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FERTILIZATION
Fertilization is the fusion of the spermatozoon and an ovum. It takes place in the fallopian tube. In human, fertilization is preceded by the process of sexual intercourse or coition, where a man discharges semen into the vagina. The semen of about 2cm3 containing about 100 million sperms is deposited in the vagina near the cervix of the uterus.
FUSION OF GAMETES
Spermatozoa discharged into the vagina during copulation swim up to the cervix into the uterus and up the fallopian tube where fertilization takes place.
When the sperm encounters the ovum, it usually digests the jelly covering of the egg by means of the enzyme contained in the acrosome. The sperm itself to the layer of follicle cells outside the secondary oocyte as corona radiate. There is a fusion of egg plasma and sperm plasma membrane. The yolk at the point where the sperm penetrate the egg bulges out to form a cone. Once the sperm penetrates the egg, the yoke change to a membrane known as vitelline membrane. One other very important event is that the tail of the sperm is lost and the middle piece disintegrates. The egg now travels down the fallopian tubes and the second meiotic division takes place consequently upon fusion of sperm with egg. It is noteworthy that once a sperm has entered the ovum, another sperm cannot enter it because of the development of a protective cortical layer.
IMPLANTATION
The zygote resulting from the fusion of the sperm cell and the egg contains the characteristic features of both the man and the woman involved in the sexual intercourse. The zygote under a number of cell division into two cells, four cells, eight cells, etc. until a ball of cell known as blastocyst is obtained as it passed further down the fallopian tube on it way to the uterus. On reaching the uterus, it becomes embedded in the uterus wall which is thick and highly vascularised. This process is known as implantation and as a result the woman is pregnant
Implantation is the process by which the fertilised egg or zygote enter into an intimate relationship which leads to its embedment in the uterine wall such that it cannot be flushed out. It is the first stage of development of placenta. It involves the adhesion of the embryonic trophoblast (mitotically divided mass of cell at the early stage of embryonic development) and micorvilli of the mother and embryonic cells.
EVALUATION
- Define fertilization and implantation
- Explain the process involved in fusion of gametes
SUB-TOPIC 3: DEVELOPMENT OF EMBRYO
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