Class 12 Notes

Class 12 Biology Notes Chapter 3 (Human reproduction) – Biology Book

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Philoid

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Biology
Detailed Notes with MCQs of Chapter 3: Human Reproduction. This is a crucial chapter, not just for your board exams but also for various government competitive exams where Biology is a component. We need to understand the processes involved in detail.

Chapter 3: Human Reproduction - Detailed Notes

1. The Male Reproductive System

  • Location: Pelvis region.
  • Components:
    • Primary Sex Organ: A pair of testes.
    • Accessory Ducts: Rete testis, vasa efferentia, epididymis, and vas deferens.
    • Accessory Glands: Paired seminal vesicles, a prostate gland, and paired bulbourethral glands.
    • External Genitalia: Penis.
  • Testes:
    • Located outside the abdominal cavity within a pouch called the scrotum.
    • Scrotum: Maintains a temperature 2-2.5°C lower than the normal internal body temperature, essential for spermatogenesis (sperm formation).
    • Each testis has about 250 compartments called testicular lobules.
    • Each lobule contains 1-3 highly coiled seminiferous tubules where sperm are produced.
    • Seminiferous Tubules (Lining):
      • Spermatogonia (male germ cells): Undergo meiosis to form sperm.
      • Sertoli cells: Provide nutrition to the germ cells, secrete inhibin (hormone).
    • Interstitial Spaces (outside tubules): Contain Leydig cells (interstitial cells).
    • Leydig cells: Synthesize and secrete androgens, mainly testosterone.
  • Accessory Ducts:
    • Seminiferous tubules open into rete testis through vasa efferentia.
    • Vasa efferentia leave the testis and open into the epididymis (located along the posterior surface of each testis).
    • Epididymis leads to vas deferens, which ascends to the abdomen and loops over the urinary bladder.
    • Vas deferens receives a duct from the seminal vesicle and opens into the urethra as the ejaculatory duct.
    • Urethra: Originates from the urinary bladder and extends through the penis to its external opening (urethral meatus). Carries both urine and semen (in males).
  • Accessory Glands:
    • Seminal Vesicles (paired): Contribute fluid rich in fructose (energy for sperm), calcium, and certain enzymes to form seminal plasma. Constitutes the major part of semen.
    • Prostate Gland (single): Secretes a milky, slightly alkaline fluid containing citrate (nutrient) and enzymes (e.g., prostate-specific antigen) that helps activate sperm.
    • Bulbourethral Glands (Cowper's glands - paired): Secrete mucus that helps in the lubrication of the penis.
    • Semen: Seminal plasma along with sperm.
  • External Genitalia (Penis):
    • Male copulatory organ. Made of special erectile tissue that facilitates insemination.
    • Enlarged end called glans penis, covered by a loose fold of skin called foreskin (prepuce).
  • Hormonal Control:
    • GnRH (Gonadotropin-releasing hormone): Secreted by the hypothalamus, stimulates the anterior pituitary.
    • LH (Luteinizing Hormone): Acts on Leydig cells to stimulate testosterone synthesis and secretion.
    • FSH (Follicle-stimulating Hormone): Acts on Sertoli cells to stimulate factors helping in spermiogenesis.
    • Testosterone: Regulates development of secondary sexual characters, spermatogenesis, libido.

