22. REPRODUCTIVE SYSTEM

REPRODUCTIVE SYSTEM

Humans are sexually reproducing, unisexual, and viviparous (development of the embryo inside the body of the parent). Both males and females have distinct reproductive organs (gonads), reproductive ducts, and accessory structures.

MALE REPRODUCTIVE SYSTEM

The male reproductive system produces male gametes (sperm) and delivers them to the female reproductive tract. Its secondary function is secretion of hormones (e.g., androgens).

Scrotum and Testes

• Scrotum: A pouch of pigmented skin, fibrous tissue, and smooth muscle; maintains temperature 2-2.5°C lower than body (required for spermatogenesis).
• Testes: Covered by three layers – tunica vaginalis, tunica albuginea, tunica vasculosa. Divided into 200-300 lobules containing seminiferous tubules.
• Seminiferous Tubules: Contain spermatogonia (germ cells) and Sertoli cells (nourish germ cells). Interstitial spaces contain Leydig cells (secrete testosterone).

Accessory Ducts

• Rete Testes: Seminiferous tubules open into vasa efferentia through rete testis
• Vasa Efferentia: Transport sperm from testes to epididymis
• Epididymis: Stores and matures sperm (highly coiled tubule)
• Spermatic Cords: Contain ductus deferens, nerves, blood vessels; connect testes to abdomen
• Vas Deferens (Ductus Deferens): Muscular tube carrying sperm from epididymis to ejaculatory duct
• Seminal Vesicles: Paired glands behind bladder; produce alkaline fluid (fructose, proteins, mucus) to neutralize vaginal acidity
• Ejaculatory Duct: Unites vas deferens with urethra; ejects sperm and seminal fluid during ejaculation

Accessory Glands

• Prostate Gland: Largest accessory sex gland; surrounds urethra origin; secretes fibrinolysin (makes semen liquid); produces PSA (marker for prostate cancer)
• Bulbourethral (Cowper’s) Glands: Pea-sized glands; secrete alkaline mucus for lubrication and pH neutralization of urine residue

External Genitalia (Penis)

• Penis: Male external genitalia; contains three cylindrical masses of cavernous tissue – corpora cavernosa penis (2 lateral) and corpus spongiosum (contains urethra)
• Glans Penis: Enlarged end covered by foreskin (prepuce)
• Functions: Passage for urine; deposits sperm in female reproductive tract

FEMALE REPRODUCTIVE SYSTEM

External Genitalia (Vulva)

• Mons Pubis: Fatty pad over pubic bone; covered with pubic hair after puberty
• Labia Majora: Hair-covered folds surrounding vaginal opening
• Labia Minora: Pigmented folds under labia majora; protect urethra and vaginal entrance
• Clitoris: Finger-like structure at upper junction of labia minora (homologous to glans penis)
• Hymen: Thin membrane partially covering vaginal opening
• Bartholin’s Glands: Outlets at vaginal opening borders

Internal Genitalia

Vagina

• Elastic muscular tube (~3 inches long) joining uterine cervix with vulva
• Lined with non-keratinized stratified squamous epithelium (acidic pH prevents infection)
• Functions: Receives penis, passage for menstruation and childbirth

Uterus

• Hollow muscular organ (7.5×5×2.5 cm; weight 30-40 g)
• Parts: Fundus (upper rounded part), Body, Cervix (lower narrow part with internal and external os)
• Layers: Perimetrium (outermost), Myometrium (middle smooth muscle), Endometrium (inner lining; has straight and coiled arteries)
• Functions: Receives fertilised ovum, retains and nourishes foetus, expels foetus during parturition, involved in menstruation

Fallopian Tubes (Oviducts)

• 10-12 cm long, 1 cm diameter
• Parts: Isthmus (closest to uterus), Ampulla (site of fertilisation), Infundibulum (funnel-shaped with fimbriae)
• Functions: Collect ovum, site of fertilisation, convey fertilised egg to uterus

Ovaries

• Oval-shaped paired organs (2-4 cm long)
• Parts: Germinal epithelium, Tunica albuginea, Ovarian cortex (contains follicles), Ovarian medulla (connective tissue, blood vessels)
• Ovarian Follicles: Contain oocytes; follicular cells (single layer) → granulosa cells (multiple layers)
• Graafian Follicle: Mature follicle ready for ovulation
• Corpus Luteum: Remnant after ovulation; produces progesterone, oestrogen, relaxin, inhibin

