Class 12 Notes Biology for Chapter 2 Human Reproduction

Human reproduction is a sexual and viviparous process involving the formation of gametes (sperm in males, ovum in females), their fusion (fertilisation), and the subsequent development of an embryo and foetus within the female. Key stages include gametogenesis, insemination, fertilisation, implantation, gestation, and parturition. Significant differences exist between male and female reproductive events, such as continuous sperm formation in older men versus the cessation of ovum formation around age fifty in women. Hormonal regulation plays a crucial role in all these processes, particularly during the menstrual cycle, pregnancy, and childbirth.

1. The Male Reproductive System

• Located in the pelvis region.
• Includes:
  • Pair of testes: Produce sperm.
    • Situated outside the abdominal cavity in the scrotum to maintain a lower temperature (2-2.5°C below body temperature) essential for spermatogenesis.
    • Oval shape, 4-5 cm long and 2-3 cm wide in adults.
    • Covered by a dense covering.
    • Divided into about 250 testicular lobules.
    • Each lobule contains 1-3 highly coiled seminiferous tubules where sperm are produced.
    • Lined by:
      • Male germ cells (spermatogonia): Undergo meiosis to form sperm.
      • Sertoli cells: Provide nutrition to germ cells. Sperm heads become embedded in Sertoli cells during spermiogenesis. FSH acts on Sertoli cells to stimulate secretion of factors aiding spermiogenesis.
    • Interstitial spaces (regions outside seminiferous tubules) contain:
      • Small blood vessels.
      • Interstitial cells or Leydig cells: Synthesise and secrete testicular hormones called androgens. LH acts on Leydig cells to stimulate androgen production.
      • Immunologically competent cells.
  • Accessory ducts: Store and transport sperm.
    • Rete testis: Seminiferous tubules open into vasa efferentia through this.
    • Vasa efferentia: Leave the testis and open into the epididymis.
    • Epididymis: Located along the posterior surface of each testis. Involved in sperm maturation and motility.
    • Vas deferens: Ascends to the abdomen, loops over the urinary bladder, receives a duct from the seminal vesicle, and opens into the urethra as the ejaculatory duct. Also involved in sperm maturation and motility.
  • Glands: Secrete seminal plasma.
    • Paired seminal vesicles: Secrete fluid rich in fructose, calcium, and certain enzymes.
    • Prostate: Secretes components of seminal plasma.
    • Paired bulbourethral glands: Secrete mucus that helps in lubrication of the penis.
    • Secretions form seminal plasma, which along with sperm constitutes semen.
    • Functions of ducts and glands are maintained by androgens.
  • External genitalia: Penis.
    • Made of special tissue that helps in erection for insemination.
    • Enlarged end is the glans penis, covered by a loose fold of skin called foreskin.
    • Urethra originates from the urinary bladder, extends through the penis, and its external opening is the urethral meatus.

2. The Female Reproductive System

• Located in the pelvic region.
• Includes:
  • Pair of ovaries: Primary female sex organs that produce the female gamete (ovum) and steroid hormones (ovarian hormones).
    • Located on each side of the lower abdomen.
    • 2-4 cm in length, connected to pelvic wall and uterus by ligaments.
    • Covered by a thin epithelium enclosing the ovarian stroma.
    • Stroma divided into a peripheral cortex and an inner medulla.
    • Contain ovarian follicles in different stages of development.
  • Pair of oviducts (fallopian tubes): Extend from the periphery of each ovary to the uterus.
    • About 10-12 cm long.
    • Part closer to the ovary is the funnel-shaped infundibulum with finger-like projections called fimbriae that help in the collection of the ovum after ovulation.
    • Infundibulum leads to a wider part called ampulla. Fertilisation typically occurs in the ampullary region.
    • Last part is the isthmus, with a narrow lumen, joining the uterus.
  • Uterus (womb): Single, shaped like an inverted pear.
    • Supported by ligaments attached to the pelvic wall.
    • Opens into the vagina through a narrow cervix.
    • Cavity of the cervix is the cervical canal, which along with the vagina forms the birth canal.
    • Wall has three layers:
      • Perimetrium: External thin membranous layer.
      • Myometrium: Middle thick layer of smooth muscle; exhibits strong contractions during delivery (parturition).
      • Endometrium: Inner glandular layer lining the uterine cavity; undergoes cyclical changes during the menstrual cycle and is necessary for implantation of the fertilised ovum.
  • Vagina: Part of the female accessory ducts; forms the birth canal with the cervical canal.
  • External genitalia (vulva): Include mons pubis, labia majora, labia minora, hymen, and clitoris.
    • Mons pubis: Cushion of fatty tissue covered by skin and pubic hair.
    • Labia majora: Fleshy folds of tissue surrounding the vaginal opening.
    • Labia minora: Paired folds of tissue under the labia majora.
    • Hymen: Membrane often partially covering the vaginal opening; its presence or absence is not a reliable indicator of virginity or sexual experience.
    • Clitoris: Tiny finger-like structure at the upper junction of the labia minora above the urethral opening.
  • Mammary glands (breasts): Paired structures containing glandular tissue and fat.
    • Glandular tissue divided into 15-20 mammary lobes containing clusters of cells called alveoli.
    • Alveoli secrete milk, stored in their lumens.
    • Alveoli open into mammary tubules, which join to form mammary ducts.
    • Several mammary ducts join to form a wider mammary ampulla, connected to the lactiferous duct through which milk is sucked out.
    • Functional mammary gland is characteristic of all female mammals.

