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                                                                                                            A sperm has just fertilised a human egg in the Fallopian tube. Trace the events that the fertilised egg will undergo upto the implantation of the blastocyst in the uterus.

Answer:

The fertilised egg will undergo the following events upto the implantation of the blastocyst in the uterus.

(i) The mitotic division called cleavage, starts as the zygote moves through the isthmus of the oviduct towards the uterus and forms 2, 4, 8, 16 daughter cells called blastomeres.

(ii) The embryo with 8-16 blastomeres is called a morula.

(iii) The morula continues to divide and transforms into blastocyst as it moves further into the uterus.

(iv) Blastocyst is composed of an outer envelope of blastomeres called trophoblast and an inner group of cells attached to trophoblast, called inner cell mass.

(v) The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as an embryo.

(vi) After attachment, the uterine cells divide rapidly and cover the blastocyst.

(vii) As a result, the blastocyst becomes embedded in the endometrium of the uterus. This is called implantation and it leads to pregnancy.