NCERT Exemplar Class 12 Biology Ch 2 Human Reproduction - PDF

Correct option: (c) Polyspermy in mammals is prevented by the chemical changes in the egg surface.

Concept used. Polyspermy means the entry of more than one sperm into a single egg, which would give an abnormal chromosome number. Mammals stop this with a fast electrical block (depolarisation of the egg plasma membrane the moment the first sperm fuses) followed by a slow zona reaction (the zona pellucida hardens and its sperm receptors are destroyed). The block is membrane-electrical and physical, not a "chemical change of the egg surface" as worded in option (c). We must judge each statement true or false against textbook biology and pick the false one.

1. Test (a): In birds and mammals fertilisation is internal: the sperm meets the egg inside the female body. This statement is true.
2. Test (b): Colostrum is the first thick yellowish milk after childbirth. It is rich in antibodies (mainly IgA) and nutrients, giving the newborn passive immunity. This statement is true.
3. Test (c): The block to polyspermy is by a change in the electrical potential of the egg plasma membrane and a physical hardening of the zona pellucida, not by "chemical changes in the egg surface." This statement is false, so it is the incorrect statement asked for.
4. Test (d): In humans the blastocyst implants into the uterine endometrium about 6 to 7 days after fertilisation. This statement is true.

Option (c) is the incorrect statement.

Correct option: (d) Progesterone level is high during the post ovulatory phase of the menstrual cycle.

Concept used. In the menstrual cycle, after ovulation the ruptured Graafian follicle becomes the corpus luteum, which secretes large amounts of progesterone. This post-ovulatory part is the luteal phase. We test each statement against the textbook sequence of events and hormone levels.

1. Test (a): The ovulatory LH surge is triggered by a peak (high level) of estrogen, but the surge itself is the rise in LH, not estrogen. The statement says high estrogen "triggers the ovulatory surge"; while estrogen does cause the LH surge, NCERT credits the LH surge as the trigger of ovulation, so this wording is taken as not the best correct statement.
2. Test (b): Oogonia do not keep proliferating from puberty. All oogonia are formed before birth; no new oogonia are added after birth. So (b) is false.
3. Test (c): Sperms in the seminiferous tubules are immature and not highly motile; they gain motility later in the epididymis. So (c) is false.
4. Test (d): After ovulation the corpus luteum secretes progesterone, so progesterone is high in the post-ovulatory (luteal) phase. This is true and is the correct answer.

Option (d) is the correct statement.

Correct option: (d) Isthmus.

Concept used. The sperm transport pathway in the male is: seminiferous tubules \(\rightarrow\) rete testis \(\rightarrow\) vasa efferentia \(\rightarrow\) epididymis \(\rightarrow\) vas deferens. The isthmus is a part of the female fallopian tube. We find the structure that does not belong to the male system.

1. Rete testis: a network of tubules in the testis that collects sperm. This is a male structure.
2. Epididymis: the coiled tube on the testis where sperm mature and gain motility. This is a male structure.
3. Vasa efferentia: small ducts carrying sperm from rete testis to epididymis. This is a male structure.
4. Isthmus: the narrow region of the fallopian tube next to the uterus, part of the female system. It is the odd one out.

Option (d) Isthmus is the odd one out.

Correct option: (b) i, ii and iv.

Concept used. Seminal plasma is the fluid part of semen (semen minus the sperm). It is made by the male accessory glands: the seminal vesicles, the prostate gland and the bulbourethral (Cowper's) glands. The urethra is only a passage (a duct), not a gland, so it does not contribute fluid. We pick the option listing exactly the three accessory glands.

1. Seminal vesicle (i): secretes fructose-rich fluid; contributes to seminal plasma.
2. Prostate gland (ii): secretes a thin milky fluid; contributes to seminal plasma.
3. Urethra (iii): a tube carrying semen out; it is a duct, not a secretory gland, so it does not contribute the plasma.
4. Bulbourethral gland (iv): secretes lubricating mucus; contributes to seminal plasma.
5. The contributors are i, ii and iv, which is option (b).

Option (b): i, ii and iv.

Correct option: (a) Seminiferous tubules.

Concept used. Spermiation is the final step of spermatogenesis: the mature spermatozoa are released from the Sertoli cells into the lumen of the seminiferous tubules. (Do not confuse it with spermiogenesis, the transformation of spermatids into spermatozoa.) We match the definition to the correct site.

1. Recall the place where sperm are made: the seminiferous tubules of the testis.
2. Spermiation = release of the formed sperms from the Sertoli cells into the tubule lumen. So the release site is the seminiferous tubules, option (a).
3. Eliminate the rest: vas deferens and epididymis only transport/store sperm later; the prostate is a gland, not a site of sperm release.

Option (a) Seminiferous tubules.

Correct option: (b) 11–17 day of menstrual cycle.

