(a) Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place. [5]
OR
(b) (i) Case studies of some couples are given below. They were not able to have kids even though the parents were not taking any precautions. They also do not wish to adopt a child or take the help of donors. Study these cases and suggest an appropriate Assisted Reproductive Technology that could help them.
Case I – Female – Normal Reports. Male – Normal sperms but no connection between epididymis and vas deferens. [1]
Case II – Female – Normal eggs but Fallopian tube blocked. Male – Normal sperms. Male reports are normal. [1]
Case III – Female – Normal Reports. Male – Low sperm count with less motility. [1]
(ii) Copper releasing intrauterine devices are very effective and popular contraceptive devices. Name any two copper releasing IUDs. Write two reasons that make them effective contraceptives. [2]
(a) Trace the events from fertilization till implantation of blastocyst in human female. Also mention the site where fertilization takes place. [5]
OR
(b) (i) Case studies of some couples are given below. They were not able to have kids even though the parents were not taking any precautions. They also do not wish to adopt a child or take the help of donors. Study these cases and suggest an appropriate Assisted Reproductive Technology that could help them.
Case I – Female – Normal Reports. Male – Normal sperms but no connection between epididymis and vas deferens. [1]
Case II – Female – Normal eggs but Fallopian tube blocked. Male – Normal sperms. Male reports are normal. [1]
Case III – Female – Normal Reports. Male – Low sperm count with less motility. [1]
(ii) Copper releasing intrauterine devices are very effective and popular contraceptive devices. Name any two copper releasing IUDs. Write two reasons that make them effective contraceptives. [2]
PRIMARY (a) — Fertilisation to implantation
Site of fertilisation: the ampullary-isthmic junction of the fallopian tube (oviduct). Fertilisation is not certain unless the ovum and sperms are transported simultaneously to this region.
Sequence of events
- Sperm–ovum contact: A sperm reaches the ovum and its head contacts the zona pellucida; the acrosomal reaction releases enzymes that let the sperm penetrate.
- Zona reaction: Entry of a sperm induces changes in the zona pellucida that block polyspermy (prevent further sperms entering).
- Completion of meiosis: Sperm entry stimulates the secondary oocyte to complete its second meiotic division, forming a haploid ovum (ootid) and a second polar body.
- Syngamy: The haploid sperm nucleus fuses with the haploid egg nucleus to form a diploid zygote (2n).
- Cleavage: As the zygote moves through the isthmus toward the uterus, it undergoes rapid mitotic divisions (cleavage) forming 2, 4, 8, 16 blastomeres.
- Morula: The 8–16 cell stage forms a solid ball called the morula, which continues to divide and moves into the uterus.
- Blastocyst: The morula transforms into a blastocyst; its blastomeres arrange into an outer trophoblast and an inner group, the inner cell mass (embryoblast).
- Implantation: The trophoblast attaches to the uterine endometrium; the uterine cells divide and envelop the blastocyst, which becomes embedded in the endometrium — this is implantation, leading to pregnancy.
OR (b) — briefly
- Case I (normal sperm, no epididymis–vas deferens connection): ICSI — sperm collected surgically and injected directly into the ovum.
- Case II (blocked fallopian tube, normal eggs and sperm): IVF followed by embryo transfer (ZIFT/IUT) — fertilisation done in the lab, embryo placed in the uterus.
- Case III (low sperm count, low motility): ICSI (or IUI/artificial insemination) — a single viable sperm injected into the ovum.
- (ii) Copper IUDs: CuT and Cu7 (also Multiload-375). They are effective because the released Cu ions suppress sperm motility and fertilising capacity and increase phagocytosis of sperms in the uterus, making it unsuitable for fertilisation.
Marking Scheme
- 11 mark: site of fertilisation correctly stated as the ampullary-isthmic junction of the fallopian tube (oviduct).
- 21 mark: acrosomal/zona reaction and completion of second meiotic division of the secondary oocyte forming ovum + second polar body; syngamy forming diploid zygote.
