Embryology and foetal development

It only takes 12 to 24 hours for the egg to live after which shedding of the unfertilized egg occurs (menstruation), compared to the sperms that can last up to 48 hours depending on individuals (Hatasaka, n.d.). Unlike one egg that is released in each ovulation, there are many sperms released during the sexual intercourse. The sperms ascend into the ampulla section of the fallopian tube to the matured egg. The egg is covered by protective layer called zona pellucid, mainly composed of glycoproteins, which the sperm that comes into encounter with the ovum penetrates through biochemical events. It entails the release of a digestive enzyme onto the layer that the sperm plasma membrane penetrates to fuse with that of the egg, causing the sperm nucleus to move into the ovum. When the nuclear membrane of the sperm and egg fuse, their nuclear genomes combine together to form a zygote.
The zygote is a diploid cell and has to undergo differentiation as the development of the embryo occurs. The zygote is moved down to the uterus for implantation and within few days, the zygote undergoes cell division to form a group of cells called blastula, whose cavity is filled with blastocoel fluid. It is termed the morula stage and comprises of first cell differentiation where the inner cells of the blastula separate to form the embroblast and outer the trophoblast, which develop into the embryo and placenta respectively (Embryology.ch, n.d). More hundreds of cells are cleaved from the blastula, some of which contain maternal chemicals, while others do not. The presence of chemicals influences the gene expression of each cell in the embryo’s development. Certain genes in the cells are turned on while others off, and depending on the location of the cell during the early embryo development, the type of the cell is determined. As the patterns of gene expressions are altered, the cells are differentiated into brain, blood, bone and