Unit 1: Reproduction

Chapter 1.2: Human Reproduction

Male and Female Reproductive Systems: Organs and Functions

Male Reproductive System

• Primary Sex Organ: Testes (produce sperm and testosterone).
• Accessory Ducts:
  • Rete Testis: Network of tubules within the testis.
  • Vasa Efferentia: Ducts leading from rete testis to epididymis.
  • Epididymis: Coiled tube located on the posterior side of each testis; site of sperm maturation and storage.
  • Vas Deferens (Ductus Deferens): Tube that carries sperm from the epididymis to the ejaculatory duct.
  • Ejaculatory Duct: Formed by the union of vas deferens and seminal vesicle duct; opens into the urethra.
  • Urethra: Common passage for urine and semen, extends through the penis.
• Accessory Glands:
  • Seminal Vesicles (paired): Secrete seminal plasma (fructose, prostaglandins, clotting proteins) which provides nourishment and aids in sperm motility.
  • Prostate Gland (single): Secretes a milky, slightly acidic fluid (citric acid, enzymes, prostate specific antigen - PSA) that activates sperm.
  • Bulbourethral Glands (Cowper's glands, paired): Secrete mucus that lubricates the urethra and neutralizes any acidic urine residue.
• External Genitalia:
  • Scrotum: Pouch of skin that holds the testes outside the abdominal cavity, maintaining a temperature 2-2.5°C lower than body temperature, essential for spermatogenesis.
  • Penis: Male copulatory organ, contains erectile tissue.

Female Reproductive System

• Primary Sex Organs: Ovaries (produce ova and female hormones like estrogen and progesterone).
• Accessory Ducts:
  • Oviducts (Fallopian Tubes, paired): Extend from the periphery of each ovary to the uterus.
    • Infundibulum: Funnel-shaped part closest to the ovary, with finger-like projections called fimbriae that help collect the ovum after ovulation.
    • Ampulla: Wider part of the oviduct, where fertilization usually occurs.
    • Isthmus: Narrow lumen, joins the uterus.
  • Uterus (Womb): Pear-shaped muscular organ where the embryo implants and develops.
    • Fundus: Upper dome-shaped part.
    • Body (Corpus): Main central part.
    • Cervix: Narrow lower part that opens into the vagina.
    • Uterine Walls:
      • Perimetrium: Outermost thin membranous layer.
      • Myometrium: Middle thick layer of smooth muscle, responsible for strong contractions during childbirth.
      • Endometrium: Innermost glandular layer, undergoes cyclical changes during the menstrual cycle and is shed during menstruation. Site of implantation.
  • Vagina: Muscular tube extending from the cervix to the outside, serves as the birth canal and receives sperm during coitus.
• External Genitalia (Vulva):
  • Mons Pubis: Cushion of fatty tissue covered by skin and pubic hair.
  • Labia Majora: Fleshy folds of tissue, surround the vaginal opening.
  • Labia Minora: Paired folds of tissue under the labia majora.
  • Hymen: Partially covers the vaginal opening (often torn during first coitus, but not a reliable indicator of virginity).
  • Clitoris: Tiny finger-like erectile structure, homologous to the penis.

Testis and Ovary Structure

Testis Structure

• Location: Located in the scrotum outside the abdominal cavity.
• Tunica Albuginea: Dense fibrous capsule surrounding the testis.
• Testicular Lobules: Each testis has about 250 compartments called testicular lobules.
• Seminiferous Tubules: Each lobule contains 1-3 highly coiled seminiferous tubules, which are the sites of sperm production.
  • Lined by spermatogonia (male germ cells) and Sertoli cells (nurse cells).
  • Spermatogonia: Undergo meiosis to form spermatozoa.
  • Sertoli cells: Provide nourishment to developing sperm, secrete androgen-binding protein (ABP) and inhibin.
• Interstitial Spaces (Leydig Cells): Regions outside the seminiferous tubules contain small blood vessels and Leydig cells (interstitial cells), which synthesize and secrete androgens (e.g., testosterone).