2. The Female Reproductive System

  • Location: Pelvis region.
  • Components:
    • Primary Sex Organ: A pair of ovaries.
    • Accessory Ducts: Oviducts (fallopian tubes), uterus, vagina.
    • External Genitalia: Mons pubis, labia majora, labia minora, hymen, clitoris.
    • Mammary Glands: Structurally and functionally integrated for child care.
  • Ovaries:
    • Located one on each side of the lower abdomen, connected to the pelvic wall and uterus by ligaments.
    • Almond-shaped. Each is covered by a thin epithelium enclosing the ovarian stroma.
    • Stroma: Divided into peripheral cortex and inner medulla.
    • Functions: Produce the female gamete (ovum) and steroid hormones (estrogen and progesterone).
  • Accessory Ducts:
    • Oviducts (Fallopian Tubes): ~10-12 cm long. Extend from the periphery of each ovary to the uterus.
      • Infundibulum: Funnel-shaped part closer to the ovary. Edges possess finger-like projections called fimbriae (help collect ovum after ovulation).
      • Ampulla: Wider part, site of fertilisation.
      • Isthmus: Narrow last part, joins the uterus.
    • Uterus (Womb): Single, inverted pear-shaped. Supported by ligaments attached to the pelvic wall. Site of implantation and fetal development.
      • Opens into the vagina through a narrow cervix. The cavity of the cervix is the cervical canal.
      • Uterine Wall Layers:
        • Perimetrium: External thin membrane.
        • Myometrium: Middle thick layer of smooth muscle (strong contractions during childbirth).
        • Endometrium: Inner glandular layer. Undergoes cyclical changes during the menstrual cycle; site of implantation.
    • Vagina: Muscular tube connecting the cervix to the outside. Receives semen during copulation; serves as the birth canal.
  • External Genitalia (Vulva):
    • Mons pubis: Cushion of fatty tissue covered by skin and pubic hair.
    • Labia majora: Fleshy folds of tissue, extending down from the mons pubis and surrounding the vaginal opening.
    • Labia minora: Paired folds of tissue under the labia majora.
    • Hymen: Membrane partially covering the opening of the vagina (often torn during first coitus or other activities).
    • Clitoris: Tiny finger-like structure lying at the upper junction of the two labia minora above the urethral opening. Homologous to the penis glans.
  • Mammary Glands (Breasts):
    • Paired structures containing glandular tissue and variable amounts of fat.
    • Glandular tissue: Divided into 15-20 mammary lobes, containing clusters of cells called alveoli.
    • Alveoli: Secrete milk, stored in their lumens. Open into mammary tubules.
    • Tubules of each lobe join to form a mammary duct.
    • Several ducts join to form a wider mammary ampulla, connected to a lactiferous duct through which milk is sucked out.

3. Gametogenesis

  • The process of formation of gametes (sperm and ova).
  • Spermatogenesis (Formation of Sperm):
    • Occurs in seminiferous tubules, starts at puberty.
    • Process:
      1. Spermatogonia (diploid, 2n=46) multiply by mitosis. Some grow to become primary spermatocytes (2n).
      2. Primary spermatocytes undergo Meiosis I to form two equal, haploid secondary spermatocytes (n=23).
      3. Secondary spermatocytes undergo Meiosis II to form four equal, haploid spermatids (n=23).
      4. Spermiogenesis: Transformation of spermatids into spermatozoa (sperm).
      5. Spermiation: Release of sperm from Sertoli cells into the lumen of seminiferous tubules.
    • Structure of Sperm:
      • Head: Contains elongated haploid nucleus; anterior portion covered by cap-like acrosome (contains enzymes like hyaluronidase for fertilisation).
      • Neck: Connects head and middle piece.
      • Middle Piece: Contains numerous mitochondria (energy for tail movement).
      • Tail: Long flagellum, facilitates sperm motility.
    • About 200-300 million sperm ejaculated during coitus. For normal fertility, at least 60% must have normal shape/size, and 40% must show vigorous motility.
  • Oogenesis (Formation of Ovum):
    • Occurs in the ovary, initiated during embryonic development.
    • Process:
      1. Oogonia (diploid, 2n) formed in the fetal ovary, multiply by mitosis. No more oogonia formed after birth.
      2. Oogonia enter Meiosis I and get arrested in Prophase I, now called primary oocytes (2n).
      3. Primary oocytes get surrounded by granulosa cells, forming primary follicles. Many degenerate from birth to puberty.
      4. At puberty, primary follicles develop into secondary follicles (more granulosa layers, new theca).
      5. Secondary follicles transform into tertiary follicles (fluid-filled cavity called antrum appears; theca differentiates into inner theca interna and outer theca externa).
      6. Inside the tertiary follicle, the primary oocyte grows and completes Meiosis I, forming a large, haploid secondary oocyte (n=23) and a tiny first polar body (n). (Unequal division).
      7. Tertiary follicle matures into Graafian follicle. Secondary oocyte forms a new membrane, zona pellucida.
      8. Ovulation: Graafian follicle ruptures, releasing the secondary oocyte (ovum) from the ovary.
      9. Secondary oocyte begins Meiosis II but arrests in Metaphase II.
      10. Meiosis II completes only upon entry of a sperm into the cytoplasm of the secondary oocyte, forming a large ootid (ovum) (n) and a second polar body (n).
    • Key Differences: Spermatogenesis produces 4 motile sperm; Oogenesis produces 1 large non-motile ovum and 2-3 polar bodies. Oogenesis starts in fetus, arrests, resumes at puberty, ceases at menopause; Spermatogenesis starts at puberty and continues throughout life.

