Zoology

Fundamentals of Human Reproduction

• Definition of Reproduction: Biological process involving the production of offspring to ensure the continuity of a species generation after generation.
• Characteristics of Human Reproduction:
  • Sexual Reproduction: Involves two distinct sexes producing haploid gametes.
  • Male Gamete: Sperm ($\sigma^7$).
  • Female Gamete: Ovum or Egg (\female).
  • Viviparous Nature: Humans directly give birth to live young ones rather than laying eggs.

Sequence of Reproductive Events

• Gametogenesis: Formation of male and female gametes.
  • Spermatogenesis: Production of sperms in the testes.
  • Oogenesis: Production of ova/eggs in the ovaries.
• Insemination: Transfer of male gametes (sperms) into the female reproductive tract.
• Fertilisation: Fusion of male and female haploid gametes to form a single-celled diploid Zygote.
• Implantation: Attachment of the developing Blastocyst to the inner uterine wall (Uterine Endometrium).
  • Developmental Transition: Single-celled Zygote $\rightarrow$ 2-celled stage $\rightarrow$ 4-celled stage $\rightarrow$ 8--16 celled stage $\rightarrow$ Blastocyst.
• Gestation: Total time duration of embryonic and foetal development within the womb (approximately 9 months in humans).
• Parturition: Act of delivering the fully developed baby from the mother's body.

Classification and Types of Sex Organs

• Primary Sex Organs:
  • Organs directly responsible for gamete production and sex hormone synthesis.
  • Male Primary Sex Organ: Testes (1 pair) — secretes male androgen hormones such as Testosterone.
  • Female Primary Sex Organ: Ovaries (1 pair) — secretes female steroid hormones such as Estrogen and Progesterone.
• Secondary (Accessory) Sex Organs:
  • Organs not involved in gametogenesis directly, but crucial for supporting reproduction, gamete transport, storage, and sustaining pregnancy.
  • Male Accessory Organs: Vas deferens, epididymis, prostate gland, seminal vesicles, bulbourethral glands.
  • Female Accessory Organs: Fallopian tubes, uterus, cervix, vagina.
• Comparative Developmental Timing:
  • Male: Spermatogenesis initiates at puberty (around $10\text{--}14$ years of age) due to physical and hormonal shifts, continuing continuously even into old age.
  • Female: Oogenesis initiates during embryonic life (before birth), pauses, resumes at puberty, and permanently halts at Menopause (cessation of menstrual cycles at approximately $45\text{--}50$ years of age).

Male Reproductive System

• Anatomical Location: Situated in the pelvic region.
• Components:
  1. Primary Sex Organs: Testes (1 pair).
  2. External Genitalia: Scrotum and Penis.
  3. Male Accessory Duct System: Rete testis, Vasa efferentia, Epididymis, Vas deferens.
  4. Male Accessory Glands: Seminal vesicles (paired), Prostate gland (single), Bulbourethral / Cowper's glands (paired).
• Scrotum and Testicular Topography:
  • Scrotum: A pouch-like sac hanging outside the abdominal cavity, divided internally into two scrotal sacs.
  • Thermoregulation: Positioned outside the body cavity because spermatogenesis requires a temperature $2\text{--}2.5^\times$ lower than normal core body temperature (i.e., $34.5\text{--}35^\times\text{C}$).
  • Testis Dimensions: Oval-shaped structure with a length of $4\text{--}5\text{\,cm}$ and a width of $2\text{--}3\text{\,cm}$.
  • Tunica Albuginea: Dense fibrous connective tissue capsule enclosing each testis and forming internal septa that divide the organ into compartments.