Concept used. In a typical 28-day menstrual cycle, the follicular phase (days 1–13) is when a primary follicle grows into a mature Graafian follicle. Ovulation occurs around day 14. So a mature Graafian follicle is present in the days just before ovulation. We match this window to the options.

1. Days 1–5: menstrual flow; only early follicle growth.
2. Days 6–13: follicular phase; the follicle matures into a Graafian follicle, fully mature just before day 14.
3. Around day 14: ovulation releases the secondary oocyte and the follicle is converted into the corpus luteum.
4. So the mature Graafian follicle is present roughly in the 11–17 day window (peaking just before ovulation), which is option (b).

Option (b): 11–17 day of the menstrual cycle.

Correct option: (a) Its contact with zona pellucida of the ova.

Concept used. The acrosome is a cap on the sperm head filled with hydrolytic enzymes (sperm lysins). The acrosomal reaction is the release of these enzymes, triggered when the sperm touches the zona pellucida (the glycoprotein layer around the egg). The enzymes dissolve a path through the zona so the sperm can reach the egg membrane. We match the trigger to the correct cause.

1. Identify the trigger event: physical contact of the sperm with the zona pellucida of the ovum.
2. This contact makes the acrosome release its enzymes (the acrosomal reaction), digesting a path through the zona pellucida.
3. Eliminate the others: the uterine or epididymal environment relates to capacitation/maturation, not the acrosomal reaction itself; uterine androgens are not the trigger.

Option (a): contact with the zona pellucida.

Correct option: (b) Ampulla.

Concept used. The male accessory glands are exactly three: the seminal vesicles, the prostate, and the bulbourethral (Cowper's) glands. The ampulla is the wide central part of the female fallopian tube (the usual site of fertilisation); it is not a male gland. We pick the structure that is not in the list of three.

1. Seminal vesicle: a male accessory gland (fructose-rich secretion).
2. Prostate: a male accessory gland (thin milky secretion).
3. Bulbourethral gland: a male accessory gland (lubricating mucus).
4. Ampulla: a part of the female oviduct, not a gland and not male. So it is the answer.

Option (b) Ampulla is not a male accessory gland.

Correct option: (c) Secondary spermatocytes have 23 chromosomes and undergo second meiotic division.

Concept used. Spermatogenesis sequence: spermatogonium (\(2n=46\)) divides by mitosis; some become primary spermatocytes (\(2n=46\)); a primary spermatocyte undergoes meiosis I to give two secondary spermatocytes (\(n=23\)); each secondary spermatocyte undergoes meiosis II to give two spermatids (\(n=23\)); spermatids change shape into spermatozoa (spermiogenesis). We test each statement against this order and chromosome count.

1. Test (a): Spermatogonia have 46 chromosomes (true) but they divide by mitosis, not "always meiotic." So (a) is false.
2. Test (b): Primary spermatocytes divide by meiosis I, not mitosis. So (b) is false.
3. Test (c): A secondary spermatocyte is haploid with 23 chromosomes and it undergoes the second meiotic division (meiosis II) to form spermatids. This is true.
4. Test (d): The direction is reversed: spermatids are transformed into spermatozoa, not the other way round. So (d) is false.

Option (c) is correct.

Correct option: (b) A-iv, B-iii, C-i, D-ii.

Concept used. A spermatozoon has four parts. The head holds the haploid nucleus (genetic material). The acrosome is a cap over the head storing hydrolytic enzymes. The middle piece is packed with mitochondria that supply energy (ATP). The tail (flagellum) beats to give motility. We match each part to its function.

1. A. Head \(\rightarrow\) carries the nucleus, i.e. genetic material \(=\) iv.
2. B. Middle piece \(\rightarrow\) has mitochondria, supplies energy \(=\) iii.
3. C. Acrosome \(\rightarrow\) stores hydrolytic enzymes \(=\) i.
4. D. Tail \(\rightarrow\) provides sperm motility \(=\) ii.
5. So A-iv, B-iii, C-i, D-ii, which is option (b).

Option (b): A-iv, B-iii, C-i, D-ii.

Correct option: (c) Secondary oöcyte.

Concept used. Diploid cells have \(2n=46\) chromosomes; haploid cells have \(n=23\). The secondary oocyte is formed when the primary oocyte completes meiosis I, so it is haploid (\(n=23\)). We check the chromosome number of each option.

1. Spermatogonia: diploid germ cells, \(2n=46\). Not 23.
2. Zygote: formed by fusion of sperm (\(n=23\)) and egg (\(n=23\)), so it is diploid, \(2n=46\). Not 23.
3. Secondary oocyte: produced after meiosis I of the primary oocyte, so it is haploid, \(n=\mathbf{23}\). This is the answer.
4. Oogonia: diploid germ cells, \(2n=46\). Not 23.

Option (c) Secondary oöcyte has 23 chromosomes.

Correct option: (b) A-iii, B-iv, C-ii, D-i.

