Localization of replicated DNA‐containing sites in preimplantation bovine embryo in relation to the onset of RNA synthesis

Abstract

The distribution of embryonic DNA in nuclei of blastomeres of early-preimplantation cow embryos was studied by autoradiography. Two-, 4-, 8-, and 16-cell embryos were cultured for 18-20 h in the presence of [methyl-3H]thymidine to ensure that all replicated embryonic DNA was labeled. In nuclei of blastomeres before the onset of transcription, taking place in the progressed 8-cell cleavage division of the cow embryo, labeled DNA was distributed excentrically at the nuclear periphery. After transcription started, the distribution of the labeled DNA was uniform throughout the nucleus. The progressive association of DNA with the nucleolus-precursor body (NPB) also correlated with the expected onset of rRNA synthesis. The penetration of the labeled DNA into NPB was seen to start contemporaneously with the formation of a big central vacuole in NPB, but remained restricted, at this stage, to a few points of contact with the adjacent nucleolus-associated chromatin. No labeled DNA was detected inside this big vacuole, still showing a uniform nucleoplasmic texture. The inside of the nucleolus was penetrated by DNA in the next step of nucleologenesis when secondary small vacuoles were formed and when the first signs of nucleolar transcription were detected. These small vacuoles contained a coarse fibrillar component which was a frequent site of labeled DNA detection. This study of shifts of DNA-containing sites during transition from maternal to embryonic genome expression correlates closely with previous evidence obtained by a study of genome reactivation which was situated in the late 8-cell cleavage division of cattle early embryogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)