Human Reproduction (NEET UG)

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Male Reproductive System Primary sex organs: a pair of testes. Accessory ducts: rete testis, vasa efferentia, epididymis, vas deferens. Accessory glands: paired seminal vesicles, a prostate gland, paired bulbourethral glands. Testes: located outside abdominal cavity within a pouch called scrotum. Scrotum maintains $2-2.5^\circ C$ lower temperature than normal body temperature for spermatogenesis. Each testis has about 250 compartments called testicular lobules. Each lobule contains 1-3 highly coiled seminiferous tubules. Seminiferous tubules are lined by spermatogonia (male germ cells) and Sertoli cells. Sertoli cells provide nutrition to the germ cells. Interstitial spaces (outside seminiferous tubules) contain Leydig cells (interstitial cells). Leydig cells synthesize and secrete testicular hormones (androgens). Epididymis: stores sperm temporarily and aids in maturation. Vas deferens: ascends to the abdomen, loops over urinary bladder, receives duct from seminal vesicle to form ejaculatory duct. Urethra originates from urinary bladder and extends through penis to its external opening (urethral meatus). Seminal plasma + sperm = semen. Bulbourethral glands' secretion helps in lubrication of the penis. Female Reproductive System Primary sex organs: a pair of ovaries. Accessory ducts: oviducts (fallopian tubes), uterus, vagina. Accessory glands: Bartholin's glands, mammary glands. Ovaries: located one on each side of the lower abdomen. Ovaries are connected to pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium, enclosing ovarian stroma (cortex and medulla). Oviducts (Fallopian tubes): about 10-12 cm long. Parts of oviduct: infundibulum (funnel-shaped, with fimbriae), ampulla (wider part), isthmus (narrow lumen, joins uterus). Fimbriae help in collection of ovum after ovulation. Uterus (womb): inverted pear shape. Attached to pelvic wall by ligaments. Uterine wall layers: perimetrium (external thin membranous), myometrium (middle thick smooth muscle), endometrium (inner glandular layer, undergoes cyclical changes). Cervix: narrow opening of the uterus into the vagina. Cervical canal + vagina = birth canal. External genitalia (vulva): mons pubis, labia majora, labia minora, hymen, clitoris. Clitoris: tiny finger-like structure at upper junction of labia minora above urethral opening. Hymen can be torn during first coitus, but also by other activities. Its presence/absence is not a reliable indicator of virginity. Gametogenesis Formation of gametes: spermatogenesis (sperm), oogenesis (ovum). Spermatogenesis: Spermatogonia (2n) $\xrightarrow{\text{mitosis differentiation}}$ Primary spermatocytes (2n) Primary spermatocytes (2n) $\xrightarrow{\text{meiosis I}}$ Secondary spermatocytes (n) Secondary spermatocytes (n) $\xrightarrow{\text{meiosis II}}$ Spermatids (n) Spermatids $\xrightarrow{\text{spermiogenesis}}$ Spermatozoa (sperm) Spermiation: release of sperm from seminiferous tubules. Sperm head contains elongated haploid nucleus. Acrosome (cap-like, anterior) contains enzymes for fertilization. Middle piece contains numerous mitochondria for energy (tail movement). Oogenesis: Initiated during fetal development. No more oogonia formed/added after birth. Oogonia (2n) $\xrightarrow{\text{mitosis}}$ Primary oocytes (2n) - arrested at prophase I of meiosis. At puberty, primary oocytes complete meiosis I $\rightarrow$ Secondary oocyte (n) + First polar body. Secondary oocyte arrested at metaphase II of meiosis. Ovulation: release of secondary oocyte. Meiosis II completed only upon sperm entry $\rightarrow$ Ovum (n) + Second polar body. Ovum is female gamete. Menstrual Cycle Cyclical changes in female primates. Average duration 28/29 days. Begins at puberty (menarche). Ceases around 50 years (menopause). Phases: Menstrual Phase (Day 1-5): Breakdown of endometrial lining, blood vessels, forms liquid that exits through vagina. Caused by low progesterone and estrogen. Lack of fertilization. Follicular/Proliferative Phase (Day 5-13): Primary follicles grow into Graafian follicle. Endometrium regenerates. FSH and LH increase. Estrogen from growing follicles increases. Ovulatory Phase (Day 14): LH surge induces rupture of Graafian follicle and release of ovum (ovulation). Luteal/Secretory Phase (Day 15-28): Ruptured Graafian follicle transforms into Corpus Luteum. Corpus Luteum secretes large amounts of progesterone (essential for maintaining endometrium). If fertilization occurs, pregnancy begins, menstrual cycle stops. If no fertilization, corpus luteum degenerates, progesterone levels fall, leading to menstruation. Fertilization and Implantation Coitus $\rightarrow$ insemination (sperm released into vagina). Sperm travels through cervix, uterus, to isthmic-ampullary junction of fallopian tube. Fertilization occurs only if ovum and sperm are simultaneously transported to the ampullary-isthmic junction. Acrosome enzymes (hyaluronidase, acrosin) help sperm penetrate corona radiata and zona pellucida. Zona pellucida block polyspermy (entry of multiple sperm). Completion of Meiosis II of secondary oocyte forms ovum and second polar body. Fusion of male and female pronuclei $\rightarrow$ Zygote (2n). Zygote undergoes cleavage (mitotic division) $\rightarrow$ blastomeres $\rightarrow$ morula $\rightarrow$ blastocyst. Blastocyst has trophoblast (outer layer) and inner cell mass. Trophoblast cells attach to endometrium. Inner cell mass differentiates into embryo. Implantation: attachment of blastocyst to uterine wall (around 7 days after fertilization). After implantation, trophoblast cells form chorionic villi. Pregnancy and Embryonic Development Chorionic villi and uterine tissue form placenta. Placenta: Facilitates nutrient, oxygen supply to embryo. Removes $\text{CO}_2$, excretory waste from embryo. Acts as an endocrine gland: produces hCG, hPL, estrogen, progesterone, relaxin (late pregnancy). hCG, hPL, relaxin are produced only during pregnancy. Inner cell mass (embryoblast) differentiates into outer ectoderm, inner endoderm, and mesoderm. Stem cells: totipotent cells in inner cell mass. Gestation periods: 1 month: Heart formed. 2 months: Limbs and digits develop. 12 weeks (first trimester): Most major organ systems formed. 5 months: First movements, appearance of hair on head. 24 weeks (second trimester): Body covered with fine hair, eyelids separate, eyelashes form. 9 months: Fetus fully developed, ready for delivery. Parturition and Lactation Parturition (childbirth): vigorous contraction of uterus at end of pregnancy. Induced by complex neuroendocrine mechanism. Signal for parturition originates from fully developed fetus and placenta $\rightarrow$ mild uterine contractions (fetal ejection reflex). Fetal ejection reflex triggers oxytocin release from maternal pituitary. Oxytocin acts on uterine muscles $\rightarrow$ stronger contractions. Relaxin (from ovary) helps in loosening pelvic ligaments. Lactation: production of milk by mammary glands towards end of pregnancy. Colostrum: first milk, rich in antibodies (IgA) to provide passive immunity to newborn.