15.1 Reproductive System of Humans
Reproduction ensures the continuity of species. The human reproductive system consists of male and female structures specialized for gamete production, fertilization, and embryo development.
1. Male Reproductive System
- Testes: Produce sperm (male gametes) and male sex hormones (testosterone). Located inside the scrotum.
- Scrotum: Holds testes outside the abdominal cavity to maintain a lower temperature (~2°C lower) necessary for spermatogenesis.
- Sperm Duct (Vas Deferens): Transports sperm from the epididymis to the urethra.
- Seminal Vesicles, Prostate Gland & Cowper's Gland: Secretes nutrient-rich fluids (fructose, alkaline substances) that nourish, activate, and protect sperm to form semen.
- Urethra: Canal passing through the penis for the release of semen and urine (at different times).
2. Female Reproductive System
- Ovaries: Produce secondary oocytes (female gametes) and female sex hormones (estrogen and progesterone).
- Fallopian Tubes (Oviducts): Site of fertilization. Lined with cilia to move the secondary oocyte or zygote toward the uterus.
- Uterus: Pear-shaped muscular organ where embryo implantation and fetal development occur. Lined with vascularized endometrium.
- Cervix: Narrow neck of the uterus that opens into the vagina; secretes mucus to assist sperm transport.
- Vagina: Canal through which semen is deposited during copulation and acts as the birth canal.
15.2 Gametogenesis in Humans
Gametogenesis is the process of gamete formation via meiosis in the gonads, producing haploid gametes (n = 23).
1. Spermatogenesis (Occurs in Seminiferous Tubules of Testes)
- Spermatogonia (2n) multiply by mitosis to form primary spermatocytes (2n).
- Primary spermatocyte undergoes Meiosis I to produce two secondary spermatocytes (n).
- Secondary spermatocytes undergo Meiosis II to produce four spermatids (n).
- Spermatids undergo differentiation (growing tails, losing cytoplasm) to become mature spermatozoa (n).
2. Oogenesis (Occurs in Ovaries)
- Before birth, oogonia (2n) undergo mitosis to form primary oocytes (2n), which remain arrested at Prophase I of meiosis.
- At puberty, FSH stimulates primary oocytes to resume Meiosis I, producing one large secondary oocyte (n) and one tiny first polar body (n).
- The secondary oocyte starts Meiosis II but remains arrested at Metaphase II until fertilization occurs.
- If fertilized by sperm, Meiosis II completes to produce an ovum (n) and a second polar body (n).
15.3 Menstrual Cycle
The menstrual cycle (~28 days) involves monthly changes in ovary structure (ovarian cycle) and uterine lining thickness (uterine cycle) controlled by pituitary and ovarian hormones.
Hormonal Control
- Follicle-Stimulating Hormone (FSH): Stimulates follicle development in the ovary and triggers estrogen secretion.
- Estrogen: Repairs and thickens the endometrium; stimulates LH production while inhibiting FSH via negative feedback.
- Luteinizing Hormone (LH): Causes a surge that triggers ovulation (Day 14) and forms the corpus luteum from the ruptured follicle.
- Progesterone: Maintained by corpus luteum; maintains thick vascular endometrium for implantation and inhibits FSH & LH release.
- Menstruation: If fertilization does not occur, corpus luteum degenerates, progesterone and estrogen levels drop, leading to endometrial shedding (Days 1–5).
15.4 Development of Human Foetus
- Fertilization: Nucleus of sperm fuses with nucleus of ovum in the Fallopian tube to form a diploid zygote (2n).
- Cleavage & Blastocyst Formation: Zygote undergoes rapid mitotic divisions forming a solid ball of cells (morula), then a hollow ball of cells called a blastocyst.
- Implantation: Blastocyst embeds into the thickened endometrium about 7 days after fertilization.
- Placenta Formation: Formed from embryonic tissue (chorion) and maternal uterine tissue. Functions include:
- Exchange of gases ($O_2, CO_2$), nutrients (glucose, amino acids), and metabolic wastes between mother and foetus.
- Secretion of hCG (Human Chorionic Gonadotropin) initially to maintain corpus luteum, and later secretes estrogen and progesterone.
- Acts as a selective barrier preventing mixing of maternal and fetal blood to protect foetus from high blood pressure and pathogens.
- Umbilical Cord: Contains two umbilical arteries (carry deoxygenated blood and metabolic waste from foetus to placenta) and one umbilical vein (carries oxygenated blood and nutrients from placenta to foetus).
15.5 Formations of Twins
- Identical Twins (Monozygotic): One ovum fertilized by one sperm to form one zygote. Embryo splits into two early during cleavage. Twins share 100% DNA, identical gender, and usually share one placenta.
- Fraternal Twins (Dizygotic): Two separate ova fertilized by two separate sperm. Twins share 50% DNA, may have different genders, and possess separate placentas.
- Siamese Twins: Identical twins where early embryonic separation is incomplete, leaving twins physically joined at specific body organs.
15.6 Health Issues Related to Human Reproductive System
- Male Infertility Causes: Low sperm count, poor sperm motility, erectile dysfunction, abnormal sperm structure.
- Female Infertility Causes: Anovulation (failure to release secondary oocyte), blocked Fallopian tubes, endometriosis, uterine fibroids.
- Assisted Reproductive Technologies (ART): In Vitro Fertilization (IVF), Intracytoplasmic Sperm Injection (ICSI), hormone therapy.
15.7 Growth in Humans and Animals
Growth is an irreversible, permanent increase in size, mass, and number of cells accompanied by differentiation.
1. Sigmoid Growth Curve
- Lag Phase: Slow growth rate; cells adapting to environment.
- Exponential (Log) Phase: Rapid growth rate due to high mitotic cell division.
- Decelerating Phase: Growth rate slows down; limited resources or aging cells.
- Stationary Phase: Zero net growth rate; rate of cell division equals rate of cell death.
- Senescence Phase: Negative growth rate due to aging and degeneration.
2. Intermittent Growth Curve in Insects (Ecdysis)
- Insects with hard exoskeletons undergo step-like growth interrupted by periodic shedding of the exoskeleton (ecdysis).
- During ecdysis, the insect swallows air to swell its body, breaking the old cuticle. A soft new exoskeleton underneath expands and hardens.
- Actual body growth occurs rapidly while the new cuticle is soft, whereas measured dimensions remain constant during instar stages.