Ovary Structure

• Location: Paired, located one on each side of the lower abdomen, connected to the pelvic wall and uterus by ligaments.
• Shape and Size: Almond-shaped, about 2-4 cm in length.
• Ovarian Cortex: Outer region containing ovarian follicles at various stages of development.
• Ovarian Medulla: Inner region containing blood vessels, lymphatic vessels, and nerves.
• Ovarian Follicles: Structures containing an oocyte surrounded by follicular cells.
  • Primary Follicle: Oocyte surrounded by a single layer of granulosa cells.
  • Secondary Follicle: Oocyte surrounded by multiple layers of granulosa cells and a new layer called the theca interna.
  • Tertiary Follicle: Characterized by a fluid-filled cavity called the antrum. The oocyte is surrounded by the cumulus oophorus.
  • Graafian Follicle: Mature tertiary follicle, ready for ovulation.
  • Corpus Luteum: Formed from the ruptured Graafian follicle after ovulation; secretes progesterone.
  • Corpus Albicans: Degenerated corpus luteum.

Gametogenesis: Spermatogenesis, Spermiogenesis, Spermiation; Oogenesis

Spermatogenesis

• Definition: The process of formation of spermatozoa (sperm) from spermatogonia in the seminiferous tubules of the testis.
• Phases:
  1. Multiplication Phase: Spermatogonia (diploid, 2n) undergo repeated mitotic divisions to increase in number.
  2. Growth Phase: Some spermatogonia grow in size to become primary spermatocytes (diploid, 2n).
  3. Maturation Phase:
     • Each primary spermatocyte undergoes Meiosis I (reductional division) to form two haploid secondary spermatocytes (n).
     • Each secondary spermatocyte undergoes Meiosis II (equational division) to form two haploid spermatids (n).
     • Thus, one primary spermatocyte produces four spermatids.

Spermiogenesis

• Definition: The process of transformation of non-motile spermatids into motile spermatozoa (sperm).
• Process: Spermatids undergo morphological changes (e.g., formation of head, neck, middle piece, tail) without further division.
• Sperm Structure:
  • Head: Contains the haploid nucleus and is capped by the acrosome (a cap-like structure derived from the Golgi apparatus, containing hydrolytic enzymes like hyaluronidase and acrosin, essential for penetrating the egg).
  • Neck: Short region connecting the head to the middle piece.
  • Middle Piece: Contains numerous mitochondria that provide energy for sperm motility.
  • Tail: Long, slender, flagellum-like structure responsible for motility.

Spermiation

• Definition: The process of release of mature spermatozoa from the Sertoli cells into the lumen of the seminiferous tubules.

Oogenesis

• Definition: The process of formation of a mature female gamete (ovum) from oogonia in the ovaries.
• Initiation: Oogenesis is initiated during fetal development.
• Phases:
  1. Multiplication Phase: Oogonia (diploid, 2n) are formed in the fetal ovary by mitotic divisions. No more oogonia are formed after birth.
  2. Growth Phase: Oogonia grow in size and get surrounded by granulosa cells to form primary follicles. The primary oocyte (diploid, 2n) within the primary follicle enters Meiosis I but gets arrested at the prophase I stage.
  3. Maturation Phase:
     • At Puberty: Many primary follicles degenerate (follicular atresia). The remaining primary follicles develop into secondary, then tertiary follicles. The primary oocyte in the tertiary follicle completes Meiosis I, resulting in two unequal haploid cells:
       • A large secondary oocyte (n).
       • A tiny first polar body (n).
     • The secondary oocyte retains most of the cytoplasm and enters Meiosis II, but gets arrested at the metaphase II stage.
     • Ovulation: The secondary oocyte is released from the Graafian follicle during ovulation.
     • Fertilization: Meiosis II is completed only upon the entry of a sperm. This results in:
       • A large ovum (ootid) (n).
       • A tiny second polar body (n).
     • The first polar body may or may not divide further.
• Comparison with Spermatogenesis: Oogenesis produces only one functional ovum and two or three polar bodies, while spermatogenesis produces four functional spermatozoa.

