Human Reproduction

(a) Humans reproduce sexually.

(b) Humans are viviparous (they give birth to young ones directly, without laying eggs).

(c) Fertilisation is internal in humans (it occurs inside the female reproductive tract).

(d) Male and female gametes are haploid (they contain half the chromosome number, $n = 23$).

(e) Zygote is diploid (formed by fusion of two haploid gametes, $2n = 46$).

(f) The process of release of ovum from a mature follicle is called ovulation.

(g) Ovulation is induced by a hormone called Luteinising Hormone (LH) (the LH surge triggers ovulation).

(h) The fusion of male and female gametes is called fertilisation.

(i) Fertilisation takes place in the ampulla of the fallopian tube (oviduct).

(j) Zygote divides to form blastocyst which is implanted in the uterus.

(k) The structure which provides vascular connection between foetus and uterus is called placenta.

Diagram of Male Reproductive System

Although a hand-drawn diagram is required in the board exam, the key structures to be labelled are described below:

Key structures and their positions:

1. Testes — paired oval organs located in the scrotal sac; site of spermatogenesis and androgen production.
2. Epididymis — coiled tube on the posterior surface of each testis; site of sperm maturation and storage.
3. Vas deferens (Ductus deferens) — muscular tube that carries sperms from epididymis upward into the pelvic cavity.
4. Seminal vesicles — paired glands that secrete fructose-rich fluid forming part of semen.
5. Prostate gland — single gland surrounding the urethra; secretes slightly alkaline fluid.
6. Bulbourethral glands (Cowper's glands) — paired glands that secrete lubricating fluid.
7. Urethra — common passage for urine and semen.
8. Penis — male external genitalia; contains erectile tissue.
9. Scrotum — pouch of skin housing the testes, maintains temperature ~2–3°C below body temperature.
10. Ejaculatory duct — formed by union of vas deferens and duct of seminal vesicle.

Note to student: Draw a neat diagram showing the above structures in correct anatomical positions and draw arrows with labels for each part. The urethra should be shown passing through the prostate and penis.

Diagram of Female Reproductive System

Key structures to be labelled in the board exam diagram:

1. Ovaries — paired organs located in the pelvic cavity; produce ova and ovarian hormones (estrogen, progesterone).
2. Fallopian tubes (Oviducts) — paired tubes connecting ovaries to uterus; each has:
- Infundibulum — funnel-shaped opening with finger-like fimbriae that collect the ovum.
- Ampulla — wider region; site of fertilisation.
- Isthmus — narrow region joining the uterus.
3. Uterus (Womb) — pear-shaped muscular organ; site of implantation and foetal development.
4. Cervix — narrow lower part of uterus opening into vagina.
5. Vagina — fibromuscular tube connecting cervix to the exterior; birth canal.
6. External genitalia — mons pubis, labia majora, labia minora, clitoris, hymen.
7. Mammary glands — shown separately; produce milk after parturition.

Note to student: Draw a neat mid-sagittal section view showing ovaries connected to the uterus via fallopian tubes. Label all parts with clear leader lines.

Functions of Testis:

1. Spermatogenesis: The seminiferous tubules of the testis produce male gametes (spermatozoa) by the process of spermatogenesis.
2. Androgen secretion: The Leydig cells (interstitial cells) present in the testis synthesise and secrete male sex hormones called androgens (primarily testosterone), which regulate the development of secondary sexual characters and accessory reproductive structures.

Functions of Ovary:

1. Oogenesis: The ovary produces female gametes (ova/eggs) by the process of oogenesis.
2. Hormone secretion: The ovary secretes female sex hormones — estrogens (by developing follicles) and progesterone (by corpus luteum) — which regulate the menstrual cycle, development of secondary sexual characters, and maintenance of pregnancy.

Structure of a Seminiferous Tubule:

Given: Each testis has about 250 testicular lobules, and each lobule contains 1–3 highly coiled seminiferous tubules.

