Human Reproduction

Human Reproduction

1. Overview & Male Reproductive System

A. Sequence of Reproductive Events

1. Gametogenesis: Formation of gametes (Sperm in males, Ova in females).
2. Insemination: Transfer of sperms into female genital tract.
3. Fertilization: Fusion of male and female gametes to form Zygote.
4. Implantation: Attachment of Blastocyst to the uterine wall (Endometrium).
5. Gestation: Embryonic/fetal development inside the womb (uterus).
6. Parturition: Delivery of the fully developed fetus out of the mother’s body.
7. Lactation: Production and release of milk by mammary glands to feed the newborn.

B. Male Reproductive System: Testes Anatomy

• Primary Sex Organ: Pair of Testes (Length: 4–5 cm, Width: 2–3 cm). Located extra-abdominally within the Scrotum to maintain temperature 2 to 2.5°C lower than normal body temperature for spermatogenesis.
• Testicular Descent: During the 7th month of fetal development, testes descend from abdomen to scrotum via the Inguinal canal under the influence of Testosterone.• Cryptorchidism: Failure of testes to descend into scrotum; leads to sterility.

• Monorchidism: Presence of only one testis in scrotum (e.g., Ascaris).
• Temperature Regulation Muscles: Darter muscle (scrotal wall) & Cremaster muscle (spermatic cord) contract in winter (bring testes closer to body) and relax in summer (move testes away).
• Internal Structure: Each testis contains 250–300 testicular lobules. Each lobule has 1 to 3 highly coiled Seminiferous Tubules (structural unit of spermatogenesis).
• Duct Pathway:$$\text{Seminiferous Tubules} \to \text{Tubuli Recti} \to \text{Rete Testis} \to \text{Vasa Efferentia} \to \text{Epididymis} \to \text{Vas Deferens}$$

2. Accessory Glands, Sperm Structure & Female Organs

A. Male Accessory Glands

• Seminal Vesicles (Paired): Contributes 60–70% of semen volume. Secretes fructose (energy source for sperms), inositol, prostaglandins, citric acid, and fibrinogen (clotting protein).
• Prostate Gland (Single): Chestnut-shaped, lies below urinary bladder. Secretes alkaline fluid containing $\text{Ca}^{2+}$, citrate, and fibrinolysin to neutralize acidic vaginal environment. Has characteristic odor.
• Bulbourethral / Cowper’s Gland (Paired): Pea-sized, secretes alkaline viscous fluid for lubrication of penis during coitus.

B. Structure of Mature Sperm

• Head: Contains haploid nucleus ($n=23$) capped by Acrosome (derived from Golgi body, contains hydrolytic enzymes / sperm lysins).
• Neck: Contains proximal centriole (initiates cleavage in zygote) and distal centriole (gives rise to axial filament).
• Middle Piece: Packed with spiral mitochondria (Nebenkern) that provide energy (ATP) for sperm motility.
• Tail: Long axial filament facilitating flagellar movement toward the ovum.

C. Female Reproductive System Overview

• Primary Sex Organ: Pair of Ovaries (produce ova and steroid hormones: Estrogen & Progesterone).
• Accessory Ducts: Fallopian tubes (Oviducts), Uterus (Womb), Cervix, Vagina.
• Fallopian Tube Divisions (10–12 cm long):$$\text{Infundibulum (Fimbriae)} \to \text{Ampulla (Site of Fertilization)} \to \text{Isthmus}$$

3. Ovary Anatomy, Folliculogenesis & Uterus Layers

A. Ovarian Anatomy & Attachments

• Almond-shaped organs (2–4 cm length) attached to pelvic wall and uterus by ligaments:• Mesovarium / Broad Ligament: Attaches ovary to abdominal wall.• Ovarian Ligament: Connects ovary to uterus.

  • Suspensory Ligament: Connects ovary to pelvic wall.

• Ovarian Stroma: Divided into outer Cortex (contains follicles) and inner Medulla (contains blood vessels, elastic fibers, and connective tissue).

