Abstract
The pattern of cell proliferation of early rat embryos in vivo was examined. Naturally mated female rats were sacrificed every 2 to 3 h between 0000 h on Day 1 (day of estrus) and 2200 h on Day 5 of pregnancy. Embryos were obtained by flushing the oviducts and uterus with M2 medium. Embryonic stages were morphologically categorized into 12 stages, i.e. 1-cell, 2-cell, 3-cell, 4-cell, 5-7 cell, 8-cell, 8-cell-morula, morula, early blastocyst, blastocyst, large expanded blastocyst, and hatched blastocyst. The 2-cell stage lasted 31 h and was longer than any other stage. Duration of the 1-, 4-, 5-7-and 8-cell stage was 23, 11, 5, and 4 h, respectively. Embryonic shape at the 4-cell stage was either square or rhomboidal, and embryos at the 8-cell stage were either circular or rectangular. After the 8-cell stage, an increase in cell numbers was more dependent on time than the progression of developmental stages; thus, the number of blastomeres in each embryonic stage after the 8-cell stage varied considerably. Understanding the chronology of rat embryonic development in vivo will provide the basic knowledge that is required for manipulating the rat embryo.
