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Human Reproduction

Anatomy: Male Reproductive System

  • Testis (Testicle): produces sperm cells and the hormone testosterone.

  • Epididymis: sperm cells mature and are stored here.

  • Vas deferens: transports sperm from the epididymis to the urethra.

  • Seminal vesicle: produces an alkaline fluid that neutralises acids in the vagina, helping sperm survive.

  • Prostate gland: produces a nutrient-rich fluid that provides energy for sperm cells.

  • Bulbourethral (Cowper’s) gland: produces mucus that assists the movement of sperm.

  • Scrotum: skin sac that protects the testes and holds them outside the body at a temperature about 2°C below body temperature, which is optimal for sperm production.

  • Testes: produce sperm cells and the hormone testosterone.

  • Urethra: transports semen and urine out of the body.

  • Penis: the organ through which semen is ejaculated; contains erectile tissue (corpus cavernosum and corpus spongiosum) and the urethral opening; includes the foreskin and glans.

  • Seminal structures and ducts visible in side/front views include: ejaculatory duct, vas deferens, epididymis, prostate gland, bulbourethral gland, testis, seminiferous tubules, urethra, bladder, rectum, anus.

  • Additional functional notes:

    • Sperm duct (vas deferens) and epididymis are involved in maturation and transport of sperm to the urethra during ejaculation.

    • Sperm travel pathway: testis → epididymis → vas deferens → ejaculatory duct → urethra → outside body; semen also contains secretions from seminal vesicles and prostate.

Anatomy: Female Reproductive System

  • Ovary: produces ova (eggs); secretes the hormones oestrogen (estrogen) and progesterone.

  • Fallopian tube (oviduct): connects ovaries to the uterus; transports ova from ovary; site of fertilisation.

  • Fimbriae: finger-like projections at the end of the fallopian tube that help capture the ovulated ovum.

  • Uterus: organ that carries the embryo and foetus during pregnancy.

  • Endometrium: inner lining of the uterus; thickens and provides the site for implantation of an embryo and placenta formation.

  • Cervix: lower, narrow part of the uterus; expands to allow passage of the baby during childbirth.

  • Vagina: receives penis and semen during intercourse; passage through which the baby is born.

  • Front view features: Uterine cavity, fallopian tubes, ovaries, fimbriae, endometrium, cervical canal.

  • Side view features: Ureter, fallopian tube, ovary, uterus, cervix, urinary bladder, vagina, pubic bone, clitoris, urethra, labia (minora and majora), anus.

  • Supporting structures include ligaments around the ovaries.

The Main Processes Involved in Human Reproduction

  • Maturation: birth occurs after 40 weeks; growth continues for 15-20 years through puberty until individuals are sexually mature and able to reproduce.

  • Growth and development of the embryo involves cell division and differentiation, increasing size and complexity during the embryonic stage.

  • Implantation: a ball of cells embeds into the wall of the uterus, resulting in pregnancy.

  • Ovulation: an ovum is released from the ovary roughly once a month.

  • Copulation: the male penis is inserted into the female vagina during sexual intercourse.

  • Ejaculation: semen containing sperm is released from the urethra of the penis into the vagina.

  • Sperm swims to the Fallopian tube.

  • Fertilisation: a male sperm fuses with a female ovum in the Fallopian tube to form a zygote.

  • If fertilisation does not occur: the ball of cells moves to the uterus and continues to divide.

  • Cell division: the fertilised ovum (zygote) divides repeatedly to form a ball of cells.

  • Menstruation: part of the uterine lining (including the unfertilised ovum) is discharged via the vagina.

  • Overall process: ovulation, fertilisation, implantation, and pregnancy may occur if a zygote forms; otherwise, menstruation occurs.

Structure and Function of the Human Sperm Cell

  • Structure components:

    • Head: contains the nucleus with the haploid set of chromosomes; contains the acrosome (a sac of enzymes on the front of the head).

    • Midpiece: contains mitochondria that provide energy for movement.

    • Tail: enables swimming.

