Cytological and Autoradiographic Study on Ribonucleic Acid Synthesis in Preimplantation Eggs of the Syrian Hamster

Ribonucleic acid synthesis in preimplantation stage embryos of the Syrian hamster was studied cytologically and autoradiographically using whole-mount squash preparations. Cytologically, the first cleavage chromosomes were large and extended, and the relational coiling was detected. Pronucleus and the early 2-cell stage nucleus were morphologically similar and contained large amount of heteropycnotic blocks, but no ribonucleoprotein was demonstrated either with May-Grünwald-Giemsa staining or with acridine orange. In the late 2-cell stage, however, the morphology of the nucleus differed considerably from that of pronucleus and of the early 2-cell stage. Several ribonucleoprotein-containing nucleoli appeared in each nucleus, and the rest of nuclear area showed coarse chromatin network with scattered heteropycnotic blocks. Chromosomes, which were extended in the first cleavage, became gradually shorter in the course of preimplantation development. In some metaphases in the blastocyst, chromosomes were approximately as compact as those of adult tissues. Nucleolar RNA synthesis was demonstrated in the late 2-cell stage. Heavy H3-UR incorporation appeared in the nucleoli of blastomeres from the late 2-cell stage to the blastocyst. Chromatin was also active in RNA synthesis. No grains above the background level were observed in the cytoplasm of metaphase and anaphase cells of these embryos.