Concept used. After fertilisation the zygote undergoes cleavage (rapid mitotic divisions) to form a morula, then a blastocyst. The blastocyst has an outer trophoblast (attaches to the endometrium) and an inner cell mass (forms the embryo proper). Implantation is the embedding of the blastocyst in the uterine endometrium. We match each term to its definition.

1. A. Trophoblast \(\rightarrow\) outer layer of blastocyst attached to the endometrium \(=\) iii.
2. B. Cleavage \(\rightarrow\) mitotic division of the zygote \(=\) iv.
3. C. Inner cell mass \(\rightarrow\) group of cells that would differentiate as embryo \(=\) ii.
4. D. Implantation \(\rightarrow\) embedding of blastocyst in the endometrium \(=\) i.
5. So A-iii, B-iv, C-ii, D-i, which is option (b).

Option (b): A-iii, B-iv, C-ii, D-i.

Correct option: (d) LH.

Concept used. The placenta acts as an endocrine gland during pregnancy. It secretes hCG (human chorionic gonadotropin), hPL (human placental lactogen), estrogens and progestogens (progesterone). LH (luteinising hormone) is secreted by the anterior pituitary, not the placenta. We pick the hormone the placenta does not make.

1. hCG: secreted by the placenta (trophoblast); maintains the corpus luteum early in pregnancy. Placental.
2. Estrogens: secreted by the placenta in increasing amounts during pregnancy. Placental.
3. Progesterone: secreted by the placenta, maintaining the endometrium. Placental.
4. LH: a pituitary gonadotropin, not secreted by the placenta. So (d) is the answer.

Option (d) LH is not secreted by the placenta.

Correct option: (b) Ejaculatory duct.

Concept used. In the male tract, the vas deferens carries sperm upward, then joins the duct of the seminal vesicle. The combined channel is the ejaculatory duct, which opens into the urethra. We identify this combined duct.

1. The vas deferens ascends and reaches near the urinary bladder.
2. There it is joined by the duct of the seminal vesicle.
3. The short channel formed by this union is the ejaculatory duct.
4. The ejaculatory duct opens into the urethra, so the answer is (b).

Option (b) Ejaculatory duct.

Correct option: (c) External opening of the urinogenital duct.

Concept used. The word meatus means a body opening or passage. The urethral meatus is specifically the external opening through which the urethra (the urinogenital duct in the male) opens to the outside. We match the term to its precise meaning.

1. "Urethral" refers to the urethra (the urinogenital duct in the male).
2. "Meatus" means an external opening.
3. Combining, urethral meatus \(=\) the external opening of the urinogenital duct, option (c).
4. Eliminate the rest: (a) is the whole duct, (b) is an internal junction, (d) refers to muscles, none is an external opening.

Option (c): the external opening of the urinogenital duct.

Correct option: (a) Between the zygote and blastocyst.

Concept used. After fertilisation the zygote divides by cleavage into 2, 4, 8, 16 cells. The compact 8 to 16-celled solid ball is the morula. The morula then develops a cavity to become the blastocyst. So the morula sits between the zygote and the blastocyst. We place the morula in the developmental sequence.

1. Sequence: zygote \(\rightarrow\) cleavage \(\rightarrow\) morula (8–16 cells, solid) \(\rightarrow\) blastocyst (with cavity) \(\rightarrow\) implantation.
2. The morula comes after the zygote and before the blastocyst.
3. Therefore it is the stage between the zygote and the blastocyst, option (a). Implantation happens later, ruling out (c) and (d); gastrula is much later, ruling out (b).

Option (a): between the zygote and the blastocyst.

Correct option: (a) Corona radiata.

Concept used. At ovulation the secondary oocyte is released surrounded by coats. From inside out: the plasma membrane, then the zona pellucida (a glycoprotein layer), then the corona radiata, the outermost layer of follicular (granulosa) cells radiating around the egg. The outermost membranous cover at ovulation is therefore the corona radiata. We identify the outermost cover present at ovulation.

1. List the coats of the released oocyte from inside out: oocyte membrane \(\rightarrow\) zona pellucida \(\rightarrow\) corona radiata.
2. "Zona radiata" is not a standard human term here; "chorion" is an extra-embryonic membrane formed much later, not a cover at ovulation.
3. The outermost cover present at ovulation is the corona radiata, option (a).

Option (a) Corona radiata.

Correct option: (a) Labia minora.

Concept used. The fallopian tube (oviduct) has, from the ovary towards the uterus: fimbriae (finger-like projections), infundibulum (funnel), ampulla, and isthmus. The labia minora are folds of the external genitalia (vulva), not part of the fallopian tube. We find the structure that is not part of the oviduct.

1. Fimbriae: finger-like ends of the oviduct that collect the ovum. Oviduct part.
2. Infundibulum: the funnel-shaped opening of the oviduct near the ovary. Oviduct part.
3. Isthmus: the narrow part of the oviduct near the uterus. Oviduct part.
4. Labia minora: skin folds of the external genitalia, not part of the oviduct. So it is the odd one out.

