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Field notes · chapter 51

The Reproductive System

pp. 1133–1154 · Raven Part VII

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  1. Sexual reproduction: meiosis makes haploid gametes, fertilization restores diploidy → zygote, which grows by mitosis. Asexual reproduction: mitosis only, offspring genetically identical to the one parent. Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters
  2. Three asexual routes: fission (one cell splits into two organisms), budding (a piece of the parent separates and differentiates — cnidarians), parthenogenesis (offspring from unfertilized eggs). Watch Asexual and Sexual Reproduction on YouTube, from Amoeba Sisters Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  3. All parthenogenic parents are female by definition — only a female makes eggs. Honeybees: unfertilized egg → haploid drone (16 chromosomes); fertilized egg → diploid female (32). That is the haploid-diploid sex system, no sex chromosomes needed. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  4. Hermaphroditism = one individual with both testes and ovaries. Sequential hermaphroditism = changing sex over a lifetime: protogyny = female first (bluehead wrasse), protandry = male first. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  5. Sex determination is either genetic or environmental. Mammals: XY male, XX female, so the male is heterogametic. Birds: reversed — the female is heterogametic. Many fish and reptiles use temperature. Watch Sex Determination: More Complicated Than You Thought on YouTube, from TED-Ed
  6. Human male and female systems look identical for the first ~40 days. The SRY gene on the Y turns indifferent gonads into testes. No SRY → ovaries. Female is the default pathway.
  7. Evidence for SRY: Turner syndrome (XO) develops female, Klinefelter (XXY) develops male — so it is the presence of Y/SRY, not the number of X chromosomes. Transgenic XY mice with mutated SRY develop as female.
  8. Once testes form they secrete testosterone, which builds the penis and scrotum. Without testosterone the same embryonic tissue becomes clitoris and labia majora — homologous structures. Watch Reproductive System, Part 2 - Male Reproductive System: Crash Course Anatomy & Physiology #41 on YouTube, from CrashCourse
  9. Three post-fertilization strategies: oviparity (eggs laid, develop outside), ovoviviparity (eggs retained inside, fed by yolk, born live), viviparity (fed by maternal blood through a placenta). Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed
  10. Live birth requires internal fertilization. It evolved many times and, once evolved, is essentially never lost — a one-way trend. Watch The three different ways mammals give birth - Kate Slabosky on YouTube, from TED-Ed Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  11. External fertilization dominates in bony fish and amphibians (frogs use amplexus); internal fertilization is exclusive in reptiles, birds, and mammals — it arose once in the amniote ancestor. Watch Sexual & Asexual Reproduction: How Animals Do It: Crash Course Biology #47 on YouTube, from CrashCourse
  12. Male spermatogenesis: spermatogonium (diploid) → mitosis keeps one spermatogonium in reserve → primary spermatocyte (diploid, 46) → meiosis I → 2 secondary spermatocytes (haploid) → meiosis II → 4 spermatids → 4 spermatozoa. Adult males make 100–200 million sperm per day. Watch Male Reproductive System | Spermatogenesis on YouTube, from Ninja Nerd
  13. Sertoli cells nurse the sperm (stimulated by FSH, secrete inhibin). Leydig cells sit between the tubules and make testosterone (stimulated by LH). Memorize: FSH → Sertoli, LH → Leydig. Watch Male Reproductive System | Spermatogenesis on YouTube, from Ninja Nerd
  14. Male feedback loop: hypothalamic GnRH → anterior pituitary FSH + LH. Inhibin suppresses FSH only; testosterone suppresses LH (and GnRH). Remove the testes and FSH/LH shoot up. Watch Male Reproductive System | Spermatogenesis on YouTube, from Ninja Nerd
  15. Sperm path: seminiferous tubule → epididymis → vas deferens → ejaculatory duct → urethra. Sperm are non-motile until they sit in the epididymis at least 18 hours. Watch Reproductive System, Part 2 - Male Reproductive System: Crash Course Anatomy & Physiology #41 on YouTube, from CrashCourse
  16. Semen volume: seminal vesicles 60% (fructose-rich), prostate 30% (milky alkaline), bulbourethral/Cowper 10% (lubricating). Sperm are only about 1% of the volume. Watch Reproductive System, Part 2 - Male Reproductive System: Crash Course Anatomy & Physiology #41 on YouTube, from CrashCourse
  17. One ejaculation = 2–5 mL holding ~300 million sperm. Under 20 million sperm/mL is generally considered sterile. Erection: parasympathetic neurons release nitric oxide → arterioles dilate → veins compressed so blood cannot leave. Watch Reproductive System, Part 2 - Male Reproductive System: Crash Course Anatomy & Physiology #41 on YouTube, from CrashCourse
