Cleavage, Blastocyst Formation and Implantation

Immediately after fertilisation, the zygote begins a rapid series of mitotic cell divisions called cleavage as it is moved along the fallopian tube toward the uterus, a journey that takes roughly three to four days. Cleavage divisions are unusual in that the resulting daughter cells, called blastomeres, become progressively smaller with each division, so that the overall size of the whole embryo stays roughly the same as the original zygote even as the number of individual cells increases — the divisions increase cell number without an accompanying growth phase between divisions. The zygote first divides into a 2-celled embryo, then successively into 4-celled and 8-celled stages, and continues dividing while still enclosed within the protective zona pellucida inherited from the oocyte.

By about the time the developing embryo reaches the uterine cavity, cleavage has produced a solid ball of roughly 8 to 16 blastomeres called the morula (named for its resemblance to a mulberry). The morula continues to divide, and its cells begin to reorganise: a fluid-filled cavity, the blastocoel (or blastocoele), forms within the ball of cells, and the structure is now called a blastocyst. Within the blastocyst, the cells differentiate into two distinct populations with different developmental fates: an outer, single-cell-thick layer called the trophoblast, which surrounds the blastocoel and will go on to form the foetal contribution to the placenta and the foetal membranes, and a small, compact cluster of cells attached to the inner surface of the trophoblast at one pole, called the inner cell mass, which will develop into the embryo proper. …