Option (a) Labia minora is the odd one out.

Concept used. Human reproduction proceeds in a fixed order: gametes are first formed (gametogenesis), sperm are deposited in the female tract (insemination), the sperm fuses with the egg (fertilisation), the embryo embeds in the uterus (implantation), the embryo develops over the pregnancy period (gestation), and finally the baby is delivered (parturition). We arrange the six terms in this natural cause-to-effect order.

1. Gametes must exist before anything else: gametogenesis comes first.
2. Sperm must be delivered into the female tract next: insemination.
3. Sperm and egg then fuse: fertilisation.
4. The resulting blastocyst embeds in the uterine wall: implantation.
5. The embryo grows for about nine months: gestation.
6. The fully developed baby is delivered: parturition.

Gametogenesis \(\rightarrow\) Insemination \(\rightarrow\) Fertilisation \(\rightarrow\) Implantation \(\rightarrow\) Gestation \(\rightarrow\) Parturition.

Concept used. The sperm transport pathway in the male begins where sperm are made and ends where semen leaves the body: seminiferous tubules \(\rightarrow\) rete testis \(\rightarrow\) vasa efferentia \(\rightarrow\) epididymis \(\rightarrow\) vas deferens \(\rightarrow\) ejaculatory duct \(\rightarrow\) urethra. We fill the missing links between the seminiferous tubules and the epididymis (and beyond).

1. Sperm leave the seminiferous tubules and collect in the rete testis (a network of tubules).
2. From the rete testis they pass through the vasa efferentia (small ducts).
3. The vasa efferentia open into the epididymis, where sperm mature and gain motility.
4. From the epididymis sperm move on through the vas deferens, then the ejaculatory duct, and finally the urethra.

Seminiferous tubules \(\rightarrow\) rete testis \(\rightarrow\) vasa efferentia \(\rightarrow\) epididymis \(\rightarrow\) vas deferens \(\rightarrow\) ejaculatory duct \(\rightarrow\) urethra.

Concept used. The cervix is the narrow lower part of the uterus that opens into the vagina through the cervical canal. Its functions follow from this position between the uterus and the vagina.

1. It forms the passage that connects the uterus to the vagina, so sperm deposited in the vagina can enter the uterus through the cervical canal.
2. During childbirth the cervix dilates widely to form part of the birth canal through which the baby is delivered.

The cervix connects the uterus to the vagina; its canal is the route for sperm entry, and it forms part of the birth canal during parturition.

Concept used. Menstruation happens only when the endometrium is shed because no pregnancy has occurred. During pregnancy, high progesterone (from the corpus luteum, then the placenta) maintains the endometrium and suppresses the gonadotropins (FSH, LH) that would otherwise start a new follicular cycle. We explain why no cycle runs during pregnancy.

1. After fertilisation and implantation, the corpus luteum (later the placenta) secretes large amounts of progesterone.
2. High progesterone maintains the endometrium (it is not shed) and, by negative feedback, suppresses FSH and LH from the pituitary.
3. Without the FSH/LH-driven follicular development and ovulation, no new menstrual cycle can occur, so menstruation stops throughout pregnancy.

High progesterone during pregnancy maintains the endometrium and suppresses FSH/LH, so no follicular cycle or menstruation occurs.

Concept used. Each major female reproductive organ has a defining function: ovaries (ovulation, i.e. release of the ovum), oviduct (site of fertilisation), uterus (pregnancy/implantation and development), vagina (birth canal). We fill the blanks by pairing each organ with its function.

1. Ovaries \(\rightarrow\) Ovulation (blank a): the ovary releases the secondary oocyte.
2. Oviduct \(\rightarrow\) Fertilisation: the third blank in column B; fusion of sperm and egg occurs at the ampullary-isthmic junction.
3. Column A blank b \(\rightarrow\) Uterus, whose function is Pregnancy (implantation and development).
4. Vagina \(\rightarrow\) Birth: it serves as the birth canal.

(a) Ovulation; (b) Uterus; missing column-B function \(=\) Fertilisation (Oviduct).

Concept used. Parturition (childbirth) is triggered by signals that originate from the fully developed foetus and the placenta. These set off a neuroendocrine foetal ejection reflex, in which oxytocin from the maternal pituitary is the main hormone driving uterine contractions.

1. The signals for parturition arise from the foetus (and placenta), not the mother. They build up only when the foetus is fully developed.
2. These signals trigger the foetal ejection reflex; in response the maternal posterior pituitary releases oxytocin.
3. Oxytocin causes strong contractions of the uterine muscles, leading to delivery of the baby.

Parturition signals arise from the foetus (and placenta); the main hormone is oxytocin.

Concept used. The epididymis is a long coiled tube on the back of each testis that receives sperm from the vasa efferentia. Sperm leaving the testis are immature and non-motile; the epididymis is where they finish maturing.

