👶 Human Reproduction Study Notes

3.1 Male Reproductive System

Located in the pelvis region. Comprises a pair of testes, accessory ducts, accessory glands, and external genitalia.

1. Testes & Scrotum

• Testes are situated outside the abdominal cavity within a pouch called the scrotum, which maintains a low temperature (2–2.5°C lower than internal body temperature) necessary for **spermatogenesis**.
• Each testis has about 250 testicular lobules. Each lobule contains 1 to 3 highly coiled seminiferous tubules.
• Seminiferous tubules are lined with:
  • Male germ cells (Spermatogonia): Undergo meiosis to form sperms.
  • Sertoli cells: Provide nutrition to the dividing germ cells.
• Leydig cells (Interstitial cells): Located outside the tubules; synthesise and secrete testicular hormones called **androgens (testosterone)**.

2. Accessory Ducts & Glands

• Duct pathway: Seminiferous tubules → Rete testis → Vasa efferentia → Epididymis (stores sperms) → Vas deferens → joins duct from seminal vesicle → **Ejaculatory duct** → Urethra.
• Accessory Glands: Paired **Seminal vesicles**, a single **Prostate gland**, and paired **Bulbourethral (Cowper's) glands**.
• Secretions form **seminal plasma**, which is rich in **fructose, calcium, and certain enzymes**. Bulbourethral secretions also help in lubricating the penis.

3.2 Female Reproductive System

Comprises a pair of ovaries, accessory ducts (fallopian tubes, uterus, vagina), external genitalia, and mammary glands.

1. Ovaries & Fallopian Tubes

• Ovaries are the primary female sex organs producing the **ovum** and ovarian hormones (estrogen, progesterone).
• Fallopian tubes (10–12 cm long):
  • Infundibulum: Funnel-shaped part closest to the ovary. Has finger-like projections called fimbriae to collect the ovum after ovulation.
  • Ampulla: Wider middle part where **fertilisation** takes place.
  • Isthmus: Narrow end joining the uterus.

2. Uterus (Womb)

Inverted pear-shaped organ. Its wall consists of three layers:

• Perimetrium: Outer thin membranous layer.
• Myometrium: Middle thick layer of smooth muscle; exhibits strong contractions during parturition.
• Endometrium: Inner glandular layer lining the cavity; undergoes cyclical changes during the menstrual cycle.

3. Mammary Glands

Glandular tissue divided into 15–20 mammary lobes containing clusters of cells called alveoli which secrete milk. Pathway of milk: Alveoli → mammary tubules → mammary duct → mammary ampulla → **lactiferous duct** → expressed at the nipple.

3.3 Gametogenesis (Spermatogenesis & Oogenesis)

1. Spermatogenesis (Starts at Puberty)

Triggered by Hypothalamic **GnRH**, which stimulates anterior pituitary to release **LH** (stimulates Leydig cells to secrete androgens) and **FSH** (stimulates Sertoli cells to release factors helping spermiogenesis).

• Spermiogenesis: Transformation of spermatids into spermatozoa (sperms).
• Spermiation: Release of sperms from Sertoli cells into the seminiferous tubule lumen.
• Sperm Structure: Head (haploid nucleus + cap-like **acrosome** containing enzymes for egg entry), Neck, Middle piece (with many **mitochondria** providing energy for tail movement), and Tail.

2. Oogenesis (Starts in Fetal Life)

Markedly different from spermatogenesis. Couple of million **oogonia** are formed in fetal ovaries; no more form after birth.

• Oogonia start meiosis I and get arrested at Prophase I as **primary oocytes**.
• Primary oocyte + granulosa cells = **primary follicle**. Large numbers degenerate from birth to puberty; only **60,000–80,000 primary follicles** remain in each ovary at puberty.
• Follicular progression: Primary follicle → Secondary follicle → Tertiary follicle (characterised by a fluid-filled cavity called **antrum**).
• Within the tertiary follicle, the primary oocyte completes meiosis I (unequal division) to form a large haploid **secondary oocyte** and a tiny **first polar body**.
• Tertiary follicle matures into a **Graafian follicle**. The secondary oocyte forms a new membrane called **zona pellucida**.
• Graafian follicle ruptures to release the secondary oocyte (ovum) during **ovulation**.

3.4 Menstrual Cycle

Reproductive cycle in female primates (monkeys, apes, humans). Repeated at an average interval of **28/29 days**.

Phases of Menstrual Cycle

1. Menstrual Phase (Days 1–5): Breakdown of the endometrial lining and blood vessels occurs due to low progesterone. Menstrual flow lasts 3-5 days.
2. Follicular/Proliferative Phase (Days 6–13): Primary follicles mature into Graafian follicles; endometrium regenerates through proliferation. Induced by increasing estrogen and gonadotropins (LH/FSH).
3. Ovulatory Phase (Day 14): LH and FSH attain peak levels. A rapid rise in LH (called **LH surge**) causes the Graafian follicle to rupture and release the secondary oocyte (ovulation).
4. Luteal/Secretory Phase (Days 15–28): Remnants of the Graafian follicle transform into the **corpus luteum**, which secretes large amounts of **progesterone** (essential for maintaining the endometrium for implantation).

In the absence of fertilisation, the corpus luteum degenerates, causing endometrial disintegration and marking the start of a new cycle. **Menarche** is the first cycle at puberty; **Menopause** is the permanent cessation of menstrual cycles (~50 years of age).

3.5 Fertilisation, Implantation & Pregnancy

1. Fertilisation

Occurs in the ampullary region of the fallopian tube. Sperm comes in contact with the zona pellucida layer of the ovum, inducing changes that block polyspermy. Acrosome enzymes help sperm penetrate the cytoplasm, triggering the secondary oocyte to complete meiosis II (unequal division, forming a second polar body and haploid ootid/ovum). Fusion of nuclei forms the diploid **zygote**.

2. Cleavage & Implantation

As the zygote moves towards the uterus, it undergoes mitotic divisions called **cleavage** to form 2, 4, 8, 16 blastomeres.

• Embryo with 8–16 blastomeres is a **morula**.
• Morula transforms into a **blastocyst**. The blastomeres are arranged into an outer layer called **trophoblast** and an inner group called **inner cell mass**.
• Trophoblast attaches to the endometrium, and the inner cell mass differentiates into the embryo. Blastocyst gets embedded in the endometrium (**implantation**).

3. Pregnancy & Hormones

After implantation, trophoblast forms **chorionic villi** which, with uterine tissue, form the **placenta** (vascular connection between mother and fetus). The placenta acts as an endocrine tissue, secreting:

• Human chorionic gonadotropin (hCG)
• Human placental lactogen (hPL)
• Estrogens and Progestogens
• Relaxin (secreted by the ovary in later phases)

4. Parturition & Lactation

• Parturition (childbirth): Induced by a neuroendocrine mechanism. Developed fetus + placenta trigger **fetal ejection reflex**, causing maternal pituitary to release **oxytocin**, which stimulates strong uterine contractions.
• Lactation: Mammary glands produce milk. The milk produced during the initial few days is called **colostrum** and is rich in **IgA antibodies**, essential for infant immunity.