Human Reproduction — NCERT Solutions

Definition:
The process of formation of a mature female gamete (ovum/egg) from the oogonial cells in the ovary is called oogenesis.

Brief Account of Oogenesis:

Step 1 — Foetal stage (Multiplication and Growth):

• During foetal development, certain cells in the ovary differentiate into oogonia ($2n$) — the primary female germ cells.
• Oogonia undergo mitotic divisions to increase in number.
• Each oogonium grows into a primary oocyte ($2n$) by accumulating nutrients.
• The primary oocyte gets surrounded by a layer of granulosa cells to form a primary follicle.
• The primary oocyte enters Meiosis I but gets arrested at the prophase I stage. This arrest continues until puberty.

Step 2 — At puberty (Meiosis I completion):

• At puberty, under hormonal influence, some primary follicles develop into Graafian follicles.
• The primary oocyte within the Graafian follicle completes Meiosis I (just before ovulation), producing:
• One large secondary oocyte ($n$) — receives most of the cytoplasm.
• One small first polar body ($n$) — receives very little cytoplasm.

Step 3 — Meiosis II (at fertilisation):

• The secondary oocyte enters Meiosis II but gets arrested at metaphase II.
• At ovulation, the secondary oocyte (along with the first polar body) is released from the Graafian follicle.
• Meiosis II is completed only if fertilisation occurs, producing:
• One large ootid (which matures into the ovum, $n$).
• One small second polar body ($n$).
• The first polar body may also divide to give two polar bodies.
• Thus, one primary oocyte gives rise to one functional ovum and three polar bodies (which degenerate).

Key point: Unlike spermatogenesis (which produces 4 functional sperms), oogenesis produces only 1 functional ovum per primary oocyte due to unequal cytokinesis.

Labelled Diagram of a Section Through Ovary:

Key structures to be labelled in the diagram:

1. Germinal epithelium — outermost layer covering the ovary.
2. Tunica albuginea — dense connective tissue layer beneath the germinal epithelium.
3. Stroma — connective tissue framework of the ovary in which follicles are embedded.
4. Primordial follicle — smallest follicle; primary oocyte surrounded by a single layer of flat granulosa cells.
5. Primary follicle — primary oocyte surrounded by a single layer of cuboidal granulosa cells; zona pellucida begins to form.
6. Secondary follicle — multiple layers of granulosa cells (stratum granulosum); theca interna and theca externa begin to form.
7. Tertiary follicle — fluid-filled spaces (antrum) appear among granulosa cells; oocyte is pushed to one side.
8. Graafian follicle (mature follicle) — large follicle with a well-developed antrum; secondary oocyte surrounded by corona radiata; cumulus oophorus present.
9. Corpus luteum — yellowish body formed after ovulation from the ruptured Graafian follicle; secretes progesterone.
10. Corpus albicans — white scar tissue formed after degeneration of corpus luteum.
11. Blood vessels — present in the stroma.

Note to student: Draw a neat oval-shaped ovary in section, showing follicles at different stages of development embedded in the stroma. Label all structures with clear leader lines.

Labelled Diagram of a Graafian Follicle:

Key structures to be labelled:

1. Theca externa — outermost fibrous layer of the follicle wall.
2. Theca interna — inner vascular layer; secretes estrogens.
3. Stratum granulosum (Granulosa cells) — multiple layers of granulosa cells lining the follicle; secrete estrogens and follicular fluid.
4. Antrum — large fluid-filled cavity containing follicular fluid (liquor folliculi).
5. Cumulus oophorus — mound of granulosa cells on which the oocyte rests, projecting into the antrum.
6. Secondary oocyte — the female germ cell arrested at metaphase II; large cell in the centre of the cumulus.
7. Zona pellucida — thick, transparent glycoprotein layer surrounding the secondary oocyte.
8. Corona radiata — innermost layer of granulosa cells closely surrounding the zona pellucida; remains with the oocyte after ovulation.
9. First polar body — small cell present in the perivitelline space between the oocyte and zona pellucida.

Note to student: Draw a large, circular follicle with a prominent antrum. Show the oocyte surrounded by zona pellucida and corona radiata, resting on the cumulus oophorus. Label all parts clearly.

(a) Corpus luteum:

• The corpus luteum secretes large amounts of progesterone, which is essential for the maintenance of the endometrium during the secretory phase of the menstrual cycle.
• Progesterone is also essential for the maintenance of pregnancy (prevents uterine contractions and supports implantation).
• It also secretes small amounts of estrogen.