1. Sperm from the seminiferous tubules are immature and immotile.
2. In the epididymis they undergo maturation and acquire motility and the ability to fertilise.
3. The epididymis also stores sperm temporarily until ejaculation.

The epididymis matures and stores sperm, giving them motility and fertilising ability, which is essential for male fertility.

Concept used. Spermatogenesis is controlled by the hypothalamo-hypophyseal axis. GnRH from the hypothalamus stimulates the anterior pituitary to release FSH and LH; LH acts on Leydig cells to make androgens (testosterone), and FSH acts on Sertoli cells. We name each hormone, its gland and its role.

1. GnRH (gonadotropin releasing hormone), from the hypothalamus: stimulates the anterior pituitary to secrete FSH and LH.
2. LH (luteinising hormone), from the anterior pituitary: acts on Leydig (interstitial) cells to secrete androgens (testosterone).
3. Testosterone/androgens, from the Leydig cells of the testis: stimulate the process of spermatogenesis.
4. FSH (follicle stimulating hormone), from the anterior pituitary: acts on Sertoli cells, which release factors needed for spermiogenesis (sperm maturation).

GnRH (hypothalamus) \(\rightarrow\) FSH & LH (anterior pituitary); LH \(\rightarrow\) androgens from Leydig cells (drive spermatogenesis); FSH \(\rightarrow\) Sertoli cells (support sperm maturation).

Concept used. In oogenesis, the oogonia (mother germ cells) multiply and enter meiosis I to become primary oocytes. A primary oocyte gets surrounded by granulosa cells to form a primary follicle, which grows into a secondary follicle and then a tertiary follicle with a fluid-filled antrum, finally becoming the Graafian follicle. We supply the missing stages.

1. Oogonia divide and enlarge into primary oocytes (which enter and pause in meiosis I).
2. The primary oocyte completes meiosis I (around ovulation) to give the secondary oocyte.
3. The oocyte surrounded by a granulosa layer is the primary follicle; this grows into a secondary follicle, then a tertiary follicle (with antrum), and finally the Graafian follicle.

Oogonia \(\rightarrow\) primary oocyte \(\rightarrow\) secondary oocyte \(\rightarrow\) primary follicle \(\rightarrow\) secondary follicle \(\rightarrow\) tertiary follicle \(\rightarrow\) Graafian follicle.

Concept used. Meiosis is a reduction division that halves the chromosome number, producing haploid (\(n\)) gametes from diploid (\(2n\)) germ cells. Fertilisation is the fusion of two haploid gametes, which restores the diploid (\(2n\)) number in the zygote. We name the process for each event.

1. Halving (\(2n \rightarrow n\)): occurs during gametogenesis by meiosis, which separates homologous chromosomes so each gamete gets only \(n=23\) chromosomes.
2. Restoring (\(n + n \rightarrow 2n\)): occurs at fertilisation, when a haploid sperm (\(n\)) fuses with a haploid egg (\(n\)) to form a diploid zygote (\(2n=46\)).

Reduction to \(n\): meiosis (during gametogenesis). Restoration to \(2n\): fertilisation (fusion of two haploid gametes).

Concept used. In oogenesis, the primary oocyte is the cell that enters meiosis I; the secondary oocyte is formed after meiosis I is completed. They differ in ploidy and the meiotic stage they are at.

1. Primary oocyte: diploid (\(2n=46\)); it is arrested in prophase of meiosis I until just before ovulation.
2. Secondary oocyte: haploid (\(n=23\)); it is formed when the primary oocyte completes meiosis I (with an unequal split giving one large secondary oocyte and a small first polar body). It then begins meiosis II and is arrested at metaphase II until fertilisation.

Primary oocyte: diploid (\(2n\)), in meiosis I. Secondary oocyte: haploid (\(n\)), formed after meiosis I, arrested in meiosis II until fertilisation.

Concept used. The fallopian tube has, near the ovary, a wide ampulla and, near the uterus, a narrow isthmus. Where these meet is the ampullary-isthmic junction, the normal site of fertilisation.

1. The ovum released at ovulation is picked up by the fimbriae and moves into the ampulla.
2. Sperm swim up the female tract to the same region.
3. Fertilisation (fusion of sperm and ovum) normally occurs at the ampullary-isthmic junction.

The ampullary-isthmic junction is the usual site of fertilisation in the female reproductive tract.

Concept used. The zona pellucida is the glycoprotein coat around the ovum. When the first sperm contacts and fuses with the egg, the egg's cortical reaction releases enzymes that change the zona pellucida (the zona reaction), blocking further sperm entry. This prevents polyspermy (entry of more than one sperm).

1. The first sperm binds the zona pellucida and induces the acrosomal reaction, then fuses with the egg membrane.
2. Fusion triggers the cortical reaction: cortical granules release enzymes into the zona pellucida.
3. These enzymes harden the zona pellucida and inactivate its sperm receptors (the zona reaction), so no other sperm can bind or penetrate. This blocks polyspermy.

