Class 12 Notes

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

Priyanka Rawat

— 11 min read

Dear Student, here are the detailed notes for Chapter 3: Human Reproduction from the NCERT Class 12 Biology textbook, structured for exam preparation.

Chapter 3: Human Reproduction

Introduction:
Humans reproduce sexually and are viviparous (give birth to young ones). Reproductive events include:

  1. Gametogenesis (formation of gametes: sperms in males, ovum in females)
  2. Insemination (transfer of sperms into the female genital tract)
  3. Fertilisation (fusion of male and female gametes leading to zygote formation)
  4. Implantation (attachment of blastocyst to the uterine wall)
  5. Gestation (embryonic development)
  6. Parturition (delivery of the baby)
    These events occur after puberty. Significant differences exist between male and female reproductive events.

1. The Male Reproductive System

  • Located in the pelvis region.

  • Includes: A pair of testes, accessory ducts, accessory glands, and external genitalia.

    • Testes (Primary Sex Organs):

      • Location: Situated outside the abdominal cavity within a pouch called the scrotum.
      • Scrotum Function: Maintains a temperature 2-2.5°C lower than the normal internal body temperature, essential for spermatogenesis.
      • Shape & Size: Oval, length 4-5 cm, width 2-3 cm.
      • Covering: Dense covering.
      • Internal Structure: Each testis has about 250 compartments called testicular lobules.
      • Testicular Lobules: Each lobule contains 1-3 highly coiled seminiferous tubules where sperms are produced.
      • Seminiferous Tubules: Lined internally by:
        • Spermatogonia (male germ cells): Undergo meiosis to form sperms.
        • Sertoli cells (supporting cells): Provide nutrition to the germ cells.
      • Interstitial Spaces: Regions outside seminiferous tubules contain small blood vessels and Interstitial cells or Leydig cells.
      • Leydig Cells: Synthesise and secrete testicular hormones called androgens (mainly testosterone). Other immunologically competent cells are also present.

    • Accessory Ducts:

      • Include: Rete testis, vasa efferentia, epididymis, and vas deferens.
      • Path of Sperm Transport: Seminiferous tubules -> Rete testis -> Vasa efferentia -> Epididymis (temporary storage, maturation) -> Vas deferens.
      • Vas Deferens: Ascends to the abdomen and loops over the urinary bladder. Receives a duct from the seminal vesicle.
      • Ejaculatory Duct: Formed by the union of vas deferens and the duct of the seminal vesicle. Stores and transports sperms from testes to outside through the urethra.
      • Urethra: Originates from the urinary bladder and extends through the penis to its external opening called the urethral meatus. Carries both urine and semen (in males).

    • Accessory Glands:

      • Include: A pair of seminal vesicles, a prostate gland (single), and a pair of bulbourethral glands (Cowper's glands).
      • Secretions: Constitute the seminal plasma, rich in fructose, calcium, and certain enzymes.
      • Seminal Plasma + Sperms = Semen.
      • Functions of Secretions:
        • Seminal Vesicles: Contribute the majority of seminal fluid (fructose for energy, prostaglandins).
        • Prostate Gland: Secretes a milky, slightly alkaline fluid containing citrate (nutrient) and enzymes (e.g., prostate-specific antigen - PSA). Alkalinity helps neutralize the acidity of the female reproductive tract.
        • Bulbourethral Glands: Secrete mucus that helps in the lubrication of the penis.

    • External Genitalia:

      • Penis: Male external copulatory organ. Made of special erectile tissue that helps in erection to facilitate insemination.
      • Glans Penis: The enlarged end of the penis.
      • Foreskin (Prepuce): Loose fold of skin covering the glans penis.

2. The Female Reproductive System

  • Located in the pelvis region.

  • Includes: A pair of ovaries, a pair of oviducts, uterus, cervix, vagina, and external genitalia. Mammary glands are structurally and functionally integrated.

    • Ovaries (Primary Sex Organs):

      • Function: Produce the female gamete (ovum) and several steroid hormones (ovarian hormones like estrogen and progesterone).
      • Location: Lower abdomen, one on each side.
      • Size & Shape: Almond-shaped, 2-4 cm in length.
      • Connection: Connected to the pelvic wall and uterus by ligaments.
      • Structure: Covered by a thin epithelium enclosing the ovarian stroma. Stroma divided into:
        • Cortex (peripheral): Contains ovarian follicles in various stages of development.
        • Medulla (inner): Contains blood vessels and nerves.