(b) Endometrium:

• The endometrium is the innermost glandular layer of the uterus.
• It undergoes cyclical changes during the menstrual cycle (proliferation, secretion, and menstruation).
• It provides the site for implantation of the blastocyst (embryo).
• During pregnancy, it contributes to the formation of the placenta, which nourishes the developing foetus.

(c) Acrosome:

• The acrosome is a cap-like structure present on the anterior part of the sperm head.
• It contains hydrolytic enzymes (e.g., hyaluronidase, acrosin, zona lysin).
• During fertilisation, these enzymes are released (acrosomal reaction) to help the sperm penetrate the zona pellucida and the plasma membrane of the secondary oocyte.

(d) Sperm tail (Flagellum):

• The sperm tail is responsible for the motility (movement) of the spermatozoon.
• It propels the sperm through the female reproductive tract towards the ovum.
• The axoneme (9+2 arrangement of microtubules) within the tail generates the whip-like movement.

(e) Fimbriae:

• Fimbriae are finger-like projections present at the margins of the infundibulum (the funnel-shaped opening of the fallopian tube near the ovary).
• Their function is to collect the ovum (secondary oocyte) released during ovulation from the surface of the ovary and direct it into the fallopian tube.

(a) Androgens are produced by Sertoli cells.
→ FALSE
Correction: Androgens are produced by Leydig cells (interstitial cells) present in the interstitial spaces outside the seminiferous tubules in the testis. Sertoli cells provide nutrition to developing germ cells and secrete inhibin and ABP.

(b) Spermatozoa get nutrition from Sertoli cells.
→ TRUE
Sertoli cells (nurse cells) provide nutrition to the developing spermatogonia, spermatocytes, spermatids, and spermatozoa during spermatogenesis.

(c) Leydig cells are found in ovary.
→ FALSE
Correction: Leydig cells are found in the testis (in the interstitial spaces between the seminiferous tubules), not in the ovary.

(d) Leydig cells synthesise androgens.
→ TRUE
Leydig cells (interstitial cells) of the testis synthesise and secrete androgens, primarily testosterone.

(e) Oogenesis takes place in corpus luteum.
→ FALSE
Correction: Oogenesis takes place in the ovary (specifically in the ovarian follicles). The corpus luteum is a temporary endocrine structure formed after ovulation that secretes progesterone; it is not the site of oogenesis.

(f) Menstrual cycle ceases during pregnancy.
→ TRUE
During pregnancy, high levels of progesterone (secreted by corpus luteum and later placenta) and estrogen inhibit the release of GnRH, FSH, and LH, thereby preventing ovulation and stopping the menstrual cycle.

(g) Presence or absence of hymen is not a reliable indicator of virginity or sexual experience.
→ TRUE
The hymen may be broken by various physical activities (sports, exercise, etc.) other than sexual intercourse, and in some females it may remain intact even after intercourse. Hence, it is not a reliable indicator of virginity or sexual experience.

Menstrual Cycle:

The cyclical, monthly changes in the ovary and uterus of sexually mature, non-pregnant female primates (including humans) are collectively called the menstrual cycle.

• The first menstruation begins at puberty and is called menarche.
• The average duration of one menstrual cycle is 28 days (range: 21–35 days).
• Menstrual cycles cease permanently at menopause (around 45–50 years of age).

Phases of the Menstrual Cycle:

1. Menstrual phase (Day 1–5): Shedding of the endometrial lining along with blood and mucus through the vagina (menstruation), due to fall in progesterone and estrogen levels.

2. Follicular/Proliferative phase (Day 1–13): Under the influence of FSH, primary follicles develop into Graafian follicle; estrogen levels rise, causing proliferation (thickening) of the endometrium.

3. Ovulatory phase (Day 14): LH surge (mid-cycle) triggers ovulation — release of the secondary oocyte from the Graafian follicle.

4. Luteal/Secretory phase (Day 15–28): The ruptured follicle forms the corpus luteum, which secretes progesterone (and estrogen). The endometrium becomes secretory and prepares for implantation. If fertilisation does not occur, corpus luteum degenerates, progesterone falls, and menstruation begins.

Hormones Regulating the Menstrual Cycle:

1. GnRH (Gonadotropin-Releasing Hormone) — from hypothalamus; stimulates pituitary to release FSH and LH.
2. FSH (Follicle Stimulating Hormone) — from anterior pituitary; stimulates follicular development and estrogen secretion.
3. LH (Luteinising Hormone) — from anterior pituitary; triggers ovulation (LH surge) and stimulates corpus luteum formation and progesterone secretion.
4. Estrogens — from developing follicles; cause proliferation of endometrium; positive feedback causes LH surge.
5. Progesterone — from corpus luteum; maintains secretory endometrium; inhibits GnRH/FSH/LH (negative feedback).