4. Menstrual Cycle

  • Reproductive cycle in female primates (e.g., monkeys, apes, humans).
  • First menstruation begins at puberty (menarche). Ceases around 50 years of age (menopause).
  • Cycle length: ~28/29 days. Cycle of events from one menstruation till the next.
  • Phases:
    1. Menstrual Phase (Day 1-5):
      • Cycle starts with menstrual flow (bleeding).
      • Caused by breakdown of endometrial lining and its blood vessels.
      • Occurs due to decline in progesterone and estrogen levels (corpus luteum degenerates if fertilisation doesn't occur).
    2. Follicular Phase (Proliferative Phase) (Day 5-14):
      • Primary follicles grow to become mature Graafian follicles.
      • Endometrium regenerates through proliferation.
      • Driven by increasing levels of pituitary hormones (FSH, LH) and ovarian hormone (estrogen).
      • Estrogen levels rise significantly.
    3. Ovulatory Phase (~Day 14):
      • Rapid secretion of LH (LH surge) induces rupture of Graafian follicle and release of ovum (secondary oocyte).
      • FSH also peaks.
    4. Luteal Phase (Secretory Phase) (Day 15-28):
      • Remaining parts of Graafian follicle transform into corpus luteum.
      • Corpus luteum secretes large amounts of progesterone (essential for maintaining endometrium for implantation). Estrogen levels also remain elevated.
      • Endometrium thickens further, becomes more glandular and vascularised.
      • If fertilisation occurs, corpus luteum persists (stimulated by hCG).
      • If fertilisation does not occur, corpus luteum degenerates, causing disintegration of endometrium (leading to menstruation), marking a new cycle. Progesterone/Estrogen levels fall.

5. Fertilisation and Implantation

  • Insemination: Release of semen into the vagina during coitus.
  • Sperm Transport: Motile sperm swim through the cervix, uterus, and reach the ampullary region of the fallopian tube. Ovum released by the ovary also transported to the ampulla.
  • Fertilisation: Fusion of sperm and ovum occurs only if they reach the ampulla simultaneously.
    • Capacitation: Changes sperm undergo in the female tract to become capable of fertilisation.
    • Acrosomal Reaction: Sperm contacts zona pellucida, acrosome releases enzymes (hyaluronidase, acrosin) to digest zona pellucida and plasma membrane of the ovum.
    • Sperm head enters the cytoplasm of the secondary oocyte.
    • Cortical Reaction: Entry of sperm induces completion of Meiosis II of the secondary oocyte (forming ovum and second polar body) and changes in the zona pellucida (depolarisation, zona reaction) to block entry of additional sperm (prevents polyspermy).
    • Haploid nucleus of sperm fuses with the haploid nucleus of the ovum to form a diploid zygote (2n).
  • Sex Determination: Determined at fertilisation. Ovum always carries X chromosome. Sperm carries either X or Y. XX zygote develops into female; XY zygote develops into male.
  • Cleavage: Zygote undergoes rapid mitotic divisions as it moves through the isthmus towards the uterus. Daughter cells are called blastomeres.
    • Forms 2, 4, 8, 16 daughter cells. Embryo with 8-16 blastomeres is called a morula.
  • Blastocyst Formation: Morula continues to divide and transforms into a blastocyst as it moves further into the uterus.
    • Blastomeres arrange into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass.
  • Implantation: Blastocyst embeds itself in the endometrium of the uterus.
    • Trophoblast layer gets attached to the endometrium.
    • Uterine cells divide rapidly and cover the blastocyst.
    • Occurs about 6-7 days after fertilisation. Leads to pregnancy.

6. Pregnancy and Embryonic Development

  • Placenta Formation: After implantation, finger-like projections (chorionic villi) appear on the trophoblast. These interdigitate with uterine tissue (decidua) to form the placenta - a structural and functional unit between developing embryo (fetus) and maternal body.
    • Functions of Placenta:
      • Nutrient and O2 supply to the embryo.
      • CO2 and waste removal from the embryo.
      • Acts as an endocrine tissue: Produces hCG (human chorionic gonadotropin), hPL (human placental lactogen), estrogens, progestogens. hCG maintains corpus luteum. hPL involved in fetal growth and maternal metabolism.
      • Relaxin (also secreted by ovary later) helps during parturition.
    • Umbilical Cord: Connects the placenta to the embryo, transports substances.
  • Hormonal Support: Levels of estrogens, progestogens, cortisol, prolactin, thyroxine increase several-fold in maternal blood during pregnancy, supporting fetal growth, metabolic changes in mother, and maintaining pregnancy.
  • Embryonic Development:
    • Inner cell mass differentiates into three primary germ layers: outer ectoderm, middle mesoderm, inner endoderm. This process is called gastrulation.
    • Inner cell mass contains stem cells (potency to give rise to all tissues and organs).
    • Organogenesis: Formation of organs from the three germ layers.
    • Timeline (approximate):
      • 1 Month: Heart is formed.
      • End of 2nd Month: Limbs and digits develop.
      • End of 12 weeks (First Trimester): Major organ systems formed (e.g., limbs, external genital organs well-developed). Fetus recognisable.
      • 5th Month: First movements of fetus, appearance of hair on head.
      • End of 24 weeks (Second Trimester): Body covered with fine hair (lanugo), eyelids separate, eyelashes formed.
      • End of 9 Months: Fetus is fully developed and ready for delivery.
  • Gestation Period: Duration of pregnancy, approx. 9 months in humans.