Sperm fusion triggers the cortical/zona reaction, hardening the zona pellucida and inactivating its sperm receptors, so additional sperm cannot enter, preventing polyspermy.

Concept used. LH (luteinising hormone) rises sharply around the middle of the menstrual cycle; this rapid rise is the LH surge. Its main job is to cause ovulation and to form the corpus luteum.

1. The LH surge peaks around day 14 of a 28-day cycle.
2. It causes the mature Graafian follicle to rupture and release the secondary oocyte: this is ovulation.
3. After ovulation, LH converts the empty follicle into the corpus luteum, which secretes progesterone to prepare the endometrium.

The LH surge triggers ovulation (rupture of the Graafian follicle) and the formation of the corpus luteum.

Concept used. A secondary spermatocyte is haploid (\(n=23\)), formed after meiosis I. It next undergoes the second meiotic division (meiosis II), which is like a mitotic division (separates sister chromatids without changing chromosome number), to form two spermatids.

1. The secondary spermatocyte is already haploid (\(n=23\)) after meiosis I.
2. It divides by meiosis II (the second meiotic division), separating sister chromatids.
3. This produces two haploid spermatids (\(n=23\)) from each secondary spermatocyte.

Spermatids are formed from secondary spermatocytes by the second meiotic division (meiosis II).

Concept used. Menarche is the first menstruation, marking the start of the reproductive (fertile) period. Menopause is the permanent stopping of menstruation, marking the end of the reproductive period. Their significance lies in the boundaries they set on a woman's fertile years.

1. Menarche: occurs at puberty (about 10–14 years). It signals that the ovaries have begun cyclic activity and the female has become capable of reproduction (the start of the fertile phase).
2. Menopause: occurs around 45–50 years. It signals the end of ovarian cycles and ovulation, so natural reproduction is no longer possible after it.

Menarche marks the onset of the reproductive (fertile) phase; menopause marks its end. Together they define a woman's reproductive lifespan.

Concept used. (a) In spermatogenesis, one secondary spermatocyte undergoes meiosis II to give spermatids, which mature into spermatozoa. (b) After fertilisation in the oviduct, the zygote begins cleavage while still moving through the fallopian tube towards the uterus.

1. (a) One secondary spermatocyte (\(n=23\)) undergoes meiosis II \(\rightarrow\) 2 spermatids \(\rightarrow\) each matures into 1 spermatozoon. So 2 spermatozoa are formed from one secondary spermatocyte.
2. (b) Fertilisation occurs in the ampulla of the fallopian tube. The zygote then starts cleavage as it travels down the tube, so the first cleavage division takes place in the fallopian tube (oviduct).

(a) 2 spermatozoa. (b) In the fallopian tube (oviduct), while the zygote moves towards the uterus.

Concept used. The corpus luteum secretes progesterone. Its survival depends on hCG (human chorionic gonadotropin): hCG is secreted only when an embryo has implanted, so the corpus luteum is maintained only during pregnancy.

1. If fertilisation occurs and the embryo implants, the trophoblast secretes hCG, which keeps the corpus luteum active and secreting progesterone for several months (a long life).
2. If fertilisation does not occur, there is no embryo and so no hCG.
3. Without hCG support, the corpus luteum degenerates after about 10–12 days, progesterone falls, the endometrium is shed, and menstruation begins.

hCG from the implanted embryo maintains the corpus luteum during pregnancy; without fertilisation there is no hCG, so it degenerates in 10–12 days.

Concept used. The foetal ejection reflex is the mild signal from the fully developed foetus and placenta that begins uterine contractions. It sets up a positive feedback loop with oxytocin that drives parturition (childbirth).

1. When the foetus is fully developed, signals from the foetus and placenta cause mild contractions of the uterus: this is the foetal ejection reflex.
2. These contractions stimulate the maternal posterior pituitary to release oxytocin.
3. Oxytocin causes stronger uterine contractions, which in turn trigger more oxytocin release: a positive feedback loop.
4. This loop intensifies contractions until the cervix dilates and the baby is expelled, i.e. parturition.

The foetal ejection reflex is the foetus/placenta signal that starts uterine contractions; via an oxytocin positive feedback loop it intensifies contractions until childbirth (parturition).

Concept used. The placenta is the structural and functional connection between the foetus and the mother's uterine wall. Apart from secreting hormones (its endocrine role), it serves exchange and protective roles.

1. Nutrition: it supplies the foetus with nutrients (glucose, amino acids) from the mother's blood.
2. Respiration (gas exchange): oxygen passes from mother to foetus and carbon dioxide from foetus to mother across the placenta.
3. Excretion: foetal nitrogenous wastes are removed into the mother's blood through the placenta.
4. Barrier/protection: it acts as a selective barrier that lets some materials (including some antibodies) pass while restricting others.