Parturition:

The process of delivery of the fully developed foetus (baby) from the mother's uterus at the end of gestation (pregnancy) is called parturition (childbirth).

• Normal gestation period in humans is approximately 9 months (38–40 weeks).
• Parturition is a vigorous, involuntary process involving strong uterine contractions.

Mechanism of Induction of Parturition:

Parturition is induced by a complex neuroendocrine mechanism:

1. Signals for parturition originate from the fully developed foetus and the placenta.
2. These signals induce mild uterine contractions called foetal ejection reflex.
3. This triggers the release of oxytocin from the maternal posterior pituitary.
4. Oxytocin acts on the uterine muscles (myometrium) and causes stronger uterine contractions.
5. Stronger contractions stimulate further release of oxytocin — this is a positive feedback mechanism that amplifies contractions until delivery.

Hormones involved in induction of parturition:

1. Oxytocin — secreted by the posterior pituitary; causes strong uterine contractions (most important hormone in parturition).
2. Estrogens — increase uterine sensitivity to oxytocin by increasing oxytocin receptors on myometrium; stimulate uterine contractions.
3. Cortisol — secreted by the foetal adrenal cortex; acts as a signal from the foetus indicating maturity; helps initiate parturition.
4. Prostaglandins — secreted by the uterus/placenta; stimulate uterine contractions.
5. Relaxin — relaxes the pelvic ligaments and softens the cervix to facilitate delivery.

Note: Progesterone levels fall before parturition, removing its inhibitory effect on uterine contractions.

Explanation:

The blame placed on women for giving birth to daughters is scientifically incorrect. The sex of the child is determined by the father (male partner), not the mother.

Scientific Basis:

• Human females have the sex chromosome constitution XX (homogametic). All eggs (ova) produced by a female contain only the X chromosome.

• Human males have the sex chromosome constitution XY (heterogametic). Males produce two types of sperms:
• 50% sperms carry the X chromosome
• 50% sperms carry the Y chromosome

• The sex of the child depends on which type of sperm fertilises the egg:

$$\text{X sperm} + \text{X egg} \rightarrow \text{XX} = \text{Girl (Female)}$$
$$\text{Y sperm} + \text{X egg} \rightarrow \text{XY} = \text{Boy (Male)}$$

• Since the mother can only contribute an X chromosome through the egg, she has no role in determining the sex of the child.

• It is the father's sperm (whether it carries X or Y) that determines whether the child will be a girl or a boy.

Conclusion: The sex of the child is determined by the genetic contribution of the father, not the mother. Therefore, blaming women for giving birth to daughters is completely unscientific and unjustified.

Eggs released per month:

Normally, a human ovary releases one egg (secondary oocyte) per menstrual cycle (per month) — one ovulation event.

Case 1 — Identical (Monozygotic) Twins:

• Identical twins arise from a single fertilised egg (zygote) that splits into two embryos at an early stage of development.
• Therefore, only one egg is released by the ovary.
• Both twins have the same genetic constitution and are always of the same sex.

$$\text{1 egg} + \text{1 sperm} \rightarrow \text{1 zygote} \xrightarrow{\text{splits}} \text{2 identical embryos}$$

Answer: 1 egg is released.

Case 2 — Fraternal (Dizygotic) Twins:

• Fraternal twins arise from two separate eggs that are fertilised by two different sperms simultaneously.
• Therefore, two eggs are released by the ovary (one from each ovary, or two from the same ovary) in the same cycle.
• Fraternal twins may or may not be of the same sex and are genetically different (like ordinary siblings).

$$\text{2 eggs} + \text{2 sperms} \rightarrow \text{2 separate zygotes} \rightarrow \text{2 fraternal embryos}$$

Answer: Yes, the answer changes — 2 eggs are released in the case of fraternal twins.

Answer:

Dogs (like most mammals other than primates) are polytocous — they give birth to multiple offspring (a litter) at one time.

• Each puppy develops from a separate fertilised egg (zygote).
• Since the female dog gave birth to 6 puppies, each puppy originated from one egg fertilised by one sperm.
• Therefore, the ovary of the female dog must have released 6 eggs (one for each puppy) during that reproductive cycle.

Conclusion: The ovary released 6 eggs (multiple ovulation), each of which was fertilised by a separate sperm to form 6 individual zygotes, which developed into 6 puppies.

Note: This is in contrast to humans, where normally only 1 egg is released per cycle (monovulation). In dogs, multiple ovulation is the norm, allowing multiple offspring per pregnancy.