Q.Define Fertilization. Write a short note on sequence of events in embryo development upto Blastocyst formation. OR Describe in brief hormonal control of menstrual cycle in females.
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Start your 14-day free trial to unlock the full solution →Fertilization forms a zygote that divides repeatedly to become a blastocyst before implantation — OR — the menstrual cycle is driven by the cyclical rise and fall of FSH, LH, oestrogen and progesterone.
Part 1 — Fertilization and events up to blastocyst formation
Fertilization is defined as the fusion of a haploid male gamete (sperm) with a haploid female gamete (secondary oocyte) to produce a diploid zygote. In humans, fertilization occurs in the ampullary-isthmic junction of the fallopian tube (oviduct), typically within 24 hours of ovulation. Only one sperm is allowed to fertilise the ovum, since after entry of one sperm the zona pellucida undergoes changes that block the entry of additional sperm.
Sequence of events after fertilization, up to the blastocyst stage:
- The diploid zygote begins mitotic cell divisions, called cleavage, as it moves through the fallopian tube toward the uterus.
- Cleavage produces progressively smaller cells called blastomeres — the zygote first divides into a 2-celled embryo, then 4, then 8 cells, and so on.
- The embryo with 8 to 16 blastomeres is called a morula (resembling a mulberry), which continues to divide and transforms into the blastocyst.
- In the blastocyst, the blastomeres are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass, with a central fluid-filled cavity called the blastocoel.
- The trophoblast layer then attaches to the endometrium of the uterus, and the uterine cells divide rapidly to cover the blastocyst — this is implantation, marking the beginning of pregnancy. The inner cell mass subsequently differentiates to form the embryo proper.
Part 2 (OR) — Hormonal control of the menstrual cycle
The menstrual cycle (~28-29 days) is regulated by cyclic changes in pituitary and ovarian hormones:
- Menstrual phase (Days 1-5): Falling progesterone and estrogen (due to corpus luteum degeneration in the previous cycle) cause the endometrial lining to break down and be shed as menstrual flow. …
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