Learning Objectives
- Understand the male and female reproductive systems
- Learn about gametogenesis โ spermatogenesis and oogenesis
- Study the menstrual cycle and its hormonal regulation
- Understand fertilization, implantation, and embryonic development
- Learn about pregnancy, parturition, and lactation
Key Concepts
Male Reproductive System
Testes: Primary sex organs, located in scrotum (2-2.5 degrees C below body temperature for spermatogenesis). Each testis has ~250 testicular lobules with seminiferous tubules (site of sperm formation). Seminiferous tubules lined by Sertoli cells (nourish developing sperm) and contain spermatogonia. Leydig cells (interstitial cells) in spaces between tubules produce testosterone. Accessory ducts: rete testis โ vasa efferentia โ epididymis โ vas deferens โ ejaculatory duct โ urethra. Accessory glands: seminal vesicles (fructose-rich fluid), prostate gland, bulbourethral (Cowper's) glands โ secretions form seminal plasma.
Female Reproductive System
Ovaries: Primary sex organs, produce ova and hormones (estrogen, progesterone). Oviducts (Fallopian tubes): Infundibulum (with fimbriae) โ Ampulla (site of fertilization) โ Isthmus โ Uterus. Uterus: Pear-shaped, three layers โ perimetrium (outer), myometrium (middle, smooth muscle), endometrium (inner, glandular, undergoes cyclic changes). Cervix connects uterus to vagina.
Spermatogenesis
Spermatogonia (2n) โ Primary spermatocytes (2n, by mitosis) โ Secondary spermatocytes (n, by meiosis I) โ Spermatids (n, by meiosis II) โ Spermatozoa (by spermiogenesis). One spermatogonium produces 4 sperms. Sperm structure: Head (acrosome with enzymes + nucleus with haploid DNA), Middle piece (mitochondria โ energy), Tail (flagellum โ motility). Spermiation: Release of mature sperm from Sertoli cells. Hormonal control: GnRH (hypothalamus) โ LH (stimulates Leydig cells โ testosterone) + FSH (stimulates Sertoli cells โ spermatogenesis).
Oogenesis
Begins in foetal life. Oogonia (2n) โ Primary oocytes (2n, arrested in prophase I of meiosis at birth โ about 2 million). At puberty, ~60,000-80,000 remain. Each menstrual cycle: one primary oocyte completes meiosis I โ secondary oocyte (n) + first polar body. Meiosis II completed only upon fertilization โ ovum + second polar body. One oogonium produces 1 ovum + 3 polar bodies.
Menstrual Cycle
Average duration: 28 days. Phases: (1) Menstrual phase (Day 1-5): Endometrium sheds, bleeding. (2) Follicular/Proliferative phase (Day 6-13): FSH stimulates follicular growth, estrogen increases, endometrium regenerates. (3) Ovulation (Day 14): LH surge causes release of secondary oocyte from Graafian follicle. (4) Luteal/Secretory phase (Day 15-28): Ruptured follicle โ corpus luteum โ secretes progesterone, maintains endometrium. If no fertilization, corpus luteum degenerates, progesterone drops, menstruation begins. Menarche: First menstruation. Menopause: Cessation of menstruation (~50 years).
Fertilization and Development
Fertilization occurs in ampulla of oviduct. Sperm penetrates zona pellucida using acrosomal enzymes. Cortical reaction prevents polyspermy. Zygote (2n) undergoes cleavage โ morula (16 cells) โ blastocyst (trophoblast + inner cell mass). Implantation: Blastocyst embeds in endometrium (~7 days after fertilization). Trophoblast forms chorionic villi โ contributes to placenta formation.
Placenta: Structural and functional connection between mother and foetus. Functions: nutrition, respiration, excretion, hormone production (hCG, hPL, estrogen, progesterone). hCG (human chorionic gonadotropin) is detected in pregnancy tests. Gestation period: ~9 months (280 days). Parturition: Delivery of baby. Triggered by oxytocin from posterior pituitary โ uterine contractions (positive feedback). Lactation: Milk production; colostrum (first milk) contains antibodies (IgA) โ provides passive immunity.
Summary
Human reproduction involves gametogenesis, fertilization, implantation, and embryonic development. Spermatogenesis produces 4 sperms per spermatogonium; oogenesis produces 1 ovum. The menstrual cycle prepares the uterus for implantation every 28 days. Fertilization occurs in the ampullary region. The placenta connects mother and foetus. Parturition is triggered by oxytocin.
Important Terms
- Spermiogenesis: Transformation of spermatids into spermatozoa
- Corpus luteum: Ruptured Graafian follicle that secretes progesterone
- Implantation: Embedding of blastocyst in uterine endometrium
- Placenta: Organ connecting mother and foetus for exchange of materials
- hCG: Hormone produced by placenta; basis of pregnancy tests
- Colostrum: First yellowish milk rich in antibodies
- Acrosome: Cap-like structure on sperm head with enzymes for zona pellucida penetration
- Parturition: Process of childbirth triggered by oxytocin
Quick Revision
- Spermatogenesis: 1 spermatogonium โ 4 sperms; Oogenesis: 1 oogonium โ 1 ovum + 3 polar bodies
- Leydig cells โ testosterone; Sertoli cells โ nourish sperm
- Menstrual cycle: 28 days; Ovulation at Day 14 (LH surge)
- Fertilization in ampulla of oviduct
- Zygote โ Morula โ Blastocyst โ Implantation (~Day 7)
- Placenta produces hCG, hPL, estrogen, progesterone
- Colostrum = first milk with IgA antibodies
- Parturition triggered by oxytocin (positive feedback)