- 31 mark: cleavage divisions forming blastomeres and the morula stage moving to the uterus.
- 41 mark: formation of blastocyst with trophoblast and inner cell mass (embryoblast).
- 51 mark: implantation — trophoblast attaches to and embeds in the endometrium leading to pregnancy. [OR (b): 1 mark each for correct ART in Cases I–III (ICSI; IVF-ET/ZIFT; ICSI or IUI); 1 mark naming two Cu-IUDs (CuT, Cu7, Multiload-375); 1 mark for two valid reasons — Cu ions suppress sperm motility/fertilising capacity and increase phagocytosis of sperms.]
Hint
Path: ampullary-isthmic junction → zygote → cleavage → morula → blastocyst (trophoblast + inner cell mass) → implantation; for the OR, match each defect to the right ART.
Quick Oral Answer
Fertilisation occurs at the ampullary-isthmic junction; the sperm triggers the secondary oocyte to finish meiosis, syngamy forms a diploid zygote, which cleaves into a morula, becomes a blastocyst with a trophoblast and inner cell mass, and then implants by embedding into the endometrium.
Analysis & Explanation
Concept
The primary part traces the continuum from syngamy to implantation, the core of the Human Reproduction chapter. Key ideas often tested: the exact site (ampullary-isthmic junction), the block to polyspermy, and the trophoblast/inner cell mass organisation of the blastocyst.
Sequence anchors (do not reorder)
- Fertilisation → zygote → cleavage → morula → blastocyst → implantation.
- Cleavage differs from ordinary mitosis: cell number rises but total size does not, as it happens inside the zona.
OR — ART logic
- Match the defect to the technique: a duct blockage with good sperm → ICSI; a blocked fallopian tube → IVF-ET/ZIFT (bypasses the tube); poor sperm count/motility → ICSI or IUI. GIFT is not used here because the couple refuses donors.
Exam trap
- Students forget to name the site of fertilisation, which carries an explicit mark. Also, do not confuse morula (solid) with blastocyst (hollow, with cavity).
Real-world relevance
- The world's first IVF (test-tube) baby, Louise Brown (1978), and India's Durga (1978) resulted from exactly this ART logic applied to tubal blockage.
Common Mistakes
- 1Omitting the site of fertilisation (ampullary-isthmic junction), which carries a specific mark.
- 2Confusing the morula (solid ball) with the blastocyst (hollow, with trophoblast and inner cell mass).
- 3In the OR part, mismatching ART to the defect — e.g. suggesting GIFT (a donor technique) when the couple explicitly refuses donors, or IVF for a simple sperm-duct blockage.
Interesting Facts
Cleavage divisions increase cell number without increasing the embryo's overall size because they occur within the confining zona pellucida.
India's first IVF baby, Kanupriya (Durga), was born in Kolkata on 3 October 1978, only weeks after Louise Brown, the world's first test-tube baby.
The copper ion released by a CuT does not act as a hormone; it works locally by immobilising sperm and provoking a sterile inflammatory response in the uterus.
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Frequently Asked Questions
Where does fertilisation take place in the human female?
Fertilisation takes place at the ampullary-isthmic junction of the fallopian tube (oviduct). It occurs only if the ovum and the sperms are transported to this region simultaneously, which is why timing relative to ovulation matters.
How is a morula different from a blastocyst?
A morula is a solid ball of 8–16 dividing cells (blastomeres) formed early after cleavage. A blastocyst is the later hollow stage whose cells are organised into an outer trophoblast and an inner cell mass; only the blastocyst implants into the endometrium.
Which ART suits a woman with blocked fallopian tubes?
In-vitro fertilisation followed by embryo transfer (IVF-ET), using ZIFT or IUT, is appropriate. Ova and sperm are fused in the laboratory and the resulting zygote/early embryo is placed into the fallopian tube (ZIFT) or uterus (IUT), bypassing the blocked tube.