Structure:

- Each seminiferous tubule is a highly coiled, long tubule lined by a germinal epithelium.
- The inner wall (lumen side) of the tubule is lined by two types of cells:

1. Spermatogonia (male germ cells): These are the primary male germ cells. They are large, rounded cells lying near the basement membrane. They undergo mitotic and meiotic divisions to ultimately produce spermatozoa (spermatogenesis).

2. Sertoli cells (nurse cells): These are large, columnar cells extending from the basement membrane to the lumen. They provide nutrition to the developing germ cells. They also secrete inhibin, which provides feedback control of spermatogenesis, and androgen-binding protein (ABP).

- The basement membrane surrounds the tubule from outside.
- Outside the seminiferous tubules, in the interstitial spaces, lie the Leydig cells (interstitial cells), which synthesise and secrete androgens (testosterone).
- Also present in the interstitium are blood vessels, lymph vessels, and connective tissue.

Summary: The seminiferous tubule is the functional unit of the testis where spermatogenesis occurs, supported by Sertoli cells, while Leydig cells in the surrounding interstitium produce androgens.

Definition:
The process of formation of spermatozoa (male gametes) from the spermatogonial cells (primary germ cells) in the seminiferous tubules of the testis is called spermatogenesis.

Process of Spermatogenesis:

Step 1 — Multiplication phase:
- The undifferentiated germ cells called spermatogonia ($2n$) are present on the inner wall of seminiferous tubules.
- They undergo repeated mitotic divisions to increase in number.

Step 2 — Growth phase:
- Some spermatogonia (called primary spermatocytes, $2n$) grow in size by accumulating nutrients.

Step 3 — Meiotic phase:
- Each primary spermatocyte ($2n$) undergoes Meiosis I (first meiotic division) to form two secondary spermatocytes ($n$), each with half the chromosome number.
- Each secondary spermatocyte undergoes Meiosis II (second meiotic division) to form two spermatids ($n$).
- Thus, one primary spermatocyte gives rise to 4 spermatids.

Step 4 — Spermiogenesis:
- The spermatids are transformed into mature, motile spermatozoa (sperms) by a process called spermiogenesis.
- This involves differentiation: formation of head, neck, middle piece, and tail.

Step 5 — Spermiation:
- The mature sperms are released from the Sertoli cells into the lumen of the seminiferous tubule — this is called spermiation.

Hormonal regulation: Spermatogenesis is initiated at puberty due to increased secretion of GnRH from the hypothalamus, which stimulates the anterior pituitary to secrete FSH and LH. FSH stimulates Sertoli cells to facilitate spermatogenesis; LH stimulates Leydig cells to secrete testosterone, which is essential for spermatogenesis.

The following hormones regulate spermatogenesis:

1. Gonadotropin-Releasing Hormone (GnRH): Secreted by the hypothalamus at puberty; stimulates the anterior pituitary to release gonadotropins (FSH and LH).

2. Follicle Stimulating Hormone (FSH): Secreted by the anterior pituitary; acts on Sertoli cells and stimulates them to secrete factors that help in spermatogenesis.

3. Luteinising Hormone (LH) / ICSH (Interstitial Cell Stimulating Hormone): Secreted by the anterior pituitary; acts on Leydig cells and stimulates them to secrete testosterone.

4. Testosterone (Androgens): Secreted by Leydig cells of the testis; essential for the initiation and maintenance of spermatogenesis.

5. Inhibin: Secreted by Sertoli cells; provides negative feedback to the anterior pituitary to inhibit FSH secretion, thereby regulating the rate of spermatogenesis.

Summary: GnRH → FSH + LH → Sertoli cells (spermatogenesis) + Leydig cells (testosterone) → spermatogenesis.

Spermiogenesis:

The process of transformation of non-motile, rounded spermatids into mature, motile spermatozoa (sperms) is called spermiogenesis.