  • Acrosome: enzymes dissolve the jelly membrane of the ovum to permit fertilisation.

  • Function summary: sperm cells are the male gametes responsible for delivering paternal DNA to the ovum and enabling fertilisation.

Gametogenesis

  • Gametogenesis means the formation of gametes by meiosis.

  • Male gametes: sperm cells formed by spermatogenesis.

  • Female gametes: ova formed by oogenesis.

Spermatogenesis (Process of Gametogenesis for Males)

  • Initiation: diploid germinal epithelial cells lining the seminiferous tubules divide mitotically to form diploid spermatogonia (2n).

  • Primary spermatocytes: spermatogonia grow and develop into diploid primary spermatocytes (2n).

  • Meiosis I: primary spermatocytes undergo meiosis I to yield two haploid secondary spermatocytes.

  • Meiosis II: secondary spermatocytes undergo meiosis II to form four spermatids (n).

  • Maturation: the four spermatids mature into haploid sperm.

  • Genetic diversity: each spermatid is genetically different from the original spermatogonium (2n).

  • Hormonal influence: under the influence of testosterone, diploid cells in the seminiferous tubules undergo meiosis to form haploid sperm cells.

Oogenesis (Process of Gametogenesis for Females)

  • Formation of ova occurs from germinal epithelial cells that form the outer layer of the ovaries (ovaries contain germinal epithelium).

  • Development timeline: oogenesis begins in the foetus, continues through puberty, and ends with menopause.

  • Primary follicles: oogonia (2n) are surrounded by a granular layer; together these form the primary follicle.

  • Primary oocytes: inside the primary follicle, oogonia grow to become primary oocytes (2n).

  • Meiosis I before birth: in puberty, the primary oocyte (2n) undergoes meiosis I to yield a larger haploid secondary oocyte and a smaller haploid polar body (degenerates).

  • Ovulation: the larger secondary oocyte (n) is released from the ovary during ovulation; meiosis II only occurs if sperm penetrates the ovum.

  • Meiosis II and fertilisation: upon sperm entry, the secondary oocyte undergoes meiosis II to form a larger haploid ovum and another polar body (degenerates).

  • Zygote formation: the haploid ovum fuses with the nucleus of the sperm cell to form a zygote.

Structure of the Ovum

  • Components illustrated structurally include: Corona radiata, Zona pellucida, Vitelline membrane, Nucleus, Germinal vesicle, Ooplasm.

  • Function: the ovum is the female gamete that, upon fertilisation, contributes half of the genetic material to the zygote.

The Menstrual Cycle: Overview

  • The menstrual cycle comprises two interrelated cycles:

    • Ovarian cycle: changes in the ovaries.

    • Uterine (endometrial) cycle: changes in the uterus.

Ovarian Cycle

  • General timeline: Day 1-14 — development of primary follicles into mature Graafian (vesicular) follicles; Day 14 — ovulation; Day 15 — development of the corpus luteum.

  • Important note: at Day 14, the released ovum is immature (has completed meiosis I only).

  • Hormonal control (Day 1-14): The hypophysis (pituitary) secretes Follicle Stimulating Hormone (FSH), stimulating primary follicle development into Graafian follicles. Graafian follicle secretes estrogen.

  • Ovulation (Day 14): Graafian follicle reaches the surface of the ovary and ruptures, releasing a haploid immature ovum (ovulation).

  • Post-ovulation (Day 15 onward): The remains of the Graafian follicle form the Corpus Luteum under the influence of Luteinising Hormone (LH); the corpus luteum secretes progesterone to maintain the endometrium for implantation. If fertilisation occurs, the corpus luteum continues to function and secrete progesterone until about the 12th week of gestation; if fertilisation does not occur, the corpus luteum degenerates.

Uterine Cycle (Endometrial Cycle)

  • The endometrium (uterine lining) is rich in glands and blood vessels; it thickens in preparation for implantation and nourishment of a fertilised egg.

  • Hormonal control: estrogen and progesterone drive thickening of the endometrium.

