Biology: Class 12 — Chapter 2: Human Reproduction

The testes are situated outside the abdominal cavity within a pouch called the scrotum, which helps maintain a low temperature (2–2.5°C lower than normal internal body temperature) necessary for spermatogenesis.

• Structure: Oval-shaped, length 4-5 cm, width 2-3 cm. Covered by a dense fibrous coat.
• Testicular Lobules: Each testis contains about 250 compartments called testicular lobules.
• Seminiferous Tubules: Each lobule contains 1 to 3 highly coiled tubules where sperms are produced. Lined internally by two cell types:
  • Male germ cells (spermatogonia): Undergo meiotic divisions to form sperms.
  • Sertoli cells: Provide nutrition to the dividing germ cells.
• Leydig Cells (Interstitial Cells): Located in the regions outside the seminiferous tubules. They synthesise and secrete testicular hormones called androgens.

The male sex accessory ducts store and transport sperms from the testes to the outside:

1. Rete testis
2. Vasa efferentia
3. Epididymis (located along the posterior surface of each testis)
4. Vas deferens (ascends to the abdomen and loops over the urinary bladder)

The urethra originates from the urinary bladder, extends through the penis, and opens externally at the urethral meatus.

Male Accessory Glands: Include paired seminal vesicles, a prostate gland, and paired bulbourethral glands. Their secretions constitute seminal plasma, which is rich in fructose, calcium, and enzymes. Bulbourethral secretions also aid in the lubrication of the penis.

The penis is the male external genitalia, composed of special erectile tissue that facilitates insemination. Its enlarged end is called the glans penis, covered by a loose fold of skin called the foreskin.

The primary female sex organs that produce the female gamete (ovum) and steroid ovarian hormones. Located on each side of the lower abdomen, measuring 2 to 4 cm in length. Each ovary is covered by a thin epithelium enclosing the ovarian stroma, which is divided into a peripheral cortex and an inner medulla.

• Oviducts (Fallopian Tubes): 10-12 cm long, extending from the periphery of each ovary to the uterus. Composed of:
  • Infundibulum: Funnel-shaped closest to the ovary, possessing finger-like projections called fimbriae to collect the ovum after ovulation.
  • Ampulla: Wider, middle part of the oviduct.
  • Isthmus: Narrow lumen joining the uterus.
• Uterus (Womb): Single, inverted pear-shaped organ supported by ligaments. Its wall has three layers:
  • Perimetrium: External thin membranous layer.
  • Myometrium: Middle thick layer of smooth muscle; exhibits strong contractions during delivery.
  • Endometrium: Inner glandular layer lining the uterine cavity; undergoes cyclical changes during the menstrual cycle.
• Vagina and Cervix: The uterus opens into the vagina through a narrow cervix. The cavity of the cervix is the cervical canal, which along with the vagina forms the birth canal.

Female External Genitalia: Includes the mons pubis (fatty tissue cushion), labia majora (fleshy folds), labia minora (paired tissue folds), hymen (partial vaginal membrane), and clitoris (tiny finger-like structure at the upper junction of labia minora).

Mammary Glands: Paired glandular structures containing variable fat. The glandular tissue of each breast is divided into 15-20 mammary lobes containing clusters of cells called alveoli. Alveoli secrete milk, which flows through mammary tubules, mammary ducts, mammary ampulla, and finally out through the lactiferous duct.

The production of sperms in the testes, beginning at puberty due to an increase in GnRH (Gonadotropin-Releasing Hormone).

1. Spermatogonia (diploid, 46 chromosomes) multiply by mitosis.
2. Some differentiate into primary spermatocytes, which undergo meiosis-I to form two equal haploid secondary spermatocytes (23 chromosomes).
3. Secondary spermatocytes undergo meiosis-II to produce four haploid spermatids.
4. Spermatids transform into spermatozoa (sperms) via spermiogenesis.
5. Sperms release from seminiferous tubules via spermiation.

Hormonal Control: LH acts on Leydig cells to stimulate androgen secretion. FSH acts on Sertoli cells to stimulate factors for spermiogenesis.

Sperm Structure: Composed of a head (containing a haploid nucleus and an enzyme-filled cap called acrosome), neck, middle piece (packed with mitochondria for energy), and a tail for motility.

The formation of a mature female gamete, initiated during embryonic development.

• Millions of oogonia form in the fetal ovary; no more are formed after birth.
• Oogonia enter prophase-I of meiosis and get temporarily arrested as primary oocytes.
• Primary oocytes get surrounded by granulosa cells, forming primary follicles. Many degenerate before puberty, leaving only 60,000-80,000 per ovary.
• Primary follicles develop into secondary follicles, then tertiary follicles (characterized by a fluid-filled cavity called antrum).
• The primary oocyte within the tertiary follicle completes meiosis-I, forming a large haploid secondary oocyte and a tiny first polar body.
• The tertiary follicle matures into a Graafian follicle. The secondary oocyte forms a new membrane called zona pellucida.
• The Graafian follicle ruptures to release the secondary oocyte from the ovary via ovulation.

1. Menstrual Phase (Days 1-5): Breakdown of the endometrial lining and blood vessels, resulting in menstrual flow. Occurs only if the released ovum is not fertilised.
2. Follicular / Proliferative Phase (Days 6-13): Primary follicles grow into mature Graafian follicles under the influence of FSH and LH. The endometrium regenerates through proliferation. Estrogen levels rise.
3. Ovulatory Phase (Day 14): Rapid secretion of LH leads to maximum levels (LH surge), inducing rupture of the Graafian follicle and release of the ovum.
4. Luteal / Secretory Phase (Days 15-28): Ruptured Graafian follicle transforms into the corpus luteum, which secretes high levels of progesterone to maintain the endometrium for potential implantation. In the absence of fertilisation, the corpus luteum degenerates, leading to menstruation and a new cycle.

The process of childbirth, induced by a complex neuroendocrine mechanism.

• Signals originate from the fully developed foetus and placenta, inducing mild uterine contractions called the foetal ejection reflex.
• This triggers the release of oxytocin from the maternal pituitary gland.
• Oxytocin stimulates stronger uterine contractions, establishing a positive feedback loop that leads to the expulsion of the baby through the birth canal.

The mammary glands produce milk towards the end of pregnancy.

• The milk produced during the initial few days of lactation is called colostrum.
• Colostrum is rich in essential antibodies (especially IgA) crucial for developing resistance and immunity in the newborn.