3. Gametogenesis

• Process of forming gametes (sperm and ovum) in the primary sex organs (testes and ovaries).
  • Spermatogenesis (in males): Begins at puberty.
    • Immature male germ cells (spermatogonia) in seminiferous tubules multiply mitotically. Each is diploid (46 chromosomes).
    • Some spermatogonia differentiate into primary spermatocytes (diploid, 46 chromosomes) which undergo meiosis I (reduction division) to form two equal, haploid secondary spermatocytes (23 chromosomes each).
    • Secondary spermatocytes undergo meiosis II to produce four equal, haploid spermatids (23 chromosomes each).
    • Spermiogenesis: Transformation of spermatids into spermatozoa (sperms). Sperm heads become embedded in Sertoli cells.
    • Spermiation: Release of sperms from the seminiferous tubules.
    • Initiated at puberty due to increased GnRH secretion, which stimulates LH and FSH release from the anterior pituitary.
    • LH stimulates Leydig cells to produce androgens, which stimulate spermatogenesis.
    • FSH stimulates Sertoli cells to secrete factors aiding spermiogenesis.
  • Oogenesis (in females): Markedly different from spermatogenesis.
    • Initiated during embryonic development.
    • Millions of oogonia (gamete mother cells) are formed in each fetal ovary; no more are added after birth. Oogonia are diploid.
    • Oogonia start division and enter prophase-I of meiosis I, getting temporarily arrested at this stage as primary oocytes. Each is diploid.
    • Each primary oocyte gets surrounded by a layer of granulosa cells and is called a primary follicle.
    • Large number of primary follicles degenerate from birth to puberty.
    • At puberty, only 60,000-80,000 primary follicles are left in each ovary.
    • Primary follicles get surrounded by more granulosa cells and a new theca, becoming secondary follicles.
    • Secondary follicle transforms into a tertiary follicle, characterised by a fluid-filled cavity called antrum. Theca layer organised into theca interna and theca externa.
    • Within the tertiary follicle, the primary oocyte completes its first meiotic division (unequal division) forming a large haploid secondary oocyte and a tiny first polar body. The secondary oocyte retains most of the nutrient-rich cytoplasm.
    • Tertiary follicle further changes into the mature follicle or Graafian follicle.
    • The secondary oocyte forms a new membrane called zona pellucida around it.
    • Ovulation: The Graafian follicle ruptures to release the secondary oocyte (ovum) from the ovary.

4. Menstrual Cycle

• Reproductive cycle in female primates (monkeys, apes, humans).
• First menstruation at puberty is called menarche.
• Repeated at an average interval of about 28/29 days.
• Cycle of events from one menstruation to the next.
• One ovum is released (ovulation) during the middle of each cycle (around the 14th day).
• Major phases (regulated by pituitary and ovarian hormones):
  • Menstrual phase (3-5 days): Breakdown of endometrial lining and its blood vessels, resulting in menstrual flow through the vagina. Occurs if the released ovum is not fertilised.
  • Follicular phase: Primary follicles grow into mature Graafian follicles, and the endometrium regenerates through proliferation. Gonadotropin (LH and FSH) secretion increases, stimulating follicular development and estrogen secretion.
  • Ovulatory phase: LH and FSH peak around the 14th day. Rapid secretion of LH (LH surge) induces rupture of the Graafian follicle and release of the ovum (ovulation).
  • Luteal phase: Remaining parts of the Graafian follicle transform into the corpus luteum. Corpus luteum secretes large amounts of progesterone, essential for maintaining the endometrium for implantation.
• During pregnancy, all events of the menstrual cycle stop (no menstruation).
• In the absence of fertilisation, the corpus luteum degenerates, causing endometrial disintegration and menstruation, starting a new cycle.
• Menstrual cycles cease around 50 years of age (menopause).
• Cyclic menstruation is an indicator of the normal reproductive phase between menarche and menopause.