Histology of Seminiferous Tubules and Testicular Cells

• Testicular Lobules: Each testis contains approximately $250$ internal compartments called testicular lobules.
• Seminiferous Tubules: Each lobule contains $1\text{--}3$ highly coiled structures called seminiferous tubules where sperm synthesis takes place.
• Internal Lining of Seminiferous Tubules:
  • Male Germ Cells (Spermatogonia): Diploid precursor cells that undergo meiotic division to generate spermatozoa.
  • Sertoli Cells (Nurse / Sustentacular Cells):
    • Provide structural support and nourishment to developing germ cells and sperms.
    • Phagocytose excess cytoplasm stripped away during spermatogenesis.
    • Secrete Androgen Binding Protein (ABP), which binds to testosterone to maintain high local androgen concentrations inside seminiferous tubules.
    • Secrete Sperm Maturation Factor (SMF) required for spermiogenesis.
    • Secrete Inhibin, a glycoprotein hormone that exerts negative feedback control on FSH secretion from the anterior pituitary.
• Interstitial Space (Extra-tubular region):
  • Space outside the seminiferous tubules containing rich blood capillary networks, immunocompetent cells (WBCs), and Leydig Cells (Interstitial Cells).
  • Leydig Cells: Endocrine cells stimulated by LH to synthesize and secrete male steroid hormones called Androgens (primarily Testosterone).

Male Accessory Ducts and Glands

• Duct System Pathway:
  • Seminiferous Tubules $\rightarrow$ Rete Testis (plexus of channels) $\rightarrow$ Vasa Efferentia ($10\text{--}12$ straight ductules) $\rightarrow$ Epididymis (elongated, highly coiled tube along the posterior surface of each testis) $\rightarrow$ Vas Deferens (ascends into abdominal cavity, loops over the urinary bladder).
  • Common Ejaculatory Duct: Formed by the union of the Vas Deferens and the duct of the Seminal Vesicle. It transports sperms and glandular secretions into the Urethra.
  • Urethral Opening: Opens externally at the Urethral Meatus/Orifice.
• Male Accessory Glands:
  • Seminal Vesicles:
    • Paired glands located behind the urinary bladder.
    • Contributes $60\text{--}70\%$ of total seminal plasma volume.
    • Secretory Fluid: Rich in fructose (energy source for sperms), $Ca^{2+}$, prostaglandins (stimulate smooth muscle contractions), and specific enzymes.
  • Prostate Gland:
    • Single, large doughnut-shaped gland positioned just below the urinary bladder.
    • Contributes $20\text{--}30\%$ of seminal plasma.
    • Secretes a slightly alkaline, milky fluid containing enzymes.
  • Bulbourethral (Cowper's) Glands:
    • Paired small pea-sized glands located on either side of the urethra.
    • Contributes $5\text{--}10\%$ of seminal plasma.
    • Secretes mucus that acts as a lubricant for the penis during copulation.
• Semen Composition and Normal Values:
  • $\text{Semen} = \text{Sperms} + \text{Seminal Plasma}$.
  • Ejaculate Volume: Normal single ejaculation yields $2.5\text{--}5\text{\,mL}$ of semen.
  • Sperm Count: Contains approximately $200\text{--}300 \times 10^6$ sperms per ejaculate.
  • Fertility Criteria: For normal fertility, at least $60\%$ of sperms must possess normal morphological shape and size, and at least $40\%$ of those normal sperms must exhibit vigorous motility.
• Male External Genitalia (Penis):
  • Copulatory organ made of specialized erectile tissues (Corpora Cavernosum and Corpus Spongiosum) that facilitate penile erection for insemination.
  • Glans Penis: The enlarged, sensitive distal end of the penis.
  • Foreskin (Prepuce): Loose, retractile fold of skin covering the glans penis.

Spermatogenesis and Hormonal Regulation

• Hormonal Cascade at Puberty:
  • Hypothalamus increases secretion of GnRH (Gonadotropin-Releasing Hormone).
  • GnRH stimulates the Anterior Pituitary gland to release two gonadotropic hormones: FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone / Interstitial Cell Stimulating Hormone).
  • LH Action: Binds to Leydig cells, stimulating androgen/testosterone synthesis, which directly promotes spermatogenesis.
  • FSH Action: Binds to Sertoli cells, triggering secretion of ABP, SMF, and Inhibin to support spermiogenesis and regulate hormone levels.
• Step-by-Step Mechanism of Spermatogenesis:
  1. Spermatogonia ($2n = 46$, $44+\text{XY}$): Diploid male germ cells present at the inner wall of seminiferous tubules undergo mitotic division to increase in number and differentiate into primary spermatocytes.
  2. Primary Spermatocyte ($2n = 46$, $44+\text{XY}$): Undergoes Meiosis I (reductional division) to produce two equal, haploid cells called Secondary Spermatocytes ($n = 23$, $22+\text{X}$ or $22+\text{Y}$).
  3. Secondary Spermatocyte ($n = 23$): Rapidly undergoes Meiosis II (equational division) to form four equal haploid Spermatids ($n = 23$).
  4. Spermiogenesis: Morphological transformation and differentiation of non-motile, spherical spermatids into motile, functional Spermatozoa (sperms).
  5. Spermiation: Final release of mature spermatozoa from the Sertoli cells into the central lumen of seminiferous tubules.
• Feedback Control Mechanisms:
  • Elevated androgen levels exert Negative Feedback on both the Hypothalamus (inhibiting GnRH) and the Anterior Pituitary (inhibiting LH).
  • Inhibin secreted by Sertoli cells selectively suppresses FSH release from the Anterior Pituitary.

