8.3 Reproduction: Mitosis/Meiosis, Gametogenesis, Pregnancy & Fetal Physiology

Reproduction: Mitosis, Meiosis, Gametogenesis, Pregnancy & Fetal Physiology

Quick Answer: Mitosis yields two diploid somatic cells; meiosis yields four haploid gametes and is the source of genetic diversity through crossing over and independent assortment. Spermatogenesis is continuous; oogenesis is punctuated by long arrests. The menstrual cycle is driven by FSH, LH, estrogen, and progesterone, and pregnancy is maintained by hCG then placental hormones. Fetal circulation bypasses the lungs through the foramen ovale and ductus arteriosus.

Mitosis vs Meiosis

In mitosis, DNA replicates in S phase, then sister chromatids separate in anaphase, producing two genetically identical diploid cells. In meiosis, homologous chromosomes pair in prophase I and exchange segments via crossing over (chiasmata). In metaphase I, homologous pairs align at the equator; their random orientation produces ~2²³ (over 8 million) combinations by independent assortment. Meiosis I separates homologues; meiosis II separates sister chromatids.

Spermatogenesis

Begins at puberty in the seminiferous tubules. Spermatogonial stem cells (diploid) divide mitotically; one daughter remains a stem cell, the other becomes a primary spermatocyte. It undergoes meiosis I → two secondary spermatocytes (haploid) → meiosis II → four spermatids. Spermiogenesis differentiates spermatids into mature sperm (acrosome, flagellum, midpiece mitochondria). Sertoli cells nurse the process; Leydig cells produce testosterone under LH drive. The full cycle takes ~74 days; ~300 million sperm are produced daily.

Oogenesis

Begins in fetal life. Oogonia divide mitotically and enter meiosis I, arresting in diplotene of prophase I by birth — so a female is born with all the primary oocytes she will ever have (~1–2 million, declining to ~300–400,000 by puberty, ~400 ovulated over reproductive life). At puberty, FSH rescues a cohort each cycle; one dominant follicle completes meiosis I just before ovulation, producing a secondary oocyte and the first polar body. Meiosis II arrests in metaphase II and completes only if fertilized; otherwise the secondary oocyte degenerates.

The Menstrual Cycle

Estrogen exerts negative feedback on FSH/LH at low levels and positive feedback at high sustained levels, triggering the LH surge and ovulation — the single most tested endocrine switch in reproduction.

Pregnancy and Lactation

Fertilization typically occurs in the ampulla of the fallopian tube. The zygote cleaves to a morula, becomes a blastocyst, and implants in the uterine endometrium ~6 days after ovulation.

Human chorionic gonadotropin (hCG) is secreted by the syncytiotrophoblast and rescues the corpus luteum, which continues producing progesterone until the placenta takes over at ~8–12 weeks. hCG is the basis of pregnancy tests and peaks around 8–10 weeks. The placenta subsequently secretes progesterone (maintains myometrial quiescence), estrogen (stimulates ductal development and the feto-placental unit), human placental lactogen (hPL) (maternal insulin resistance, shunting glucose to the fetus), and relaxin (loosens pelvic ligaments and inhibits uterine contraction).

Lactation: high estrogen/progesterone during pregnancy prime the mammary glands but inhibit prolactin action; after delivery, the abrupt fall in steroid hormones permits prolactin to drive milk synthesis, and oxytocin from the posterior pituitary drives milk ejection via the let-down reflex.

Fetal Circulation

The fetal lungs are not a site of gas exchange; the placenta is. Fetal hemoglobin (HbF) has higher O₂ affinity, loading O₂ efficiently at the low PO₂ of placental blood. Three shunts bypass the non-functional lungs and liver:

• Ductus venosus — shunts umbilical vein blood past the liver to the IVC.
• Foramen ovale — directs oxygenated IVC blood from the right atrium to the left atrium, bypassing the right ventricle/pulmonary circuit.
• Ductus arteriosus — connects the pulmonary artery to the aorta, bypassing the lungs; high pulmonary vascular resistance in the fetus keeps right-heart pressure high and the ductus open.

At birth, with the first breaths pulmonary vascular resistance falls, left atrial pressure rises, and the foramen ovale functionally closes (anatomical closure via the septum primum follows). Increased oxygenation and falling prostaglandins cause the ductus arteriosus to constrict (prostaglandin E2 keeps it open in utero; indomethacin can close a patent ductus in a premature infant). The ductus venosus and umbilical vessels obliterate after cord clamping. Failure of these closures produces a patent foramen ovale or patent ductus arteriosus — left-to-right shunts that may require intervention.