Fertilisation or fusion of gametes is the most vital event in the process of sexual reproduction as it results in new life. In humans, while the male gamete or sperm is motile; the female gamete or ovum is non-motile. Therefore, for fertilisation to occur the two gametes must be brought together. This is achieved through insemination where the penis releases semen, filled with thousands of sperms, into the vagina. Interestingly, while half of the total sperms released carry the X-chromosome, the other half contain Y-chromosome.
These sperms then rapidly move through the cervix, pass through the uterus and finally reach the ampullary-isthmic junction of the fallopian tube. In the meantime, the ovum, after completing the first meiotic division, gets released into the fallopian tube with the rupture of the Graafian follicle. Upon its release, the ovum begins to undergo the second meiotic division. However, the division does not go beyond the first phase. Interestingly, the ovum contains only the X-chromosome. Studies have shown that the ovum is filled with cytoplasm called the yolk or vitellus surrounded by a membrane called the vitelline membrane.
This vitelline membrane is ensconced in another membrane called zona pellucida. Once it reaches the ampullary-isthmic junction, the ovum gets bombarded by several thousand sperms. However, only one sperm penetrates the zona pellucida and initiates the fertilisation process. After gaining entry through the zona pellucida, the acrosome present over the sperm’s nucleus, starts secreting enzymes that aids the sperm head to get through the ovum’s cytoplasm, while the sperm sheds its tail. The fertilised egg now resumes the remaining phases of the second meiotic division.
About 3 days after fertilisation, the zygote contains nearly 8 to 16 cells and is called a morula. Over a period of two days, the morula descends down into the uterus and transforms itself into a blastocyst- a hollow ball composed of about 100 blastomeres which are arranged in two layers. The outer layer, called the trophoblast, eventually gives rise to the placenta while the inner layer consisting of a group of cells called the inner cell mass eventually differentiates to form the embryo. Interestingly, at the blastocyst stage, the zygote, now called the embryo gets attached to the uterus as the tropoblast grows outwards and penetrates the endometrial lining of the uterus. In response, the endometrial cells divide and start surrounding the blastocyst. This causes the blastocyst to sink and get implanted into the uterus. Once implantation has occurred, pregnancy gets initiated. Moreover, after implantation, the inner mass cells, get differentiated and begin to form the embryo. Embryo formation is thus a complex process that begins with fertilisation and ends with implantation of the embryo.