During spermiogenesis, the following changes occur in the spermatid:
- The nucleus condenses and elongates to form the head.
- The Golgi apparatus forms the acrosome (cap-like structure over the nucleus).
- The centrioles form the flagellum (tail).
- Mitochondria aggregate around the proximal part of the flagellum to form the middle piece.
- Most of the cytoplasm is shed.

Spermiation:

The process by which mature spermatozoa are released from the Sertoli cells into the lumen of the seminiferous tubule is called spermiation.

After spermiation, the sperms are transported through the male accessory ducts (rete testis → vasa efferentia → epididymis → vas deferens) to the exterior during ejaculation.

Labelled Diagram of a Human Sperm:

A human sperm is approximately 60 µm long and consists of four parts:

1. Head:
- Contains the elongated, haploid nucleus with densely packed chromatin.
- Covered anteriorly by a cap-like structure called the acrosome, which is derived from the Golgi apparatus.
- The acrosome contains hydrolytic enzymes (e.g., hyaluronidase, acrosin) that help in penetration of the egg during fertilisation.

2. Neck:
- Short region connecting the head to the middle piece.
- Contains the proximal centriole (helps in formation of the first cleavage spindle after fertilisation).

3. Middle piece:
- Contains numerous mitochondria arranged in a spiral (mitochondrial sheath) around the axial filament.
- Mitochondria provide energy (ATP) for sperm motility.

4. Tail (Flagellum):
- Long, whip-like structure responsible for the motility of the sperm.
- Contains the axoneme (9+2 arrangement of microtubules).

Note to student: Draw an elongated sperm showing the four regions clearly. Label: Acrosome, Nucleus (Head), Neck, Mitochondrial sheath (Middle piece), Axial filament, Tail/Flagellum.

Seminal plasma is the fluid component of semen (excluding spermatozoa). It is secreted by the male accessory glands.

Major components of seminal plasma:

1. Fructose — secreted mainly by seminal vesicles; provides energy (nutrition) to the spermatozoa.

2. Citric acid — secreted by the prostate gland.

3. Calcium and certain enzymes — secreted by the prostate gland; help in coagulation and liquefaction of semen.

4. Prostaglandins — secreted by seminal vesicles; help in sperm transport by stimulating uterine contractions.

5. Mucus / lubricating fluid — secreted by bulbourethral glands (Cowper's glands); lubricates the urethra and neutralises the acidic urine residue.

6. Bicarbonate buffers — maintain the slightly alkaline pH of seminal plasma (~7.4), which protects sperms from the acidic environment of the vagina.

Function: Seminal plasma provides nutrition, protection, and a suitable medium for the transport and survival of spermatozoa.

Major Functions of Male Accessory Ducts:

| Duct | Function |
|---|---|
| Rete testis | Collects sperms from seminiferous tubules and passes them to vasa efferentia. |
| Vasa efferentia | Transport sperms from rete testis to epididymis. |
| Epididymis | Site of maturation and storage of spermatozoa; sperms gain motility and fertilising capacity here. |
| Vas deferens | Transports sperms from epididymis to the ejaculatory duct; also stores sperms. |
| Ejaculatory duct | Carries sperms and secretions of seminal vesicles into the urethra. |
| Urethra | Common passage for semen and urine; carries semen to the exterior during ejaculation. |

Major Functions of Male Accessory Glands:

1. Seminal vesicles: Secrete a fructose-rich, alkaline fluid that provides energy to sperms and constitutes about 60% of semen volume.

2. Prostate gland: Secretes a slightly alkaline, milky fluid containing citric acid, calcium, and enzymes; helps in coagulation and subsequent liquefaction of semen; neutralises vaginal acidity.

3. Bulbourethral glands (Cowper's glands): Secrete a mucus-like lubricating fluid that lubricates the urethra and neutralises any residual acidic urine before ejaculation.

Overall function: The accessory ducts transport sperms, while the accessory glands produce seminal plasma that nourishes, protects, and facilitates the transport of spermatozoa.

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.

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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.

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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.