7. Parturition and Lactation

  • Parturition: Process of childbirth/delivery of the fetus.
    • A complex neuroendocrine mechanism.
    • Signals originate from the fully developed fetus and the placenta, inducing mild uterine contractions (fetal ejection reflex).
    • This triggers release of oxytocin from the maternal posterior pituitary.
    • Oxytocin acts on uterine muscle, causing stronger contractions, which stimulates further oxytocin secretion (positive feedback loop).
    • Contractions push the fetus downwards, aided by abdominal muscle contractions.
    • Relaxin hormone helps relax pelvic ligaments.
    • Expulsion of the baby, followed by expulsion of the placenta ("afterbirth").
  • Lactation: Production of milk by mammary glands towards the end of pregnancy.
    • Stimulated mainly by prolactin (from anterior pituitary).
    • Milk ejection ("let-down") stimulated by oxytocin (triggered by suckling).
    • Colostrum: Milk produced during the initial few days. Yellowish fluid, rich in antibodies (especially IgA), providing passive immunity to the newborn. Essential for the baby's health.

Multiple Choice Questions (MCQs)

  1. The primary function of the scrotum is to:
    a) Protect the testes from physical injury.
    b) Provide a site for sperm storage.
    c) Maintain a temperature lower than the body temperature for spermatogenesis.
    d) Produce androgens like testosterone.

  2. Which cells provide nutrition to the developing sperm?
    a) Leydig cells
    b) Spermatogonia
    c) Sertoli cells
    d) Interstitial cells

  3. Fertilisation in humans normally occurs in the:
    a) Uterus
    b) Vagina
    c) Cervix
    d) Ampulla of the Fallopian tube

  4. The hormone responsible for inducing ovulation (rupture of Graafian follicle) is:
    a) Estrogen
    b) Progesterone
    c) LH (Luteinizing Hormone)
    d) FSH (Follicle-stimulating Hormone)

  5. During oogenesis, the first meiotic division is completed:
    a) During fetal development
    b) Just prior to ovulation
    c) After fertilisation
    d) At the time of puberty

  6. The structure formed immediately after fertilisation is the:
    a) Blastocyst
    b) Morula
    c) Zygote
    d) Gastrula

  7. Which placental hormone is essential for maintaining the corpus luteum during early pregnancy?
    a) hPL (Human Placental Lactogen)
    b) Estrogen
    c) Progesterone
    d) hCG (Human Chorionic Gonadotropin)

  8. The fetal ejection reflex triggers the release of:
    a) Prolactin from the anterior pituitary
    b) Oxytocin from the posterior pituitary
    c) Relaxin from the ovary
    d) Progesterone from the placenta

  9. Spermiogenesis is the process where:
    a) Spermatogonia develop into primary spermatocytes.
    b) Primary spermatocytes undergo meiosis I.
    c) Spermatids transform into spermatozoa.
    d) Spermatozoa are released from Sertoli cells.

  10. Colostrum, the initial milk produced after childbirth, is rich in:
    a) Fats
    b) Lactose
    c) Antibodies (IgA)
    d) Iron

Answer Key for MCQs:

  1. c) Maintain a temperature lower than the body temperature for spermatogenesis.
  2. c) Sertoli cells
  3. d) Ampulla of the Fallopian tube
  4. c) LH (Luteinizing Hormone)
  5. b) Just prior to ovulation
  6. c) Zygote
  7. d) hCG (Human Chorionic Gonadotropin)
  8. b) Oxytocin from the posterior pituitary
  9. c) Spermatids transform into spermatozoa.
  10. c) Antibodies (IgA)

Make sure you revise these concepts thoroughly. Pay attention to the sequence of events, the hormones involved at each stage, and the specific structures and their functions. Good luck with your preparation!

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