Human Embryogenesis - Cleavage

Cleavage

A human develops from a single diploid cell called a zygote, which results from the fusion of two reproductive cells; an ovum (egg) being fertilized by a single spermatozoon (sperm). The cell is surrounded by a strong membrane of glycoproteins called the zona pellucida (membrane derived from the ovum) which the successful sperm has managed to penetrate.

The zygote undergoes cleavage, increasing the number of cells within the zona pellucida. After the 8-cell stage, embryos undergo what is called compactation, where the cells bind tightly to each other, forming a compact sphere. After compactation, the embryo is in the morula stage (32 cells). Cavitation occurs next, where the outermost layer of cells - the trophoblast - secrete fluid into the morula.

As a consequence of this, when the number of cells reaches 40 to 150, a central, fluid-filled cavity (blastocoel) has been formed. The zona pellucida begins to degenerate, allowing the embryo to increase its volume. This stage in the developing embryo, reached after four to six days, is the blastocyst (akin to the blastula stage), and lasts approximately until implantation in the uterus, and is referred to as the preimplantation phase of development.

Each cell of the preimplantation embryo is totipotent. That is, each cell has the potential to form all of the different cell types in the developing embryo. This totipotency means that some cells can be removed from the preimplantation embryo and the remaining cells will compensate for their absence. This has allowed the development of a technique known as preimplantation genetic diagnosis (PGD), whereby a small number of cells from the preimplantation embryo created by IVF, can be removed by biopsy and subjected to genetic diagnosis. This allows embryos that are not affected by defined genetic diseases to be selected and then transferred to the mother's uterus.