ABSTRACT

Cleavage-stage embryos range from the two-cell stage to the morula stage when the embryo consists of more than eight cells and each blastomere is so closely juxtaposed so as to give the appearance of no individual internal cellular boundaries as a result of the formation of tight junctions between individual blastomeres. This light microscopic appearance at the morula stage is known as compaction. During cleavage, the embryo does not increase in size but undergoes successive reduction in blastomere size as cell numbers increase with each cleavage division. For the rst 2 days of development, the embryo is under the control of stored maternal message inherited from the oocyte and it is not until the four-to eight-cell transition that the embryo is under the control of an activated embryonic genome [2]. Many embryos undergo developmental arrest between the four-to eightcell stages at the time the embryonic genome becomes activated, and this embryonic block can be exacerbated by poor in vitro culture conditions.