Structure of Human Spermatozoon

• General Organization: A microscopic, motile haploid cell enclosed entirely by a continuous plasma membrane, structured into four main regions:
  1. Head:
     • Contains an elongated haploid nucleus ($n = 23$).
     • Acrosome: Anterior cap-like structure derived from the Golgi apparatus, loaded with hydrolytic enzymes called Sperm Lysins (e.g., Hyaluronidase, Corona-penetrating enzyme, Zona lysin) necessary for penetrating ovum envelopes during fertilisation.
  2. Neck:
     • Contains two centrioles: Proximal Centriole (transferred to egg for zygotic cleavage) and Distal Centriole (gives rise to the axial filament/axoneme).
  3. Middle Piece:
     • Packed with $25\text{--}30$ spirally arranged mitochondria (Nebenkern) surrounding the axial filament.
     • Generates ATP energy to power tail movements for motility.
  4. Tail:
     • Longest region consisting of an axial filament; produces vigorous whiplash movement for sperm propulsion.

Female Reproductive System

• Components:
  1. Primary Sex Organs: Ovaries (1 pair).
  2. Secondary Sex Organs / Accessory Ducts: Fallopian tubes (1 pair), Uterus, Cervix, Vagina.
  3. Accessory Glands: Bartholin's glands, Skene's glands (paraurethral glands).
  4. External Genitalia (Vulva): Mons pubis, Labia majora, Labia minora, Clitoris, Hymen.
  5. Mammary Glands (1 pair): Functional anatomical structures supporting lactation and infant care.

Histology and Structure of Ovaries

• Anatomy: Paired primary female sex organs, $2\text{--}4\text{\,cm}$ in length, located in the lower pelvis on either side of the uterus.
• Ligamentous Attachment: Anchored to the pelvic wall and uterus by fibrous ligaments (Ovarian Ligament and Suspensory Ligament).
• Histological Layers:
  • Covered externally by a thin layer of Germinal Epithelium, followed internally by a dense connective tissue layer called Tunica Albuginea.
  • Ovarian Stroma: Enclosed inner tissue divided into two regions:
    • Outer Cortex: Granular region containing developing ovarian follicles at various developmental stages.
    • Inner Medulla: Central vascular region containing blood vessels, lymphatic vessels, and nerves.