Besides endocrine function, the placenta provides nutrition, gas exchange (respiration), excretion of foetal wastes, and acts as a protective selective barrier.

Concept used. The first milk after childbirth is colostrum, followed by mature breast milk. Both are tailored to the newborn; colostrum in particular gives passive immunity.

1. Colostrum (the first few days' milk) is rich in antibodies, especially IgA, which give the newborn passive immunity against infections.
2. Breast milk provides balanced and easily digestible nutrition ideal for the infant's growth.
3. It is clean, at the right temperature, and strengthens the mother-infant bond.

Breast milk (especially colostrum) supplies antibodies for passive immunity plus complete, easily digested nutrition, so doctors recommend it in early infancy.

Concept used. The follicular phase is the first half of the menstrual cycle (about days 1–13). FSH drives follicle growth in the ovary, the growing follicle secretes estrogen, and estrogen rebuilds the endometrium in the uterus.

1. In the ovary: FSH from the pituitary stimulates a primary follicle to grow into a mature Graafian follicle; the follicle secretes increasing estrogen.
2. In the uterus: the rising estrogen makes the endometrium proliferate (regrow and thicken) after the previous menstruation, preparing it for a possible pregnancy.
3. The phase ends with the LH surge that causes ovulation around day 14.

Ovary: FSH-driven growth of the Graafian follicle with rising estrogen. Uterus: estrogen-driven proliferation (thickening) of the endometrium.

Concept used. Ovarian changes are driven by pituitary gonadotropins. FSH matures the follicle, the LH surge causes ovulation, and LH maintains the corpus luteum. We name the hormone for each transition.

1. (a) Primary follicle \(\rightarrow\) Graafian follicle: driven by FSH (follicle stimulating hormone), which promotes follicular growth and maturation.
2. (b) Graafian follicle \(\rightarrow\) ovulation: driven by the LH surge, which ruptures the mature follicle and releases the ovum.
3. (c) After ovulation \(\rightarrow\) corpus luteum: maintained by LH, which converts the empty follicle into the corpus luteum.

(a) FSH; (b) LH surge; (c) LH.

Concept used. Oogenesis is the formation of the ovum from an oogonium through multiplication, growth and maturation, producing one ovum and three polar bodies from one primary oocyte. We present it as a labelled schematic.

1. Oogonium (\(2n\)) grows into a primary oocyte (\(2n\)).
2. Meiosis I gives an unequal split: a large secondary oocyte (\(n\)) and a small first polar body (\(n\)).
3. Meiosis II of the secondary oocyte (completed at fertilisation) gives the ovum (\(n\)) and a second polar body (\(n\)).

Oogonium (\(2n\)) \(\rightarrow\) primary oocyte (\(2n\)) \(\rightarrow\) [meiosis I] \(\rightarrow\) secondary oocyte (\(n\)) \(+\) 1st polar body \(\rightarrow\) [meiosis II] \(\rightarrow\) ovum (\(n\)) \(+\) 2nd polar body.

Concept used. As a primary follicle develops into a Graafian follicle, the oocyte inside and the surrounding follicular cells both change. We list the changes.

1. The oocyte enlarges and a thick glycoprotein layer, the zona pellucida, forms around it.
2. The single layer of follicular (granulosa) cells multiplies into many layers, and a connective tissue theca forms around the follicle.
3. A fluid-filled cavity, the antrum, develops among the granulosa cells (secondary \(\rightarrow\) tertiary follicle).
4. The follicle enlarges greatly to become the mature Graafian follicle, with the oocyte surrounded by the corona radiata and a stalk of granulosa cells, ready for ovulation.

The oocyte enlarges with a zona pellucida; granulosa cells multiply into many layers with a theca; an antrum forms; the follicle enlarges into the Graafian follicle ready for ovulation.

Concept used. The pituitary gonadotropins are FSH (follicle stimulating hormone) and LH (luteinising hormone). In the follicular phase FSH dominates and drives follicle growth; the LH surge causes ovulation. The steroids estrogen and progesterone shift in step with these gonadotropins. We trace gonadotropin and steroid changes across the first half of the cycle.

1. Follicular phase (days 1–13): role of FSH. FSH from the anterior pituitary stimulates a primary follicle to grow into a Graafian follicle. The growing follicle secretes increasing amounts of the steroid estrogen.
2. Effect of rising estrogen. Estrogen rebuilds and thickens the endometrium (proliferative phase). Estrogen rises steadily through the follicular phase and peaks just before mid-cycle.
3. Ovulatory phase (around day 14): role of LH. The high estrogen level triggers a sharp surge of LH (and a smaller FSH rise). This LH surge causes the mature Graafian follicle to rupture and release the secondary oocyte: ovulation.
4. Steroid shift after ovulation. LH converts the ruptured follicle into the corpus luteum, which now secretes large amounts of the steroid progesterone (with some estrogen). So the dominant steroid shifts from estrogen (follicular) to progesterone (post-ovulatory).

