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Human Reproduction — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYHuman Reproduction4 Marks
Question
Explain the embryonic development upto blastula formation in human ?###Draw a labelled diagram of blastocyst formation.###Define Cleavage ? Explain the development of the zygote upto blastocyst formation.
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Answer

Fertilization : The fusion of male and female gametes produces diploid zygote. This process is called  fertilization.
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Cleavage : Structures and events proceed anticlockwise. At ovulation, the secondary oocyte leaves the ovary. A single sperm penetrates the zona pellucida, and fertilization occurs in the oviduct. As the zygote moves along the
oviduct, it undergoes cleavage to produce a morula (Fig.). The blastocyst forms and implants itself in the uterine lining.
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Fig. : Cleavage : In mammals, the cleavage is holoblastic or complete. First cleavage is vertical, second is also vertical but at right angle to the first cleavage. Third cleavage is horizontal. After this, the divisions are quick and irregular. As a result of this a solid ball of cells-morula is formed.
After fertilization the zygote passes down the fallopian tube. The cleavage takes place which is holoblastic in man and a many celled morula (Fig.) is formed having a cavity, the blastocoel (Fig.). It takes about 3 days. The morula is of the size of the zygote because even though the cleavage has taken place but there has not been growth of these newly formed cells. The cleaved cells are termed blastomeres. The outer layer of blastomere is called the trophoblast. (Gk. Troph = nourish and blast = bud).
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Fig. : Blastulation-formation of blastula from morula is called blastulation. Morula is changed to blastocyst. Its outer layer is called trophoblast. A group of cells are to the inner surface of trophoblasts. This is called inner mass of cells. As a result of cleavage a cavity is produced which is called blastocoel. From the inner cell mass ectoderm and endoderm are produced. Trophoblast is extra embryonic membrane, which helps in nutrition of embryo.
The trophoblasts at one point differentiate into thickened mass of cells, the inner cell mass. The inner cells mass eventually becomes the foetus. This stage having an inner cell mass is called blastocyst. Blastocyst is formed on the 5th day.
The blastocyst on reaching the uterus remains in the lumen for about two days. Its zona pellucida is dissolved and blastocyst comes in contact with the wall of the uterus, the endometrium. The trophoblasts derive nourishment from endometrium, divide repeatedly and within 6-7 days the balstocyst sinks down in the endometrium, and is embedded in it. This whole process is termed implantation.
Now trophoblasts form finger-like projections, the trophoblastic villi which liberate enzyme and dissolve the wall of arterial and venous blood vessels in the endometrium. At this stage, the exchange of nourishment, oxygen and excretory materials between the trophoblasts and maternal blood of the uterus wall is accomplished through the trophoblastic villi. Later on this function is performed by placenta which is formed by the fusion of allantois and chorion. 

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