Female Accessory Ducts and External Genitalia

• Fallopian Tubes (Oviducts):
  • Paired muscular tubes, $10\text{--}12\text{\,cm}$ long, lined internally with ciliated columnar epithelium to drive the ovum towards the uterus.
  • Anatomical Regions:
    1. Infundibulum: Funnel-shaped section closest to the ovary. Possesses finger-like projections called Fimbriae that sweep over the ovary during ovulation to collect the released secondary oocyte.
    2. Ampulla: Wider, elongated central region where fertilisation takes place.
    3. Isthmus: Short, narrow, thick-walled terminal segment that joins the uterus.
• Uterus (Womb / Hystera):
  • Single inverted pear-shaped organ positioned between the bladder and rectum.
  • Anatomical Regions: Fundus (top dome-shaped portion above fallopian entries; primary implantation site), Body (main uterine cavity), and Cervix (narrow lower neck region).
  • Uterine Wall Composition:
    1. Perimetrium: External thin membrane composed of epithelium and connective tissue.
    2. Myometrium: Thick middle layer of smooth muscle; undergoes powerful rhythmic contractions driven by Oxytocin during parturition.
    3. Endometrium: Inner highly vascularized, glandular lining. Undergoes extensive cyclic histological changes during the menstrual cycle. Composed of two sub-layers: Stratum Basalis (permanent base layer) and Stratum Functionalis (sheds monthly as menstrual flow if pregnancy fails to occur).
• Vagina and Cervical Canal:
  • Cervical Canal: Cavity of the cervix; joins the vagina to form the Birth Canal.
  • Vagina: Muscular, elastic copulatory tube extending from cervix to the body exterior.
• Female External Genitalia (Vulva):
  • Mons Pubis: Anteriormost cushion of fatty tissue covered by skin and pubic hair.
  • Labia Majora: Paired fleshy folds of tissue extending downward from mons pubis, bearing pubic hair, surrounding the vaginal opening.
  • Labia Minora: Paired delicate tissue folds medial to labia majora, lacking hair follicles.
  • Clitoris: Tiny, erectile finger-like structure at the superior junction of the labia minora, positioned above the urethral orifice. Homologous to the male penis.
  • Hymen: Vascularized membrane partially covering the vaginal orifice. Can be torn during first sexual intercourse, physical exertion, cycling, horseback riding, or insertion of tampons/menstrual cups; hence, its presence or absence is not a sign of virginity.
  • Vestibule: Central recess enclosing both the urethral opening and vaginal orifice.

Mammary Glands and Physiology of Lactation

• Anatomy: Paired modified sudorific (sweat) glands lying over the Pectoralis major chest muscles.
• Developmental Control:
  • Growth at Puberty: Driven primarily by Estrogen.
  • Glandular Proliferation during Pregnancy: Driven by Prolactin and Progesterone.
• Internal Path of Milk Flow:
  • Glandular tissue of each gland is divided into $15\text{--}20$ Mammary Lobes.
  • Each lobe contains clusters of secretory cells called Mammary Alveoli that synthesize and store milk within their lumens.
  • Alveoli $\rightarrow$ Mammary Tubules $\rightarrow$ Mammary Ducts $\rightarrow$ Mammary Ampulla (enlarged space) $\rightarrow$ Lactiferous Ducts $\rightarrow$ Nipple (surrounded by pigmented Areola).
• Endocrine Control of Lactation:
  • PRH (Prolactin-Releasing Hormone): Secreted by Hypothalamus during pregnancy; acts on Anterior Pituitary to secrete Prolactin (PRL).
  • Prolactin (PRL): Primary lactogenic hormone promoting milk synthesis in alveoli. High levels of progesterone during pregnancy inhibit PRL's full action; post-delivery drop in progesterone initiates active milk synthesis.
  • Oxytocin: Synthesized by Hypothalamus, stored in Posterior Pituitary. Binds to myoepithelial cells surrounding alveoli, causing contraction and immediate Milk Ejection (let-down reflex) in response to infant suckling.
• Composition of Milk and Colostrum:
  • Colostrum: Deep yellow, nutrient-rich milk secreted during initial days of lactation. Packed with immunoglobulins, notably $\text{IgA}$ antibodies, providing essential passive immunity to the neonate.
  • Milk Components: Water, Casein (milk protein), Lactose (milk sugar), vitamins, electrolytes; notable for having poor iron content.

Oogenesis and Folliculogenesis

• Oogenesis Timeline: Gametogenesis in females starts during Embryonic Life (~2 million oogonia produced per fetal ovary; zero oogonia added after birth) and permanently ceases at Menopause.
• Follicular Stages and Meiotic Progress:
  1. Oogonium ($2n = 46$, $44+\text{XX}$): Fetal germ cell undergoes mitosis to form Primary Oocytes ($2n$).
  2. Primary Oocyte ($2n$): Begins Meiosis I in the fetus but gets arrested at Prophase I (Diplotene stage).
  3. Primary Follicle: Primary oocyte surrounded by a single layer of granulosa cells.
  4. Follicular Atresia: Massive degeneration of primary follicles from birth to puberty; only $60,000\text{--}80,000$ primary follicles remain in each ovary at puberty.
  5. Secondary Follicle: Primary oocyte surrounded by multiple granulosa layers and a newly formed connective tissue layer called Theca.
  6. Tertiary Follicle: Characterized by a fluid-filled cavity called the Antrum. Theca differentiates into Theca Interna (vascular, secretes Estrogen) and Theca Externa (fibrous layer).
     • Completion of Meiosis I: Inside the tertiary follicle, the arrested primary oocyte completes its Meiosis I just before ovulation via unequal cytoplasmic division, yielding a large haploid Secondary Oocyte ($n = 23$) and a tiny First Polar Body ($n = 23$).
  7. Graafian Follicle (Mature Follicle): Fully mature follicle. Secondary oocyte secretes an acellular glycoprotein layer around itself called the Zona Pellucida.
  8. Ovulation: Triggered by LH Surge on ~Day 14. Graafian follicle ruptures to release the Secondary Oocyte (which is arrested at Metaphase II of Meiosis II) into the pelvic cavity.
  9. Corpus Luteum: Yellow glandular remnant of ruptured Graafian follicle; secretes massive amounts of Progesterone and moderate Estrogen.
  10. Corpus Albicans: White fibrous scar formed when Corpus Luteum degenerates in the absence of pregnancy.