B. Follicular Development Stages

• Primary Follicle: Primary oocyte surrounded by a single layer of granulosa cells.
• Secondary Follicle: Surrounded by multiple granulosa layers and a theca layer.
• Tertiary Follicle: Characterized by a fluid-filled cavity called the Antrum (containing Liquor folliculi). Theca differentiates into Theca interna (hormonal) and Theca externa.
• Graafian Follicle (Mature): Contains secondary oocyte arrested at Metaphase II, surrounded by Zona Pellucida and Corona Radiata (Cumulus oophorus). Ruptures during ovulation under LH surge to release secondary oocyte.
• Corpus Luteum: Remnant of ruptured Graafian follicle; acts as temporary endocrine gland secreting large amounts of Progesterone. Degenerates into Corpus Albicans (white scar) if fertilization does not occur.

C. Structure of Uterus (Womb)

Inverted pear-shaped organ consisting of Fundus, Body (Corpus), and Cervix. Uterine wall consists of 3 layers:

• Perimetrium: External thin membranous layer.
• Myometrium: Middle thick layer of smooth muscle; exhibits strong contractions during labor (parturition).
• Endometrium: Inner glandular vascular layer.• Stratum Functionalis: Outer temporary layer sloughed off during menstruation.• Stratum Basalis: Permanent inner layer that regenerates functionalis.

4. Mammary Gland & Gametogenesis

A. Mammary Gland Pathway

Paired modified sweat glands characteristic of female mammals. Contain glandular tissue and variable fat.

Duct Pathway:

Mammary Lobules (15–20) $\to$ Alveoli $\to$ Mammary Tubules $\to$ Mammary Duct $\to$ Mammary Ampula $\to$ Lactiferous Duct $\to$ Nipple

B. Spermatogenesis vs Oogenesis Comparison

5. Menstrual Cycle & Hormonal Control

A. Phases of Menstrual Cycle (28–29 Days)

• 1. Menstrual Phase (Days 1–5): Breakdown of endometrial lining and blood vessels; flow lasts 3–5 days due to sudden drop in Progesterone levels.
• 2. Follicular / Proliferative Phase (Days 6–13): Primary follicles grow into Graafian follicles under FSH; Endometrium regenerates through proliferation stimulated by Estrogen.
• 3. Ovulatory Phase (Day 14): Peak levels of LH and FSH. Rapid secretion of LH leads to LH Surge, causing rupture of Graafian follicle and release of secondary oocyte (Ovulation).• Rule for Ovulation Day: $\text{Day of Ovulation} = \text{Total Cycle Length} – 14$. Luteal phase is fixed at 14 days.
• 4. Luteal / Secretory Phase (Days 15–28): Corpus luteum secretes high Progesterone to maintain endometrium for implantation. If fertilization fails, corpus luteum degenerates, causing menstruation.

B. Estrous Cycle (Non-Primate Mammals)

Occurs in cows, sheep, dogs, horses, rats. No menstrual bleeding; broken-down endometrium is reabsorbed. Females are receptive only during “Heat period” (High Estrogen).

6. Fertilization, Capacitation & Reactions

A. Sperm Capacitation & Chemoattraction

• Capacitation: Physiological conditioning of sperms in female genital tract (uterus/fallopian tube) lasting 6–7 hours. Involves removal of membrane cholesterol/glycoproteins and $\text{Ca}^{2+}$ influx, enabling hyperactive whiplash motility.
• Chemoattraction: Egg releases species-specific glycoprotein Fertilizin, which interacts with Antifertilizin (acidic protein) on sperm surface.

B. Acrosomal Reaction & Polyspermy Blocks

• Acrosomal Enzymes Released:• Hyaluronidase: Dissolves hyaluronic acid cementing corona radiata cells.• Corona Penetrating Enzyme (CPE): Dissolves corona radiata.

  • Zona Lysin / Acrosin: Digests Zona Pellucida.

• Fast Block to Polyspermy: Binding of sperm to ZP3 receptor induces $\text{Na}^+$ influx, depolarizing the oocyte plasma membrane within seconds.
• Slow Block to Polyspermy (Cortical Reaction): $\text{Ca}^{2+}$ influx triggers exocytosis of cortical granules into perivitelline space, hardening Zona Pellucida and destroying ZP3 receptors.

7. Cleavage & Implantation

A. Cleavage Dynamics

• Rapid mitotic divisions of zygote in Fallopian tube. First cleavage takes ~30 hours.
• Pattern: Holoblastic (complete division), unequal, and Rotational cleavage.
• Morula: Solid ball of 8–16 blastomeres with tight junctions. Overall size does not increase due to presence of intact Zona Pellucida.
• Blastocyst / Blastula: Formed on Day 4–5. Consists of outer Trophoblast layer (forms extra-embryonic membranes/placenta) and Inner Cell Mass (ICM / Embryoblast) (forms complete embryo body). Contains fluid cavity called Blastocoel.