Hormonal Control of Male and Female Reproductive Systems

Hormonal Control in Males

• Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH).
• Anterior Pituitary: GnRH stimulates the anterior pituitary to secrete two gonadotropins:
  • Luteinizing Hormone (LH) / Interstitial Cell Stimulating Hormone (ICSH): Acts on Leydig cells to stimulate the synthesis and secretion of androgens (e.g., testosterone).
  • Follicle-Stimulating Hormone (FSH): Acts on Sertoli cells and stimulates the secretion of Androgen Binding Protein (ABP) and inhibin. ABP concentrates testosterone in the seminiferous tubules, and inhibin suppresses FSH secretion.
• Androgens (Testosterone): Stimulate spermatogenesis and the development/maintenance of male secondary sexual characteristics.

Hormonal Control in Females

• Hypothalamus: Secretes GnRH.
• Anterior Pituitary: GnRH stimulates the anterior pituitary to secrete FSH and LH.
• FSH: Stimulates the growth and development of ovarian follicles and the secretion of estrogens by granulosa cells.
• LH: Triggers ovulation (LH surge) and stimulates the formation and maintenance of the corpus luteum, which secretes progesterone and some estrogen.
• Estrogens: Stimulate the growth of ovarian follicles, development of female secondary sexual characteristics, and proliferation of the endometrium.
• Progesterone: Essential for maintaining pregnancy, preparing the uterus for implantation, and inhibiting uterine contractions.

Menstrual Cycle: Phases, Hormones, Menarche and Menopause

• Definition: The cyclical changes that occur in the female reproductive system, primarily in the uterus and ovaries, every 28-29 days (on average).

• Phases:

  1. Menstrual Phase (Day 1-5):
     • Events: Breakdown of the endometrial lining of the uterus, leading to bleeding (menstruation). Occurs if fertilization does not happen.
     • Hormones: Low levels of progesterone and estrogen.
  2. Follicular/Proliferative Phase (Day 5-14):
     • Events: Primary follicles in the ovary grow and mature into Graafian follicles. The endometrium regenerates and proliferates due to estrogen.
     • Hormones: FSH stimulates follicular growth and estrogen secretion. Estrogen levels rise.
  3. Ovulatory Phase (Day 14):
     • Events: Rapid secretion of LH (LH surge) induces rupture of the Graafian follicle and release of the secondary oocyte (ovulation).
     • Hormones: Peak estrogen levels trigger LH surge.
  4. Luteal/Secretory Phase (Day 15-28):
     • Events: The ruptured Graafian follicle transforms into the corpus luteum, which secretes large amounts of progesterone and some estrogen. The endometrium thickens further and becomes highly vascularized and glandular, preparing for potential implantation.
     • Hormones: LH maintains the corpus luteum. Progesterone levels are high. If pregnancy occurs, hCG (human chorionic gonadotropin) from the placenta maintains the corpus luteum. If no pregnancy, corpus luteum degenerates, leading to a drop in progesterone and estrogen, triggering menstruation.

• Menarche: The first menstruation, typically occurring at puberty (around 11-13 years of age).

• Menopause: The cessation of menstrual cycles, typically occurring around 45-50 years of age.

Fertilization: Physico-chemical Events

• Definition: The fusion of male and female gametes to form a diploid zygote.
• Site: Usually occurs in the ampulla of the fallopian tube.
• Physico-chemical Events:
  1. Insemination: Release of semen into the vagina during coitus.
  2. Sperm Motility: Sperm travel through the cervix, uterus, and into the fallopian tubes.
  3. Capacitation: Physiological changes in sperm within the female reproductive tract that enable them to fertilize an egg (e.g., increased motility, changes in acrosome membrane).
  4. Approach to Ovum: Sperm encounter the secondary oocyte.
  5. Penetration of Corona Radiata: Sperm penetrate the outer layer of follicular cells (corona radiata) surrounding the ovum, aided by hyaluronidase enzyme from the acrosome.
  6. Acrosomal Reaction and Penetration of Zona Pellucida:
     • Upon contact with the zona pellucida (a glycoprotein layer around the ovum), the sperm undergoes an acrosomal reaction, releasing acrosin and other enzymes.
     • These enzymes digest a path through the zona pellucida, allowing the sperm to reach the plasma membrane of the oocyte.
  7. Fusion of Gamete Membranes: The plasma membrane of the sperm fuses with the plasma membrane of the secondary oocyte.
  8. Cortical Reaction (Block to Polyspermy):
     • Upon fusion, the oocyte's plasma membrane depolarizes, and cortical granules beneath the oocyte membrane release their contents into the perivitelline space.
     • These contents harden the zona pellucida and inactivate sperm receptors, preventing entry of additional sperm (polyspermy).
  9. Completion of Meiosis II: The entry of sperm stimulates the secondary oocyte to complete its Meiosis II, forming the ovum (ootid) and the second polar body.
  10. Formation of Pronuclei: The haploid nucleus of the sperm (male pronucleus) and the haploid nucleus of the ovum (female pronucleus) swell.
  11. Amphimixis (Fusion of Pronuclei): The male and female pronuclei fuse to form a diploid zygote.

