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Quantitative changes in cytoskeletal β‐ and γ‐actin mRNAS and apparent absence of sarcomeric actin gene transcripts in early mouse embryos

Actin is known to be synthesized both during oogenesis and in cleavage‐stage embryos in mice. Cytoskeletal β‐actin appears to be the major component, followed by γ‐actin, but the synthesis of α‐actin has also been inferred from protein electrophoretic patterns. We have studied the expression of cytoskeletal (β‐ and γ‐) and sarcomeric (α‐cardiac and α‐skeletal) actin genes at the level of the individual mRNAs in blot hybridization experiments using isoform‐specific RNA probes. The results show that there are about 2 × 104 β‐actin mRNA molecules in the fully grown oocyte; this number drops to about one‐half in the egg and less than one‐tenth in the late two‐cell embryo but increases rapidly during cleavage to about 3 × 105 molecules in the late blastocyst. The amount of γ‐actin mRNA is similar to that of β‐actin in oocytes and eggs but only about 40% as much in late blastocysts, indicating a differential accumulation of these mRNAs during cleavage. The developmental pattern of β‐ and γ‐actin mRNA provides a striking example of the transition from maternal to embryonic control that occurs at the two‐cell stage and involves the elimination of most or all of the maternal actin mRNA. There was no detectable α‐cardiac or α‐skeletal mRNA (i.e., < 1,000 molecules per embryo) at any stage from oocyte to late blastocyst, suggesting that the sarcomeric actin genes are silent during preimplantation development.