Comparative Analysis: Spermatogenesis vs. Oogenesis

• Initiation Time: Spermatogenesis starts at puberty; Oogenesis starts during embryonic life.
• Termination Time: Spermatogenesis continues into extreme old age; Oogenesis stops at menopause (~$45\text{--}50$ years).
• Continuity: Spermatogenesis is an uninterrupted continuous process; Oogenesis is a discontinuous process with prolonged meiotic arrests (Prophase I, Metaphase II).
• Gamete Yield: 1 Primary Spermatocyte yields 4 functional sperms; 1 Primary Oocyte yields only 1 functional Ovum (and 2--3 non-functional polar bodies).
• Cytoplasmic Division: Spermatogenesis features equal cytoplasmic divisions; Oogenesis features unequal divisions to reserve maximum cytoplasm for embryo nutrition.
• Differentiation Stage: Spermatogenesis includes spermiogenesis post-meiosis; Oogenesis has no spermiogenesis equivalent.

Physiology of the Menstrual Cycle

• Definition: Reproductive cycle occurring in female primates (humans, apes, monkeys) lasting average $28\text{--}29$ days. Non-primate mammals undergo an Oestrous Cycle (heat period).
• Terminology:
  • Menarche: First occurrence of menstruation at puberty.
  • Menopause: Permanent termination of menstrual cycles.
• Phases of the Menstrual Cycle:
  1. Menstrual Phase (Days 1--5):
     • Uterine Event: Breakdown and sloughing off of Endometrium (Stratum functionalis) along with blood vessels, flowing through vagina.
     • Hormonal Cause: Steep decline in both Progesterone and Estrogen due to degeneration of the Corpus Luteum of the preceding cycle.
  2. Follicular / Proliferative Phase (Days 6--13):
     • Ovarian Event: Low steroid levels remove negative feedback; FSH and LH levels gradually increase. FSH stimulates primary follicles to grow into mature Graafian follicles.
     • Uterine Event: Developing follicles secrete rising amounts of Estrogen, stimulating rapid proliferation and repair of the uterine endometrium.
  3. Ovulatory Phase (Day 14):
     • Hormonal Event: Rapid peak of Estrogen induces a dramatic spike in LH (LH Surge) and a concurrent FSH peak.
     • Ovarian Event: LH surge induces rupture of Graafian follicle and release of the secondary oocyte (Ovulation).
  4. Luteal / Secretory Phase (Days 15--28):
     • Ovarian Event: LH converts ruptured follicle into the Corpus Luteum.
     • Hormonal Event: Corpus Luteum secretes high concentrations of Progesterone (major peak) and moderate Estrogen (second peak).
     • Uterine Event: Progesterone makes the endometrium thick, highly vascularized, glandular, and secretory to support potential implantation.
     • If No Fertilisation Occurs: High Progesterone/Estrogen exerts negative feedback on LH/FSH $\rightarrow$ Corpus Luteum degenerates $\rightarrow$ Hormone levels plunge $\rightarrow$ Endometrital lining collapses $\rightarrow$ New cycle begins.
• Luteal Phase Duration Rule:
  • Luteal phase duration is fixed at 14 days.
  • $\text{Day of Ovulation} = \text{Total Cycle Duration (Days)} - 14$.
  • Example: For a 28-day cycle, ovulation occurs on Day $28 - 14 = 14$. For a 32-day cycle, Day $32 - 14 = 18$.