  18. Oogenesis: primary oocyte (diploid, arrested in prophase I since before birth) → meiosis I at each cycle → secondary oocyte + first polar body → arrested at metaphase II → meiosis II only if fertilized → ovum + second polar body. One primary oocyte → 1 ovum + up to 3 polar bodies. Watch Basics of egg development | Reproductive system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
  19. Ovaries hold about 1 million follicles at birth, each with a primary oocyte. Divisions are unequal on purpose — the oocyte keeps nearly all the cytoplasm. Watch Basics of egg development | Reproductive system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
  20. Ovarian cycle (28 days): follicular phase days 1–13 → ovulation ~day 14 → luteal phase days 15–28. Uterine cycle: menstruation days 0–5 → proliferative days 5–13 → secretory days 15–27. Watch The ovarian cycle | Reproductive system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine Watch Female Reproductive Cycle | Menstrual Cycle on YouTube, from Ninja Nerd
  21. Female hormone jobs: FSH grows the follicle and drives estradiol secretion by granulosa cells. LH triggers ovulation, converts the follicle into the corpus luteum, and drives estradiol + progesterone from it. Watch The ovarian cycle | Reproductive system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine Watch Female Reproductive System - Menstrual Cycle, Hormones and Regulation on YouTube, from Armando Hasudungan
  22. The trick everyone misses: rising estradiol in the follicular phase has a positive effect — it stimulates the midcycle LH surge. But high estradiol plus progesterone in the luteal phase is negative feedback, shutting FSH and LH down. Watch Female Reproductive System - Menstrual Cycle, Hormones and Regulation on YouTube, from Armando Hasudungan
  23. That luteal-phase shutdown is a natural contraceptive — it blocks any further follicle development that cycle. Birth control pills copy it exactly with progesterone analogs (± estrogen). Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed Watch Female Reproductive System - Menstrual Cycle, Hormones and Regulation on YouTube, from Armando Hasudungan
  24. Estradiol builds the endometrium (proliferative phase). Progesterone finishes it — vascular, glandular, glycogen-rich (secretory phase), ready for implantation. Watch Female Reproductive Cycle | Menstrual Cycle on YouTube, from Ninja Nerd
  25. No fertilization → LH falls → corpus luteum degenerates (luteal regression) → estradiol and progesterone crash → endometrium sheds = menstruation. Watch Female Reproductive Cycle | Menstrual Cycle on YouTube, from Ninja Nerd
  26. Fertilization → the embryo’s chorion secretes hCG, an LH-like hormone that keeps the corpus luteum alive, so hormone levels stay high and menstruation never comes. hCG comes from the embryo, not the mother — that is what every pregnancy test detects. Watch Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43 on YouTube, from CrashCourse
  27. Fertilization happens in the upper third of the Fallopian tube. The zygote cleaves on the way, takes about 3 days to reach the uterus, and 2–3 more days to implant as a blastocyst. An unfertilized oocyte dies within about 1 day. Watch Reproductive System, Part 3 - Sex & Fertilization: Crash Course Anatomy & Physiology #42 on YouTube, from CrashCourse Watch Reproductive System, Part 4 - Pregnancy & Development: Crash Course Anatomy & Physiology #43 on YouTube, from CrashCourse
  28. Estrous vs menstrual: estrous mammals reabsorb the lining (no bleeding) and are receptive only around ovulation ("in heat"); humans and apes bleed and can copulate at any point in the cycle. Estrous phases proestrus, estrus, metestrus, diestrus map to proliferative, midcycle, secretory, menstrual. Watch Reproductive System, Part 1 - Female Reproductive System: Crash Course Anatomy & Physiology #40 on YouTube, from CrashCourse
  29. Failure rates (pregnancies per 100 users per year): implant 0.05, IUD 1, injectable 6, oral contraceptive 9 typical (~1 perfect use), diaphragm 12, condom 18, spermicides 10–28, douching ~40. Sterilization is nearly 100% effective. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  30. Vasectomy cuts the vas deferens (male); tubal ligation cuts the Fallopian tubes (female). Infertility = no conception after 12 months of unprotected intercourse: about 40% male factor, 45% female factor, 15% unexplained. Watch How do contraceptives work? - NWHunter on YouTube, from TED-Ed
  31. Leading worldwide cause of infertility is pelvic inflammatory disease (PID) — bacterial infection scarring the Fallopian tubes. Clomiphene (Clomid) blocks estrogen receptors, killing the negative feedback so FSH and LH rise and ovulation is induced.