Tanvi Mahajan, Subha Ganguly and Nileshkumar Pagrut
Embryogenesis start from the time of fertilization of female germ cell or ovum by a sperm. The zygote formed is surrounded by a strong membrane of glycoproteins called the zona pellucida 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, mammalian embryos undergo compaction, where the cells bind tightly to each other, forming a compact sphere. After compaction, the embryo is in the morula stage (16 cells). Cavitation ocurrs next, where the outermost layer of cells - the trophoblast secrete water into the morula. As a consequence of this, when the number of cells reaches 40 to 150, a central, fluid-filled cavity known as 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 and lasts approximately until the implantation in the uterus. The blastocyst is characterized by a group of cells, called the inner cell mass (also called embryoblast) and the trophoblast (the outer cells).