Process of Fertilisation and Polyspermy Prevention

• Site of Fertilisation: Ampullary region of Fallopian tube (requires simultaneous arrival of sperms and secondary oocyte).
• Sequential Steps:
  1. Capacitation: Functional maturation of sperms inside female reproductive tract. Involves entry of $Ca^{2+}$ into sperm tail to trigger hyperactive whiplash movements, and stripping off membrane surface glycoproteins/cholesterol from the acrosome.
  2. Sperm-Egg Recognition: Sperm surface proteins bind specifically to ZP-3 Receptors on the Zona Pellucida of the secondary oocyte.
  3. Acrosomal Reaction: Release of sperm lysins from acrosome:
     • Hyaluronidase: Hydrolyzes hyaluronic acid glue binding granulosa/corona radiata cells.
     • Corona Penetrating Enzyme: Dissolves corona radiata layers.
     • Zona Lysin (Acrosin): Digests Zona Pellucida protein layer.
  4. Prevention of Polyspermy (Ensures single sperm fertilisation):
     • Fast Block to Polyspermy (Temporary): Sperm binding depolarizes oocyte plasma membrane from resting potential ($-70\text{\,mV}$) to excited potential ($+30\text{\,mV}$) via rapid $Na^+$ influx, preventing additional sperm fusion.
     • Slow Block to Polyspermy (Permanent - Cortical Reaction): Depolarization triggers $Ca^{2+}$ release from Smooth Endoplasmic Reticulum (SER) $\rightarrow$ triggers exocytosis of Cortical Granules into the perivitelline space $\rightarrow$ cortical enzymes hydrolyze remaining ZP-3 receptors and harden Zona Pellucida into an impenetrable fertilisation membrane.
  5. Completion of Meiosis II: Entry of sperm triggers completion of Meiosis II in the secondary oocyte, giving rise to a large haploid Ovum ($n$) and a tiny Second Polar Body ($n$).
  6. Syngamy / Karyogamy: Haploid sperm nucleus fuses with haploid ovum nucleus to re-establish diploidy, forming the Zygote ($2n = 46$).
• Sex Determination:
  • Determined exclusively by male gamete karyotype.
  • Ovum always carries $22+\text{X}$.
  • If sperm carries $22+\text{X}$, zygote becomes female ($44+\text{XX}$).
  • If sperm carries $22+\text{Y}$, zygote becomes male ($44+\text{XY}$).

Cleavage, Morula, and Blastocyst Formation

• Cleavage: Series of rapid mitotic divisions occurring in zygote as it travels through Isthmus towards Uterus. Characterized by absence of growth phases ($G_1, G_2$ stages absent), producing progressively smaller daughter cells called Blastomeres.
• Developmental Stages:
  • Zygote ($1\text{-cell}$) $\rightarrow$ 2-celled $\rightarrow$ 4-celled $\rightarrow$ 8-celled $\rightarrow$ 16-celled stage.
  • Morula: Solid mulberry-like sphere of $8\text{--}16$ blastomeres enclosed by Zona Pellucida.
  • Blastocyst (Blastula - 32-cell stage):
    • Trophoblast (Trophoectoderm): Outer single layer of flat cells; secretes enzymes to dissolve Zona Pellucida (hatching) and anchors to endometrium; forms foetal placenta.
    • Inner Cell Mass (ICM / Embryoblast): Cluster of interior cells attached at one pole; contains Totipotent Stem Cells capable of giving rise to all embryonic tissues and organ systems.
    • Cells of Rauber: Trophoblast cells directly in contact with ICM.
    • Blastocoel: Internal fluid-filled cavity.

