Human Reproduction

Key Concepts

The Male Reproductive System

The male reproductive system is located in the pelvis region and includes a pair of testes, accessory ducts, glands, and external genitalia.

• Testes: Situated outside the abdominal cavity in the scrotum, which maintains a temperature 2–2.5°C lower than the internal body temperature, essential for spermatogenesis.
• Testicular Lobules: Each testis has about 250 compartments. Each lobule contains 1–3 highly coiled seminiferous tubules.
• Cells in Seminiferous Tubules:
  • Male Germ Cells (Spermatogonia): Undergo meiotic divisions to form sperms.
  • Sertoli Cells: Provide nutrition to the germ cells.
  • Leydig Cells (Interstitial Cells): Located outside tubules; synthesize and secrete testicular hormones called androgens.
• Accessory Ducts: Include rete testis, vasa efferentia, epididymis, and vas deferens.
• Accessory Glands: Include paired seminal vesicles, a prostate, and paired bulbourethral glands. Their secretions form seminal plasma (rich in fructose, calcium, and enzymes).

The Female Reproductive System

Located in the pelvic region, it consists of a pair of ovaries, oviducts, uterus, cervix, vagina, and external genitalia, along with mammary glands.

• Ovaries: Primary female sex organs. Produce the ovum and steroid hormones (estrogen and progesterone).
• Accessory Ducts:
  • Oviducts (Fallopian Tubes): 10–12 cm long. Parts include the infundibulum (with finger-like fimbriae), ampulla, and isthmus.
  • Uterus (Womb): Supported by ligaments. The wall has three layers: perimetrium (outer), myometrium (middle smooth muscle), and endometrium (inner glandular layer).
• External Genitalia: Includes mons pubis, labia majora, labia minora, hymen, and clitoris.
• Mammary Glands: Paired structures containing glandular tissue and fat. Divided into 15–20 mammary lobes containing alveoli, which secrete milk.

Gametogenesis

The process of gamete formation.

• Spermatogenesis:
  • Starts at puberty due to increase in GnRH.
  • GnRH stimulates LH (acts on Leydig cells for androgens) and FSH (acts on Sertoli cells).
  • Spermiogenesis: Transformation of spermatids into spermatozoa (sperms).
  • Spermiation: Release of sperms from seminiferous tubules.
• Structure of Sperm: Composed of a head (with a haploid nucleus and an acrosome filled with enzymes), neck, middle piece (with mitochondria for energy), and a tail.
• Oogenesis:
  • Initiated during embryonic development.
  • Primary oocytes are arrested at Prophase-I.
  • At puberty, only 60,000–80,000 primary follicles remain in each ovary.
  • Ovulation: Rupture of the Graafian follicle to release the secondary oocyte (ovum).

Menstrual Cycle

The reproductive cycle in female primates (e.g., humans).

1. Menstrual Phase (Days 1–5): Breakdown of endometrial lining; occurs if the ovum is not fertilized.
2. Follicular Phase: Primary follicles grow into Graafian follicles; endometrium regenerates. Induced by LH and FSH.
3. Ovulatory Phase (Mid-cycle): LH surge induces rupture of Graafian follicle and release of ovum.
4. Luteal Phase: Remaining Graafian follicle transforms into corpus luteum, which secretes progesterone to maintain the endometrium.

Fertilisation and Implantation

• Fertilisation: Fusion of sperm and ovum in the ampullary region of the fallopian tube.
• Sex Determination: The sex of the baby is determined by the father. Sperms carry either X or Y chromosomes, while ova only carry X. XX = Female, XY = Male.
• Cleavage: Mitotic division of the zygote to form 2, 4, 8, 16 daughter cells called blastomeres.
• Implantation: The blastocyst (with trophoblast and inner cell mass) embeds into the endometrium.

Pregnancy and Embryonic Development

• Placenta: A structural and functional unit between the foetus and maternal body. Facilitates nutrient/oxygen supply and waste removal.
• Hormones: Placenta produces hCG, hPL, estrogens, and progestogens. Relaxin is secreted by the ovary in the later phase.
• Germ Layers: Inner cell mass differentiates into ectoderm, endoderm, and mesoderm.
• Developmental Milestones:
  • 1 Month: Heart formed.
  • 2 Months: Limbs and digits develop.
  • 12 Weeks (First Trimester): Major organ systems formed.
  • 5 Months: First movements and head hair.
  • 24 Weeks: Body hair, eyelids separate, eyelashes form.

Parturition and Lactation

• Parturition: The process of delivery of the foetus (childbirth). Induced by a neuroendocrine mechanism involving oxytocin.
• Lactation: Production of milk by mammary glands.
• Colostrum: The milk produced during the initial few days; rich in antibodies (IgA) essential for the newborn’s resistance.