Abstract
To investigate the regulation of cell cycle during preimplantationdevelopment, the expression of cyclins was examined in early mouse embryos.Wefirst isolated a cDNA fragment of mouse cyclin A, corresponding to cyclin boxdomain, and determined the sequence. Mouse cyclin A is highly homologous tohuman cyclin A (98.9% in amino acid).The comparison of amino acid sequence ofcyclin A with those of B-and D-type cyclins revealed the sequences specific for A-and B-type cyclins and for A-and D- type cyclins. It is possible that thesesequences are involved in the association with cdc2 and cdk2 kinases, since A- and B- type cyclins are known to be associated with cdc2 kinase and A-and D-typecyclins are with cdk2 kinase. The expression of cyclin mRNAs was exmined by theRT-PCR assay in unfertilized oocytes and the embryos at the 2-cell, 4-cell, morulaand blastocyst stages. Cyclin A mRNA was expressed in unfertilized oocytes and thelevel was gradually decreased to the blastocyst stage. The B-type cyclins whichregulate G2/M phase transition were expressed in a low level at the 2-cell stage.The level of cyclin Bl was almost constant during preimplantation develoment, except for at the 2-cell stage. The level of cyclin B2 was decreased at the 2-cellstage and then increased from the morula to blastocyst stage. The long G2 phase atthe 2-cell stage may be attributable to the decrease in the level of expression of Btypecyclins. The D- type cyclins which regulate Gl/S transition were not constantlyexpressed. The expression of cyclin Dl was detected in only 2-cell embryos andblastocysts. The expression of cyclin D2 was not detected all through the preimplantationdevelopment. Cyclin D3 was detected in unfertilized oocytes. The level wasdecreased to the 4-cell stage and then increased from the morula to blastocyststage.It is possible that the regulation of Gl phase in early embryos is differentfrom that in most somatic cells in which D-type cyclins are required for Gl/Stransition.