Embryo Development: Zygote to Blastocyst

• Cleavage: Rapid mitotic divisions of the zygote without significant growth, occurring as it moves through the isthmus of the oviduct towards the uterus.
  • 2-celled stage, 4-celled stage, 8-celled stage, 16-celled stage.
• Morula: The embryo at the 8-16 cell stage, resembling a mulberry.
• Blastocyst: As the morula continues to divide, it transforms into a blastocyst.
  • Trophoblast: Outer layer of cells that will form the extraembryonic membranes (e.g., placenta).
  • Inner Cell Mass (ICM): Group of cells attached to the trophoblast, which will differentiate to form the embryo proper.
  • Blastocoel: Fluid-filled cavity within the blastocyst.

Implantation, Placenta (Functions), Parturition (Overview), Lactation (Hormonal Control)

Implantation

• Definition: The process by which the blastocyst attaches to the endometrium of the uterus.
• Timing: Occurs about 7 days after fertilization.
• Process: The trophoblast layer of the blastocyst adheres to the endometrial wall. The trophoblast cells rapidly divide and penetrate the endometrial lining.
• Result: The blastocyst becomes embedded in the endometrium.

Placenta

• Definition: A temporary structural and functional connection between the developing embryo (fetus) and the maternal body.
• Formation: Formed by the interdigitation of chorionic villi (from the trophoblast) and uterine tissue.
• Functions:
  • Nutrient Supply: Facilitates the supply of oxygen and nutrients (glucose, amino acids, vitamins) from maternal blood to the fetus.
  • Waste Removal: Removes carbon dioxide and nitrogenous waste products from the fetal blood to the maternal blood.
  • Endocrine Gland: Acts as an endocrine gland, producing several hormones essential for maintaining pregnancy:
    • Human Chorionic Gonadotropin (hCG): Maintains the corpus luteum, preventing menstruation. (Detected in pregnancy tests).
    • Human Placental Lactogen (hPL): Involved in mammary gland development.
    • Estrogens: Support uterine growth and mammary gland development.
    • Progesterone: Maintains the endometrium, suppresses uterine contractions.
    • Relaxin: Secreted in the later stages of pregnancy, relaxes pelvic ligaments and widens the birth canal.

Parturition (Overview)

• Definition: The process of childbirth, involving the expulsion of the fetus from the uterus.
• Initiation: A complex neuroendocrine mechanism.
  • Signals originate from the fully developed fetus and the placenta, inducing mild uterine contractions called fetal ejection reflex.
  • This reflex triggers the release of oxytocin from the maternal pituitary gland.
• Role of Oxytocin: Oxytocin acts on the uterine muscles, causing stronger uterine contractions.
• Positive Feedback Loop: The contractions stimulate further secretion of oxytocin, leading to increasingly stronger contractions, eventually leading to the expulsion of the baby.

Lactation (Hormonal Control)

• Definition: The production and secretion of milk by the mammary glands.
• Initiation: Mammary glands undergo differentiation during pregnancy under the influence of hormones like estrogen, progesterone, and hPL.
• Milk Production: Primarily stimulated by prolactin (from the anterior pituitary) after childbirth.
• Milk Ejection (Let-down): Stimulated by oxytocin (from the posterior pituitary) in response to suckling. Oxytocin causes the contraction of myoepithelial cells surrounding the alveoli, forcing milk into the ducts.
• Colostrum: The yellowish fluid secreted during the initial few days of lactation. It is rich in antibodies (especially IgA) that provide passive immunity to the newborn.
• Significance: Provides essential nutrients and antibodies to the newborn, protecting them from infections.