All centromeres of bivalent four and 70% of bivalent six were negative, while remaining centromeres were always positive. 360 of the entire complement of 607 chromomeres were found to be positive for reaction product. When compared with previous data from this laboratory for human spermatocyte bivalents obtained with the identical antibody and method, it was concluded that the percentage (59.3%) of methylated chromomeres of oocytes was less than that of spermatocytes (70.7%). 135 sites of methylation were found to be identical on the bivalents of both sexes and an additional 44 found to be potentially identical. Unique sites made up 47% of the oocyte complement and 30% of the spermatocyte complement. These findings of over-all methylation at this stage of human germ cell development are discussed with reference to theories of imprinting of the genome at fragile sites and autosomal and sex chromosome locations.