    • Accessory Ducts:

      • Oviducts (Fallopian Tubes):
        • Length: 10-12 cm. Extend from the periphery of each ovary to the uterus.
        • Parts:
          • Infundibulum: Funnel-shaped part closer to the ovary. Edges possess finger-like projections called fimbriae, which help collect the ovum after ovulation.
          • Ampulla: Wider part, site of fertilisation.
          • Isthmus: Narrow last part, joins the uterus.
      • Uterus (Womb):
        • Single, hollow, muscular, inverted pear-shaped structure. Supported by ligaments attached to the pelvic wall.
        • Location: Between the bladder and rectum.
        • Layers of Uterine Wall:
          • Perimetrium: External thin membranous layer.
          • Myometrium: Middle thick layer of smooth muscle (strong contractions during parturition).
          • Endometrium: Inner glandular layer. Lines the uterine cavity. Undergoes cyclical changes during the menstrual cycle. Site of implantation.
      • Cervix: Narrow lower part of the uterus which opens into the vagina.
      • Cervical Canal: Cavity of the cervix. Forms the birth canal along with the vagina.
      • Vagina: Muscular tube extending from 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. Homologous to the scrotum.
      • Labia Minora: Paired folds of tissue under the labia majora.
      • Hymen: Membrane partially covering the opening of the vagina. Often torn during the first coitus, but can also be broken by other activities (sports, etc.) or persist even after coitus. Its presence/absence is not a reliable indicator of virginity.
      • Clitoris: Tiny finger-like structure lying at the upper junction of the two labia minora above the urethral opening. Homologous to the penis, has erectile tissue.

    • Mammary Glands (Breasts):

      • Paired structures containing glandular tissue and variable fat. Characteristic of female mammals.
      • Glandular Tissue: Divided into 15-20 mammary lobes.
      • Mammary Lobes: Contain clusters of cells called alveoli.
      • Alveoli: Secrete milk, which is stored in their lumens (cavities).
      • Duct System (Path of Milk): Alveoli -> Mammary tubules -> Mammary ducts -> Mammary ampulla -> Lactiferous ducts -> Nipple (through which milk is sucked out).
      • Nipple: Tip of the breast surrounded by a pigmented area called the areola.

3. Gametogenesis

  • Process of formation of gametes in the gonads (testes and ovaries).

  • Sperms (male gametes) and Ova (female gametes).

    • Spermatogenesis (Formation of Sperms):

      • Location: Seminiferous tubules of testes.
      • Onset: Begins at puberty due to increased Gonadotropin-releasing hormone (GnRH) secretion.
      • Process:
        1. Spermatogonia (diploid, 2n=46 chromosomes) present on the inside wall multiply by mitosis. Some spermatogonia grow and increase in size to form primary spermatocytes (diploid, 2n).
        2. Primary spermatocytes undergo Meiosis I (reductional division) to form two equal, haploid (n=23) secondary spermatocytes.
        3. Secondary spermatocytes undergo Meiosis II (equational division) to form four equal, haploid spermatids (n=23).
        4. Spermiogenesis: Transformation of spermatids into spermatozoa (sperms).
        5. Spermiation: Release of sperms from the Sertoli cells into the lumen of seminiferous tubules.
      • Hormonal Control:
        • GnRH (Hypothalamus) -> stimulates Anterior Pituitary.
        • LH (Luteinizing Hormone) (Ant. Pituitary) -> acts on Leydig cells -> stimulates synthesis and secretion of androgens (testosterone). Androgens stimulate spermatogenesis.
        • FSH (Follicle Stimulating Hormone) (Ant. Pituitary) -> acts on Sertoli cells -> stimulates secretion of factors helping in spermiogenesis.
      • Structure of a Sperm:
        • Microscopic structure. Composed of a Head, Neck, Middle Piece, and Tail.
        • Plasma Membrane: Envelops the whole body of the sperm.
        • Head: Contains an elongated haploid nucleus. Anterior portion covered by a cap-like structure called the acrosome.
        • Acrosome: Filled with enzymes (like hyaluronidase) that help in the fertilisation of the ovum (penetration). Derived from Golgi apparatus.
        • Neck: Short region connecting head and middle piece. Contains centrioles.
        • Middle Piece: Possesses numerous mitochondria, which produce energy for the movement of the tail (sperm motility).
        • Tail: Long, flagellar structure. Facilitates sperm motility essential for fertilisation.
      • Semen: Ejaculate contains 200-300 million sperms. For normal fertility, at least 60% must have normal shape/size, and at least 40% must show vigorous motility.

