Turnover of basic chromosomal proteins in fertilized eggs: a cytoimmunochemical study of events in vivo.

Abstract

The chromosomal complements of mouse oocytes, ova and fertilizing sperm have been studied by immunofluorescence with specific antisera to the basic protein fraction of sperm nuclei and to histones H2b and H4, and by staining with ethidium bromide. These studies support the hypothesis that the chromosomes of the oocyte in maturation incorporate unique basic protein(s) similar to those incorporated during spermogenesis. That similarity is characterized by immunologic cross-reactivity. The basic proteins of the fertilizing sperm nucleus and the cross-reactive moiety of the 2 haploid complements of the ovum are displaced simultaneously, shortly after sperm entry. Because the unique basic proteins incorporated into the oocyte chromosomes do not, as in the spermatogenic sequence, entirely replace the histones, the maternal chromosomes display histones H2b and H4 at all postfertilization stages examined, whereas the decondensing paternal complement, for an interval before maturation of the pronuclei, contains neither sperm basic chromosomal proteins nor histones. Sequential staining of the same specimens with ethidium bromide revealed well-organized nuclear morphology of the residual DNA complex. For an as yet undefined period in the transformation from spermatozoal to embryonic genome, the chromatin is apparently devoid of a complement of basic proteins.