5. Fertilisation and Implantation

• Insemination: During copulation (coitus), semen is released by the penis into the vagina.
• Motile sperms swim through the cervix, uterus, and reach the ampullary region of the fallopian tube, where fertilisation takes place.
• Fertilisation can only occur if the ovum and sperm are transported simultaneously to the ampullary region.
• Process of fusion of a sperm with an ovum.
• A sperm contacts the zona pellucida of the ovum, inducing changes in the membrane that block the entry of additional sperms.
• Secretions of the acrosome help the sperm enter the cytoplasm of the ovum through the zona pellucida and plasma membrane.
• This induces the completion of the meiotic division of the secondary oocyte, resulting in the formation of a second polar body and a haploid ovum (ootid).
• The haploid nucleus of the sperm and the ovum fuse to form a diploid zygote (46 chromosomes).
• Sex determination: The sex of the baby is determined at fertilisation by the sperm. Ova always carry an X chromosome, while sperm can carry either an X or a Y chromosome. XX zygote develops into a female, and XY zygote develops into a male.
• Cleavage: Mitotic divisions start as the zygote moves through the isthmus of the oviduct towards the uterus, forming 2, 4, 8, 16 daughter cells called blastomeres.
• Morula: Embryo with 8 to 16 blastomeres.
• Blastocyst: Morula continues to divide and transforms into a blastocyst as it moves into the uterus.
  • Blastomeres arrange into an outer layer called trophoblast and an inner cell mass.
  • Inner cell mass differentiates as the embryo.
• Implantation: The trophoblast attaches to the endometrium, and uterine cells rapidly cover the blastocyst, embedding it in the endometrium of the uterus. Implantation leads to pregnancy.

6. Pregnancy and Embryonic Development

• After implantation, finger-like projections called chorionic villi appear on the trophoblast, surrounded by uterine tissue and maternal blood.
• Chorionic villi and uterine tissue interdigitate to form the placenta: A structural and functional unit between the developing embryo (foetus) and maternal body.
• Placenta functions: Facilitates the supply of oxygen and nutrients to the embryo and removal of carbon dioxide and waste materials.
• Connected to the embryo through the umbilical cord for transport of substances.
• Placenta as an endocrine tissue: Produces hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens.
• Relaxin is also secreted by the ovary in the later phase of pregnancy.
• hCG, hPL, and relaxin are produced only during pregnancy.
• Levels of other hormones like estrogens, progestogens, cortisol, prolactin, thyroxine increase several-folds in maternal blood during pregnancy to support fetal growth, metabolic changes in the mother, and maintenance of pregnancy.
• Immediately after implantation, the inner cell mass differentiates into ectoderm (outer layer) and endoderm (inner layer). Mesoderm appears between them. These three layers give rise to all tissues and organs in adults.
• The inner cell mass contains stem cells with the potency to give rise to all tissues and organs.
• Human pregnancy lasts 9 months (gestation period).
• Key developmental milestones:
  • One month: Embryo's heart is formed.
  • End of second month: Foetus develops limbs and digits.
  • End of 12 weeks (first trimester): Most major organ systems are formed, limbs and external genitalia are well-developed.
  • Fifth month: First movements of the foetus and appearance of hair on the head are usually observed.
  • End of 24 weeks (end of second trimester): Body covered with fine hair, eye-lids separate, eyelashes are formed.
  • End of nine months: Foetus is fully developed and ready for delivery.

7. Parturition and Lactation

• Parturition (childbirth): Vigorous contraction of the uterus at the end of pregnancy causing expulsion/delivery of the foetus.
• Induced by a complex neuroendocrine mechanism.
• Signals originate from the fully developed foetus and the placenta, inducing mild uterine contractions called foetal ejection reflex.
• This triggers the release of oxytocin from the maternal pituitary.
• Oxytocin acts on the uterine muscle, causing stronger contractions, which in turn stimulate further oxytocin secretion (positive feedback).
• Stronger contractions lead to the expulsion of the baby through the birth canal.
• Soon after, the placenta is also expelled from the uterus.
• Lactation: Mammary glands differentiate during pregnancy and start producing milk towards the end of pregnancy.
• Milk produced during the initial few days is called colostrum, rich in several antibodies essential for the newborn's immunity.
• Breast-feeding during the initial period is recommended for a healthy baby.

Frequently Asked Questions:

1. What are the primary functions of the testes and ovaries?
   • The testes produce sperm and secrete androgens.
   • The ovaries produce the ovum and secrete ovarian hormones (estrogens and progesterone).
   • These functions are crucial for sexual reproduction and the development of secondary sexual characteristics.
2. Briefly describe the process of fertilisation and implantation.
   • Fertilisation is the fusion of a sperm and an ovum, typically occurring in the ampullary region of the fallopian tube.
   • It results in the formation of a diploid zygote.
   • The zygote undergoes cleavage to form a blastocyst, which then embeds itself in the endometrium of the uterus in a process called implantation.
3. What are the key hormonal changes during the menstrual cycle that lead to ovulation?
   • Gradual increase in LH and FSH during the follicular phase stimulates follicle development and estrogen secretion.
   • A surge in LH in the middle of the cycle (around the 14th day) triggers the rupture of the Graafian follicle and the release of the ovum (ovulation).
   • After ovulation, the remaining follicle forms the corpus luteum, which secretes progesterone.