    • Oogenesis (Formation of Ovum):

      • Location: Ovary.
      • Onset: Initiated during the embryonic development stage.
      • Process:
        1. Oogonia (diploid, 2n) are formed within the fetal ovary. No more oogonia are formed or added after birth.
        2. Oogonia start division and enter Prophase-I of Meiosis I, getting temporarily arrested at this stage. They are now called primary oocytes (diploid, 2n).
        3. Each primary oocyte gets surrounded by a layer of granulosa cells, forming a primary follicle.
        4. A large number of these follicles degenerate from birth to puberty (follicular atresia). At puberty, only 60,000-80,000 primary follicles are left in each ovary.
        5. Follicular Development (at Puberty): Primary follicles get surrounded by more layers of granulosa cells and a new theca, becoming secondary follicles.
        6. Secondary follicle transforms into a tertiary follicle, characterized by a fluid-filled cavity called the antrum. The theca layer organizes into an inner theca interna and an outer theca externa.
        7. Inside the tertiary follicle, the primary oocyte grows and completes its Meiosis I (unequal division) -> forms a large, haploid secondary oocyte (n=23) and a tiny first polar body (haploid, n). The first polar body may or may not divide further and degenerates.
        8. The tertiary follicle further changes into the mature follicle or Graafian follicle.
        9. The secondary oocyte retains the bulk of the nutrient-rich cytoplasm. It begins Meiosis II but gets arrested at Metaphase-II.
        10. Ovulation: Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary.
        11. Completion of Meiosis II: Occurs only upon the entry of a sperm into the secondary oocyte's cytoplasm during fertilisation. This results in the formation of a large ootid (ovum) (haploid, n) and a tiny second polar body (haploid, n). The second polar body also degenerates.
      • Structure of Ovum (Secondary Oocyte at Ovulation):
        • Spherical, non-motile. Size approx 0.1 mm.
        • Cytoplasm (ooplasm) contains yolk and a large nucleus.
        • Surrounded by its own plasma membrane.
        • Outside the plasma membrane is the zona pellucida (non-cellular glycoprotein layer).
        • Outside the zona pellucida is the corona radiata (layers of granulosa cells adhering to the ovum).

    • Differences between Spermatogenesis and Oogenesis:

      • Site: Testis vs Ovary
      • Onset: Puberty vs Embryonic stage
      • Gamete Production: Continuous after puberty vs Limited (ceases at menopause)
      • Meiosis I Result: 2 equal secondary spermatocytes vs 1 large secondary oocyte + 1 small polar body
      • Meiosis II Result: 4 equal spermatids vs 1 large ovum + 1 small polar body
      • Gametes Produced per Primary Cell: 4 sperms vs 1 ovum
      • Motility: Motile sperm vs Non-motile ovum
      • Size: Sperm is much smaller than ovum.