FSH drives follicular growth and rising estrogen in the follicular phase; high estrogen triggers the LH surge that causes ovulation. The dominant steroid shifts from estrogen (follicular phase) to progesterone (post-ovulatory, from the corpus luteum).

Concept used. Both oogenesis and spermatogenesis use meiosis to make haploid gametes, but the cytoplasmic division and timing differ. The difference exists to give the egg a large cytoplasmic store for the early embryo. We compare them point by point.

1. Equal vs unequal division. In spermatogenesis the meiotic divisions are equal: one primary spermatocyte \(\rightarrow\) 4 equal-sized functional spermatids \(\rightarrow\) 4 sperms. In oogenesis the divisions are unequal: one primary oocyte \(\rightarrow\) 1 large functional ovum \(+\) 3 tiny non-functional polar bodies.
2. Number of functional gametes. Spermatogenesis yields 4 functional gametes per primary cell; oogenesis yields only 1 functional gamete (the ovum) per primary cell.
3. Timing/continuity. Spermatogenesis is continuous and completed within the testis. Oogenesis is discontinuous: it begins before birth, the primary oocyte is arrested in meiosis I for years, meiosis I completes only at ovulation, and meiosis II completes only if fertilisation occurs.
4. Why the difference. The unequal division conserves almost all the cytoplasm and nutrients in one cell, so the ovum carries enough food and organelles to support the zygote and early embryo before implantation. Sperms only need to deliver a nucleus, so equal division giving many small motile cells is efficient.

Spermatogenesis: equal divisions \(\rightarrow\) 4 functional sperms, continuous. Oogenesis: unequal divisions \(\rightarrow\) 1 ovum \(+\) 3 polar bodies, discontinuous (arrested in meiosis I, finishing at ovulation/fertilisation). The unequal split conserves cytoplasm so the ovum can nourish the early embryo.

Concept used. After fertilisation in the oviduct, the zygote undergoes cleavage (rapid mitotic divisions without growth) to form a morula, then a blastocyst, which finally undergoes implantation in the uterine endometrium. We describe each stage in order.

1. Zygote: the single diploid cell (\(2n\)) formed by fusion of sperm and egg, in the ampulla of the oviduct.
2. Cleavage: the zygote divides mitotically into 2, 4, 8, 16 cells called blastomeres, while moving down the oviduct. No growth occurs, so cells get smaller.
3. Morula: a compact ball of 8–16 blastomeres, still surrounded by the zona pellucida, reaching the uterus.
4. Blastocyst: the morula develops a fluid-filled cavity; cells arrange into an outer trophoblast and an inner cell mass (the future embryo).
5. Implantation: the blastocyst loses its zona pellucida and the trophoblast attaches to and embeds in the uterine endometrium, about 7 days after fertilisation.

Zygote \(\rightarrow\) cleavage (blastomeres) \(\rightarrow\) morula \(\rightarrow\) blastocyst (trophoblast + inner cell mass) \(\rightarrow\) implantation in the endometrium (about day 7).

Concept used. In the female reproductive system, the ovary produces the gamete (ovum), the fallopian tube (ampullary-isthmic junction) is the fertilisation site, the uterus (endometrium) is the implantation site, and the vagina (with cervix) is the birth canal. We present a labelled schematic identifying each part.

1. (a) Ovary produces the gamete (ovum) by oogenesis.
2. (b) Fallopian tube (ampullary-isthmic junction) is the site of fertilisation.
3. (c) Uterus (its endometrium) is the site of implantation of the blastocyst.
4. (d) Vagina (with the cervix) forms the birth canal during parturition.

(a) Ovary \(=\) gamete production; (b) Fallopian tube \(=\) fertilisation; (c) Uterus \(=\) implantation; (d) Vagina \(=\) birth canal.

Concept used. The mammary gland is a modified sweat gland present in pairs in females. It is made of glandular tissue (mammary lobes \(\rightarrow\) lobules \(\rightarrow\) alveoli) and a duct system that carries milk to the nipple. We describe its organisation with a labelled schematic.

1. Each mammary gland has 15–20 mammary lobes, each containing clusters of lobules.
2. Each lobule is made of grape-like alveoli; the cells of the alveoli secrete milk, which is stored in the alveolar lumen.
3. Milk passes from the alveoli into mammary tubules; tubules of a lobe join to form a mammary duct.
4. Several mammary ducts join to form a wider mammary ampulla, which leads through a lactiferous duct to the nipple, from where milk is released.

Alveoli (milk-secreting) \(\rightarrow\) mammary tubules \(\rightarrow\) mammary ducts \(\rightarrow\) mammary ampulla \(\rightarrow\) lactiferous duct \(\rightarrow\) nipple. Lobules of alveoli form lobes (15–20 per gland).