Extranucleolar genome reactivation: topochemical studies on early bovine embryo. A review

A review is presented of fine-structure autoradiographic [incorporation of (5-3H)uridine] and immunoelectron microscope (localization of small nuclear ribonucleoproteins--snRNPs) data on the onset of extranucleolar transcription in early preimplantation bovine embryo. First incorporation (5-3H) uridine into blastomere nuclei nucleoplasm occurs, in the early cow embryo, during the 8-cell stage. Both the degree of chromatin condensation as well as the intensity of labeling increase as the fourth cycle of the embryonic blastomeres progress. However, neither the level of extranucleolar chromatin condensation nor the degree of labeling usually detected in somatic cells have been observed during this stage but occurred only in later stages of embryo development. The association of the label with the periphery of the condensed chromatin demonstrates, as in other cell types the sites of newly synthesized hnRNA. A further insight into the functional microarchitectural changes of the early cow embryo nuclei in relation to the onset of transcription has been obtained by immunoelectron microscope studies on Lowicryl K4M embedded material. Using this technique, the pattern of the contrast in the preparations following uranyl/lead staining was comparable to that obtained by the Bernhard's (1969) regressive staining for preferential nuclear-RNP visualization. In this way, the coordinate appearance of perichromatin fibrils on the borderline of the already condensed chromatin was evident during the 8-cell stage. Especially in the "in vitro" produced embryos the condensed chromatin occurred in marked blocks with a clearly defined perichromatin region in the thin sections. In the same nuclear area Sm-antigen (associated with a group of snRNPs) has concentrated and localized to perichromatin fibrils.(ABSTRACT TRUNCATED AT 250 WORDS)