4. Menstrual Cycle

  • Reproductive cycle in female primates (monkeys, apes, humans).
  • Menarche: First menstruation, begins at puberty.
  • Characteristics: Cyclic changes in the ovary and uterus, regulated by hormones. Cycle repeats approximately every 28/29 days. Ovulation occurs mid-cycle.
  • Phases:
    1. Menstrual Phase (Day 1-4/5):
      • Starts with menstrual flow (bleeding). Lasts for 3-5 days.
      • Cause: Breakdown of the endometrial lining of the uterus and its blood vessels. Occurs due to the decline in progesterone and estrogen levels following the degeneration of the corpus luteum (if fertilisation doesn't occur).
      • Menstruation only occurs if the released ovum is not fertilised.
    2. Follicular Phase (Proliferative Phase) (Day 5-13):
      • Follows the menstrual phase.
      • Ovarian Changes: Primary follicles grow to become fully mature Graafian follicles.
      • Uterine Changes: Endometrium regenerates through proliferation.
      • Hormonal Changes:
        • Pituitary hormones (FSH and LH) secretion gradually increases. FSH stimulates follicular development and estrogen secretion by growing follicles.
        • Estrogen levels rise, stimulating endometrial proliferation.
    3. Ovulatory Phase (Day 14 approx.):
      • Both LH and FSH attain peak levels (LH Surge).
      • LH Surge: Induces rupture of the Graafian follicle and release of the secondary oocyte (ovulation).
    4. Luteal Phase (Secretory Phase) (Day 15-28):
      • Follows ovulation.
      • Ovarian Changes: The remaining parts of the Graafian follicle transform into the corpus luteum.
      • Corpus Luteum: Secretes large amounts of progesterone, essential for maintaining the endometrium. Also secretes some estrogen.
      • Uterine Changes: Endometrium further thickens, becomes more vascularized and glandular (secretory), ready for implantation.
      • If Fertilisation Occurs: Corpus luteum persists (stimulated by hCG from the developing embryo) and continues progesterone secretion. Menstrual cycle stops.
      • If Fertilisation Does Not Occur: Corpus luteum degenerates -> sharp decline in progesterone and estrogen -> disintegration of endometrium -> menstruation -> new cycle begins.
  • Menopause: Cessation of menstrual cycles, usually around 50 years of age. Marks the end of the reproductive phase in females.

5. Fertilisation and Implantation

  • Insemination: Release of semen into the vagina during coitus.
  • Sperm Transport: Motile sperms swim rapidly through the cervix, uterus, and reach the ampullary region of the fallopian tube. Ovum released from the ovary also reaches the ampulla.
  • Fertilisation: Fusion of sperm and ovum occurs in the ampulla. Can only occur if ovum and sperms are transported simultaneously. (Not all copulations lead to fertilisation).
  • Events during Fertilisation:
    • Capacitation: Changes in sperm within the female tract that enable it to fertilise the ovum.
    • Acrosomal Reaction: Release of enzymes (e.g., hyaluronidase) from the acrosome upon contact with the ovum's outer layers (corona radiata, zona pellucida). Helps sperm penetrate these layers.
    • Cortical Reaction: Entry of sperm induces changes in the zona pellucida (making it impermeable) to prevent polyspermy (fertilisation by more than one sperm).
    • Completion of Meiosis II: Sperm entry induces the secondary oocyte to complete Meiosis II, forming the ootid (ovum) and the second polar body.
    • Syngamy: Fusion of the sperm nucleus (haploid) and the ovum nucleus (haploid) to form a diploid zygote (2n=46).
  • Sex Determination: Determined at fertilisation. Depends on whether the sperm carries an X or Y chromosome.
    • Sperm (X) + Ovum (X) -> Zygote (XX) -> Female
    • Sperm (Y) + Ovum (X) -> Zygote (XY) -> Male
  • Cleavage: Repeated mitotic divisions of the zygote as it moves through the isthmus towards the uterus. Forms 2, 4, 8, 16 daughter cells called blastomeres.
  • Morula: Embryo with 8-16 blastomeres. Solid ball of cells. Continues to divide.
  • Blastocyst: Morula transforms into a blastocyst as it reaches the uterus.
    • Structure: Blastomeres arrange into an outer layer called trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass. A fluid-filled cavity (blastocoel) is present.
    • Trophoblast: Layer that gets attached to the endometrium. Later contributes to placenta formation.
    • Inner Cell Mass: Gets differentiated as the embryo proper. Contains stem cells.
  • Implantation:
    • Attachment of the blastocyst to the uterine wall (endometrium).
    • Trophoblast cells invade the endometrium.
    • Uterine cells divide rapidly and cover the blastocyst.
    • Blastocyst becomes embedded in the endometrium.
    • Leads to pregnancy. Occurs about 6-7 days after fertilisation.