B. Zona Hatching & Implantation

• Zona Hatching: Trophoblast secretes lytic enzymes to digest Zona Pellucida, allowing sticky blastocyst to hatch out.
• Implantation: Attachment of blastocyst to uterine endometrium on 6th–7th day after fertilization.
• Ectopic Pregnancy: Implantation occurring at any site other than uterus (e.g., Tubal pregnancy in fallopian tube).
• Decidua Layers:• Decidua Basalis: Endometrium between blastocyst and myometrium (forms maternal part of placenta).• Decidua Capsularis: Endometrial layer covering blastocyst.

  • Decidua Parietalis: Remaining lining of uterine cavity.

8. Gastrulation & Extra-Embryonic Membranes

A. Gastrulation (Germ Layer Formation)

Process of morphogenetic cell movements transforming monoblastic blastula into triploblastic gastrula containing three primary germ layers.

• Inner cell mass differentiates into Epiblast (forms 3 germ layers) and Hypoblast. Formation of Primitive Streak marks commencement of gastrulation.

B. Fate of Primary Germ Layers

C. Extra-Embryonic Membranes

• Amnion (Inner): Fills with Amniotic Fluid; prevents desiccation, acts as shock absorber.
• Chorion (Outer): Forms fetal portion of placenta; secretes hCG.
• Yolk Sac: Site of early blood cell formation (Haemopoiesis).
• Allantois: Forms blood vessels of umbilical cord; excretory/respiratory in lower vertebrates.

9. Placenta & Gestation Period

A. Placental Nature & Endocrine Hormones

Structural and functional unit between developing embryo/fetus and maternal body. Human placenta is Deciduate, Hemochorial, Discoidal, and Choriosantoic.

• Hormones Secreted by Placenta:• hCG (Human Chorionic Gonadotropin): Maintains Corpus Luteum to continue progesterone release. Presence in urine confirms pregnancy (Gravindex / Pregnancy Test).• hPL / hCS (Human Placental Lactogen / Somatomammotropin): Prepares mammary glands for lactation and regulates maternal glucose metabolism.

  • Estrogen & Progesterone: Maintain uterine lining and prevent menstruation.

  • hCT & hCC: Human Chorionic Thyrotropin and Corticotropin.

  • Relaxin: Secreted by Corpus Luteum of ovary toward end of gestation; loosens pubic symphysis ligament for easier delivery.

B. Human Fetal Milestone Timeline

• Gestation Period: 266 days from fertilization (280 days / 40 weeks from Last Menstrual Period – LMP).
• 1st Month: Heart is formed (first sign of growing fetus is heart sound via stethoscope).
• 2nd Month: Limbs and digits develop.
• 3rd Month (End of 1st Trimester): Major organ systems formed, including external genital organs.
• 5th Month: First fetal movements felt by mother and appearance of fine hair on head.
• 6th Month (End of 2nd Trimester): Body covered with fine hair, eye-lids separate, and eye-lashes formed.

10. Parturition & Lactation

A. Neuroendocrine Reflex of Parturition

Process of child birth induced by a complex neuroendocrine mechanism.

1. Fully developed fetus and placenta originate signals causing mild uterine contractions called Foetal Ejection Reflex.
2. Triggers release of Oxytocin from maternal posterior pituitary gland.
3. Oxytocin acts on myometrium, stimulating stronger uterine contractions, which further stimulates more oxytocin secretion (Positive Feedback Loop).
4. Leads to cervical dilation and expulsion of baby through birth canal.
5. Within 15–20 minutes after delivery, placenta and umbilical cord remnants are expelled (Afterbirth).

B. Lactation & Colostrum Immunity

• Hormone Control:• Prolactin (Anterior Pituitary): Stimulates milk synthesis and secretion.• Oxytocin (Posterior Pituitary): Stimulates milk ejection / let-down reflex.
• Colostrum: Yellowish thick fluid produced during initial few days of lactation. Rich in proteins, minerals, and rich in IgA antibodies providing essential Passive Immunity to newborn baby.