Implantation, Pregnancy, and Placental Physiology

• Implantation: Attachment and embedding of Blastocyst into uterine endometrium (usually ~Day 7 post-fertilisation in Fundus region).
• Hormonal Control of Early Pregnancy:
  • Trophoblast secretes hCG (human Chorionic Gonadotropin).
  • hCG mimics LH action to maintain the Corpus Luteum.
  • Corpus Luteum continues secreting Progesterone and Estrogen, maintaining endometrium and preventing menstrual breakdown.
• Ectopic Pregnancy: Abnormal implantation occurring outside the uterine endometrium (e.g., inside Fallopian tube).
• Types of Twins:
  • Monozygotic (Identical) Twins: Single zygote/blastocyst divides into two separate embryos. Possess identical genetic material and are always the same sex.
  • Dizygotic (Fraternal) Twins: Two separate ova matured and ovulated simultaneously, fertilised by two distinct sperms. Can be same or different sex, with distinct genetic profiles.
• Placenta and Extra-Embryonic Membranes:
  • Formation: Syncytiotrophoblast forms finger-like projections called Chorionic Villi (foetal placenta) that interdigitate with uterine tissue pockets (maternal placenta).
  • Extra-Embryonic Membranes: Amnion (encloses Amniotic cavity containing Amniotic fluid), Chorion, Allantois, Yolk sac.
  • Umbilical Cord: Vascular cord connecting foetus to placenta. Contains 2 Umbilical Arteries and 1 Umbilical Vein carrying $100\%$ foetal blood.
  • Placental Functions: Serves as physical and metabolic barrier for diffusion of $O_2$/nutrients to foetus and excretion of nitrogenous wastes ($CO_2$, urea) to mother; functions as temporary endocrine organ.
• Placental Endocrine Hormones:
  • hCG: Maintains Corpus Luteum; excreted in maternal urine (basis of Gravidex / Pregnancy Test).
  • hPL (human Placental Lactogen): Stimulates mammary growth and maternal lipid/glucose metabolism.
  • Relaxin: Secreted by placenta/ovary near term; relaxes pubic symphysis fibrocartilage to enlarge pelvic outlet for delivery.
  • Chorionic Thyrotrophin, CRH, Estrogen, Progesterone (forms protective mucus plug in cervical canal).

Human Gestation and Embryonic Development Timeline

• Gestation Period: ~9 months ($3$ Trimesters of $3$ months each / $38\text{--}40$ weeks total).
• Species Gestation Comparison: Cats/Dogs ($52\text{--}65\text{\,days}$); Elephants ($18\text{--}22\text{\,months}$).
• Foetal Development Milestones:
  • Month 1 (4 weeks): Heart is formed. (First clinical sign of growing foetus is heartbeat heard via stethoscope).
  • Month 2 (8 weeks): Limbs and digits develop.
  • Month 3 (12 weeks / End of 1st Trimester): Major organ systems formed; external genitalia distinct.
  • Month 5 (20 weeks): First foetal movements felt by mother (Quickening); hair appears on head.
  • Month 6 (24 weeks / End of 2nd Trimester): Body covered with fine hair; eyelids separate; eyelashes formed.
  • Month 9 (36--40 weeks): Foetus fully developed, ready for delivery.

Parturition and Foetal Ejection Reflex

• Mechanism: Complex neuro-endocrine process initiating labor.
• Sequence of Parturition:
  1. Fully developed foetus and placenta send signals, causing placenta to weaken and progesterone levels to drop.
  2. Foetal Hypothalamus secretes CRH $\rightarrow$ Foetal Anterior Pituitary secretes ACTH $\rightarrow$ Foetal Adrenal Cortex secretes Cortisol (opens pulmonary alveoli) and DHEA (Androgen).
  3. Placenta converts DHEA into Estrogen.
  4. Rising Estrogen increases Oxytocin receptors on uterine smooth muscle (Myometrium) and stimulates Prostaglandins release.
  5. Mild uterine contractions triggered by foetus and placenta form the Foetal Ejection Reflex.
  6. Reflex signals maternal Posterior Pituitary to release Oxytocin.
  7. Oxytocin acts on myometrium causing stronger contractions, triggering positive feedback for more oxytocin release $\rightarrow$ vigorous labor contractions $\rightarrow$ expulsion of baby through birth canal $\rightarrow$ placenta expelled shortly after.
• Clinical Application: Synthetic Oxytocin (Pitocin) injected intravenously to induce labor when necessary.