6. Pregnancy and Embryonic Development

  • Placenta Formation: After implantation, finger-like projections appear on the trophoblast called chorionic villi, surrounded by uterine tissue and maternal blood. Chorionic villi and uterine tissue become interdigitated to form the placenta.
  • Placenta: Structural and functional unit between the developing embryo (foetus) and the maternal body.
    • Connection: Connected to the embryo by the umbilical cord, which transports substances to and from the embryo.
    • Functions:
      1. Nutrient & O2 Supply: Facilitates supply from mother to embryo.
      2. CO2 & Waste Removal: Facilitates removal from embryo to mother.
      3. Endocrine Tissue: Produces several hormones:
        • Human chorionic gonadotropin (hCG): Maintains corpus luteum. Basis for pregnancy tests.
        • Human placental lactogen (hPL): Involved in fetal growth and milk gland preparation.
        • Estrogens & Progestogens: Essential for maintaining pregnancy and fetal growth.
        • Relaxin (late pregnancy): Secreted by the ovary also; helps relax pelvic ligaments for parturition.
      • Note: hCG, hPL, and relaxin are produced only during pregnancy. Other hormones like cortisol, prolactin, thyroxine increase several-fold in maternal blood, supporting fetal growth and metabolic changes in the mother.
  • Embryonic Development:
    • Gastrulation: Inner cell mass differentiates into three primary germ layers:
      • Ectoderm (outer): Forms nervous system, epidermis, etc.
      • Mesoderm (middle): Forms muscle, connective tissue, circulatory system, reproductive organs, kidneys, etc.
      • Endoderm (inner): Forms lining of digestive tract, respiratory system, liver, pancreas, bladder, etc.
    • The inner cell mass contains stem cells which have the potency to give rise to all tissues and organs.
  • Key Developmental Milestones (Human Pregnancy - Gestation Period ~9 months):
    • End of 1st Month: Heart is formed. (Listen to heart sound via stethoscope).
    • End of 2nd Month: Limbs and digits develop.
    • End of 1st Trimester (12 weeks): Major organ systems formed (e.g., limbs, external genital organs). Foetus is recognizable.
    • During 5th Month: First movements of the foetus; appearance of hair on the head.
    • End of 2nd Trimester (24 weeks): Body covered with fine hair (lanugo), eyelids separate, eyelashes formed.
    • End of 9 Months: Foetus is fully developed and ready for delivery.

7. Parturition and Lactation

  • Gestation Period: Duration of pregnancy (approx. 9 months in humans).
  • Parturition: Process of childbirth/delivery of the foetus.
    • Mechanism: Complex neuroendocrine mechanism.
    • Signals: Originate from the fully developed foetus and the placenta, inducing mild uterine contractions called the foetal ejection reflex.
    • Hormonal Role:
      • Foetal ejection reflex triggers the release of Oxytocin from the maternal posterior pituitary.
      • Oxytocin: Acts on uterine muscles, causing stronger contractions. Stimulates further oxytocin secretion (positive feedback loop).
      • Relaxin: Helps dilate the cervix and relax pelvic ligaments.
    • Process: Strong uterine contractions push the foetus downward -> expulsion of the baby through the birth canal (cervix + vagina). Followed by the expulsion of the placenta ("afterbirth").
  • Lactation: Production of milk by mammary glands towards the end of pregnancy.
    • Hormonal Role:
      • Prolactin (PRL): Stimulates milk production (lactogenesis). Secretion increases during pregnancy but milk production is inhibited by high progesterone/estrogen. After birth, these levels drop, allowing prolactin to act.
      • Oxytocin: Causes milk ejection ("let-down") from alveoli into ducts in response to suckling.
    • Colostrum: Milk produced during the initial few days of lactation.
      • Composition: Yellowish fluid, rich in proteins and antibodies (especially IgA). Low in fat.
      • Importance: Provides essential nutrients and passive immunity (antibodies) to the newborn, protecting against infections. Breast-feeding during the initial period is highly recommended.

This covers the essential details from NCERT Chapter 3 for government exam preparation. Remember to correlate this information with diagrams in the textbook for better understanding and retention. Focus on hormone names, their sources, target organs, and functions, as well as the sequence of events in each process.

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