Breasts (Mammary Glands)

• Paired glandular, fibrous, and fatty tissues (functional only in females)
• Structure: 20 lobes → lobules → alveoli (milk secretion) → mammary tubules → mammary ducts → lactiferous sinus → lactiferous ducts → nipple
• Areola: Dark pigmented area around nipple; contains Montgomery’s tubercles (sebaceous glands lubricating nipple)

PHYSIOLOGY OF MENSTRUATION

The menstrual cycle is a periodic physiological change in females. Menarche: first menstruation (8-16 years). Menopause: permanent cessation of menstruation. Average cycle length: 28/29 days.

Phases of Menstrual Cycle

1. Menstrual Phase (Days 1-5): Vaginal bleeding; shedding of endometrium (if egg not fertilized); lasts 3-5 days
2. Follicular Phase (Days 5-13): Uterus lining thickens (oestrogen); ovarian follicles develop; one dominant follicle matures
3. Ovulation Phase (Day 14): LH surge triggers follicle rupture; mature ovum released; captured by fallopian tube
4. Luteal Phase (Days 15-28): Corpus luteum forms; produces progesterone and oestrogen; prepares endometrium for implantation

Fertile Period: 5 days before ovulation to 1-2 days after ovulation (peak pregnancy probability).

GAMETOGENESIS

Gametogenesis is the process of meiotic division for gamete formation. Spermatogenesis (males) produces sperm; Oogenesis (females) produces ovum.

Spermatogenesis

• Occurs in seminiferous tubules
• Meiosis: Spermatogonium (2n) → primary spermatocyte → secondary spermatocytes → 4 haploid spermatids
• Spermiogenesis: Spermatid → mature sperm (head with acrosome, midpiece with mitochondria, tail for motility)
• Healthy male produces ~400 million sperm/day

Oogenesis

• Begins before birth; continues until menopause
• Oogonia (2n) → primary oocyte (arrested in prophase I until puberty) → secondary oocyte (after meiosis I) → ovum (after meiosis II, only if fertilised)
• At birth: ~60,000-80,000 primary follicles per ovary
• Produces 1 functional ovum + polar bodies (non-functional)

REPRODUCTION PROCESS

Fertilisation

• Occurs in ampulla of fallopian tube
• Fertilizin (ovum) + anti-fertilizin (sperm) → species-specific attraction
• Acrosome reaction: Sperm releases hyaluronidase to penetrate corona radiata and zona pellucida

Cleavage and Blastocyst Formation

• Zygote undergoes rapid mitotic divisions (cleavage) → blastomeres → morula (32-cell stage) → blastocyst (inner cell mass + trophoblast)
• Inner cell mass → embryo; Trophoblast → foetal portion of placenta

Implantation

By day 6 after fertilisation, the blastocyst implants in the endometrium (posterior portion of fundus).

Pregnancy (Gestation)

• Duration: ~38 weeks from fertilisation, ~40 weeks from last menstruation (9 months)
• Trimesters:
  • First (1-12 weeks): Conception, implantation, placenta formation
  • Second (13-28 weeks): Foetal movement (20th week), footprints/fingerprints develop
  • Third (29-40 weeks): Bones fully developed; lung, brain, liver mature by 39th week
• Terminology: Preterm (<37 weeks), Early term (37-38 weeks), Full term (39-40 weeks), Late term (41 weeks), Post term (≥42 weeks)

Parturition (Childbirth)

Parturition is the final stage of pregnancy (labour). Occurs within 15 days of the due date (280 days from last menstrual period).

SUMMARY: DIFFERENCES BETWEEN SPERMATOGENESIS AND OOGENESIS

Feature	Spermatogenesis	Oogenesis
Begins at	Puberty (continues lifelong)	Before birth (continues till menopause)
Number of gametes	1 primary spermatocyte → 4 sperms	1 primary oocyte → 1 ovum + polar bodies
Cytoplasm	Lost	Conserved
Continues without interruption	Oocyte completes meiosis II only after fertilisation
Gamete types	X and Y sperms	Only one type of oocyte