  • If fertilisation does not occur: the thickened and vascularised endometrium is shed (menstruation), along with unfertilised ovum, via the cervix and vagina.

  • After menstruation: the endometrium thickens again in preparation for a possible pregnancy.

Menstruation: Specifics and Timing

  • Menstruation lasts about 4-6 days.

  • The endometrium degenerates and is shed; the body immediately begins thickening the endometrium again in preparation for the next possible fertilisation cycle.

  • Menstruation calendar and timing cues are used in the study material to illustrate cycle length and hormone fluctuations.

Hormonal Regulation of the Menstrual Cycle

  • Follicle Stimulating Hormone (FSH): stimulates development of primary follicles in the ovary during the follicular phase.

  • Estrogen (oestragen/estrogen): secreted by the Graafian follicle; stimulates development and thickening of endometrial tissue; inhibits FSH release to ensure only one follicle matures at a time; rising estrogen toward day 14 helps trigger the LH surge.

  • Luteinising Hormone (LH): surge around day 14 triggers ovulation; in the presence of estrogen, LH promotes rupture of the Graafian follicle and formation of the corpus luteum.

  • Progesterone: produced by the corpus luteum; supports endometrial thickening and maintains the endometrium for implantation; inhibits the release of FSH and LH, preventing additional ovulations.

  • Feedback and cycles: the hormonal interplay ensures the timing of ovulation, preparation of the endometrium, and the regulation of the ovarian cycle.

Short Notes on Key Timelines and Phases

  • Gestation: birth occurs after approximately 40\text{ weeks}.

  • Puberty and maturation: growth to reproductive maturity occurs over 15-20 years after birth for humans.

  • Menstrual cycle length: typically around 28 days; phases and hormone fluctuations can be traced in cycle charts (e.g., basal body temperature changes and hormone levels across days 1, 7, 14, 21, 28).

  • Menstrual duration: typically 4-6 days per cycle.

Connections to Foundational Concepts and Real-World Relevance

  • Variation through sexual reproduction: the fusion of different gametes introduces genetic variation, which is essential for a population’s adaptability to changing environments.

  • Gametogenesis as a regulated developmental process: meiosis and maturation ensure that haploid gametes are produced, enabling fertilisation and the formation of a diploid zygote.

  • Endocrine control of reproduction: hormones such as FSH, LH, estrogen, and progesterone coordinate gamete development, ovulation, and preparation of the uterus for implantation.

  • Clinical relevance: understanding the menstrual cycle is foundational for fertility awareness, contraception, and diagnosing reproductive health issues; the menstrual cycle chart and hormonal roles provide a framework for interpreting common reproductive conditions.

Ethical, Philosophical, or Practical Implications (Not Explicitly Discussed in the Transcript)

  • The material focuses on biological mechanisms and does not explicitly address ethical considerations; students should consider the broader context of reproductive health, consent, and access to healthcare when applying this knowledge in real-world settings.

  • Practical implications include using knowledge of the menstrual cycle for family planning, understanding fertility windows, and recognizing signs of potential reproductive health problems that may require medical attention.

Quick Reference: Key Terms and Their Roles

  • Gametes: sperm (male) and ova (female).

  • Zygote: fertilised ovum formed by fusion of sperm and egg.

  • Graafian follicle: mature ovarian follicle that secretes estrogen and releases the ovum at ovulation.

  • Corpus luteum: remnant of the follicle after ovulation; secretes progesterone to maintain the endometrium.

  • Endometrium: uterine lining that thickens for implantation and is shed during menstruation if there is no fertilisation.

  • Acrosome: enzymatic cap on the sperm head that helps penetrate the ovum.

  • Meiosis I and II: stages of cell division that halve the chromosome number to produce haploid gametes.

  • Follicular phase: part of the ovarian cycle when follicles mature and estrogen rises.

  • Luteal phase: part of the ovarian cycle after ovulation when the corpus luteum secretes progesterone.

  • Menstruation: shedding of the endometrium if fertilisation does not occur.

  • Hormones: FSH, LH, estrogen, progesterone – regulators of the menstrual and ovarian cycles.