Practice Questions and Assessment Solutions

• Question 1: Which is correct sequence of male accessory ducts starting from the testis?
  • Correct Sequence: Rete testis $\rightarrow$ vasa efferentia $\rightarrow$ epididymis $\rightarrow$ vas deferens.
• Question 2: In which embryonic stage does implantation take place?
  • Correct Answer: Blastocyst.
• Question 3: In human beings, at which month of pregnancy are most organ systems formed?
  • Correct Answer: 1st trimester (3rd month).
• Question 4: Which hormone is NOT secreted by human placenta?
  • Correct Answer: Thyroxine (Placenta secretes Estrogen, Progesterone, hCG, hPL, Relaxin).
• Question 5: Spermiogenesis is the transformation of:
  • Correct Answer: Spermatids into functional spermatozoa.
• Question 6: Meiosis in oogenesis starts in:
  • Correct Answer: Embryonic life.
• Question 7: At the time of birth, a human female is born with:
  • Correct Answer: Primary follicles containing Primary oocytes arrested at Prophase I.
• Question 8: Assertion (A): Not all copulations lead to pregnancy. Reason (R): Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampulla region.
  • Correct Evaluation: Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
• Question 9: Matching List-I and List-II:
  • (a) Birth canal $\rightarrow$ (iii) Vagina along with cavity of cervix.
  • (b) Endometrium $\rightarrow$ (i) Inner glandular layer.
  • (c) Antrum $\rightarrow$ (ii) Fluid filled cavity of tertiary follicle.
  • (d) Uterus $\rightarrow$ (iv) Womb.
• Question 10: Identify the common duct formed by the fusion of the duct of seminal vesicle and vas deferens:
  • Correct Answer: Common Ejaculatory Duct.
• Question 11: Diagrammatic sketch labelling of male reproductive system:
  • A = Vas deferens, B = Seminal vesicle, C = Prostate, D = Bulbourethral gland.
• Question 12: Statement I: The inner cell mass contains certain cells called stem cells which have the potency to give rise to all tissues and organs. Statement II: The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.
  • Correct Evaluation: Both Statement I and Statement II are correct.
• Question 13: Matching List-I and List-II:
  • (a) hPL $\rightarrow$ (ii) Placenta.
  • (b) LH $\rightarrow$ (i) Anterior pituitary.
  • (c) Oxytocin $\rightarrow$ (iv) Posterior pituitary.
  • (d) Progesterone $\rightarrow$ (iii) Corpus luteum.
• Question 14: Matching List-I and List-II:
  • (a) Embryo with 8 to 16 blastomeres $\rightarrow$ (iv) Morula.
  • (b) Morula transforms in $\rightarrow$ (ii) Blastocyst.
  • (c) Attaches to endometrium $\rightarrow$ (i) Trophoblast.
  • (d) Blastocyst gets embedded in endometrium $\rightarrow$ (iii) Implantation.
• Question 15: Statement I: Corpus luteum secretes large amounts of progesterone essential for endometrium maintenance. Statement II: Endometrium is necessary for implantation.
  • Correct Evaluation: Both Statement I and Statement II are correct.
• Question 16: If 40 sperms are formed, how many secondary spermatocytes have completed meiosis?
  • Calculation: Each secondary spermatocyte undergoes Meiosis II to yield 2 sperms. To form 40 sperms, $40 / 2 = 20$ secondary spermatocytes must complete Meiosis II.
  • Correct Answer: 20.
• Question 17: Statement I: Milk produced during initial days of lactation is called colostrum. Statement II: Colostrum contains several antigens absolutely essential to develop resistance for newborn babies.
  • Correct Evaluation: Statement I is correct, but Statement II is incorrect (Colostrum contains antibodies such as IgA, not antigens).
• Question 18: Assertion (A): Doctors inject oxytocin to induce delivery. Reason (R): Oxytocin causes uterine contraction which helps in delivery.
  • Correct Evaluation: Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
• Question 19: Statement I: Ovaries are located on each side of the lower abdomen. Statement II: Each ovary is connected to pelvic wall and uterus by ligaments.
  • Correct Evaluation: Both Statement I and Statement II are correct.
• Question 20: Matching List-I and List-II:
  • (a) Provide nutrition to spermatogonia $\rightarrow$ (iii) Sertoli cells.
  • (b) Sperm formation $\rightarrow$ (i) Male germ cells.
  • (c) Synthesise androgens $\rightarrow$ (iv) Leydig cells.
  • (d) Synthesise GnRH $\rightarrow$ (ii) Hypothalamus.