Human Reproduction & Development – Lecture Overview

Ovarian & Uterine Cycle (Quick Recall)

• Know the three ovarian phases (follicular → ovulation → luteal) and the three uterine phases (menstrual → proliferative → secretory).
• High \uparrow estradiol + progesterone during the luteal/secretory phase = thick, glycogen-rich endometrium ready for implantation.
• Exam tip: you will not be asked to redraw the full hormone chart, but you must connect each phase to its hormone profile & endometrial status.

Fertilization

• Timing: occurs \approx 24\,\text{h} around ovulation if sperm arrive in the correct uterine tube.
• Sperm navigation: guided by chemotactic molecule "allurin" released from the oocyte/oviduct; tells sperm which tube to enter.
• Anecdotal sex-selection tale: “male (Y-carrying) sperm” = sprinters; “female (X-carrying) sperm” = marathoners. Sex closer to ovulation may slightly favor male offspring; later intercourse may favor female—unverified but conceptually ties into sperm endurance vs. speed.

Attrition of Sperm En Route

Ovum Barriers & Acrosomal Reaction

• Oocyte is wrapped by:
– Zona pellucida (white glycoprotein shell).
– Corona radiata (pinkish follicular cells).
• Sperm head houses acrosomal cap (digestive enzymes). Enzymes dissolve corona + zona so the head can fuse with the oolemma.
• Once one sperm succeeds, the oocyte membrane depolarizes ⇒ "fast block" to polyspermy. No other sperm can fuse.

Genetic & Cytoplasmic Contribution

• Only the sperm nucleus enters; cytoplasm & mitochondria come solely from the egg ⇒ mitochondrial DNA is maternal. Example: tracing ancestry via mtDNA.
• The sperm tail and midpiece are essentially a delivery system.

From Zygote → Blastocyst → Implantation

• Fertilized egg (zygote) travels down tube, becomes a blastocyst, then embeds in endometrium.
• Blastocyst secretes human chorionic gonadotropin (hCG)—the hormone detected by pregnancy tests.
• hCG rescues the corpus luteum ⇒ maintains high estrogen & progesterone until placenta takes over.

Placenta: Architecture & Function

• Think “interlaced fingers”:
– Fetal side = chorion frondosum (villus projections).
– Maternal side = decidua basalis (modified endometrium).
• Fetal & maternal vessels do not fuse; they approach closely within placental tissue so nutrients/gases diffuse but cells (incl. immune cells) do not.
• Acts as selective filter + immune-privileged zone: prevents maternal T-cells from recognizing fetal antigens; allows mismatched blood types to coexist.

Embryonic Development Timeline

• Day 6: Blastocyst contacts endometrium.
• Week 2: Neural plate forms; crest cells begin to converge.
• Week 4: Limb buds visible; heart tube beating.
• Week 8: All major organs present; obvious human form.
• Weeks 12–20: Organogenesis mostly done; emphasis shifts to growth & fat deposition.

Hormonal Cascade Leading to Labor (Parturition)

1. Fetal hypothalamus releases corticotropin-releasing hormone (CRH).

2. Fetal pituitary → adrenocorticotropic hormone (ACTH).

3. Fetal adrenal cortex ↑ cortisol + DHEAS.

4. Signals maternal uterus/placenta: “baby fully baked.”

5. Maternal posterior pituitary secretes oxytocin.

6. Positive-feedback loop: uterine contractions ⇒ cervical stretch ⇒ more oxytocin ⇒ stronger contractions until delivery.

Lactation Physiology

• Pregnancy: Estrogen & progesterone prime mammary tissue.
• Post-delivery: E & P levels drop ⇒ prolactin rises (milk synthesis).
• Infant suckling: sensory input → hypothalamus:
– Posterior pituitary releases oxytocin → myoepithelial cells contract → milk ejection ("let-down" reflex).
– Anterior pituitary releases more prolactin → replenishes milk.
• Supply-Demand Rule: Milk left in ducts exerts negative feedback; empty breast triggers higher production next feed.

Sexually Transmitted Diseases (STDs)

• Rising incidence of gonorrhea, syphilis, chlamydia—especially in the U.S. South.
• Chlamydia: >90\% asymptomatic; single unprotected contact is enough for transmission.
• Importance: can progress to pelvic inflammatory disease (PID) ⇉ scarring, infertility.

Selected Reproductive Disorders

• Cryptorchidism – undescended testis; intra-abdominal heat impairs spermatogenesis unless testes descend or are surgically placed in scrotum.
• Endometriosis – endometrial tissue migrates (often via open fimbrial end) & implants on peritoneal surfaces. Symptoms: pain, abnormal bleeding, infertility. Tx: laparoscopic removal, hormonal suppression.
• Prostatitis – bacterial/idiopathic inflammation; pain, discharge; antibiotics if bacterial.
• PID – infection of uterine tubes; fever, pelvic pain, ↑ WBC; \approx8.5\times10^{5} U.S. cases/yr; antibiotics.

Male Erectile Dysfunction (Impotence)

• Causes: neural, vascular, psychogenic, poor circulation.
• Physiology: Parasympathetic input → NO → ↑ cGMP (from GTP) → smooth-muscle relaxation → blood engorges corpora cavernosa.
• Viagra (sildenafil) inhibits cGMP-specific phosphodiesterase ⇒ cGMP stays high ⇒ sustained erection.
\text{GTP} \xrightarrow[\text{NO}]{\text{guanylate cyclase}} c\text{GMP} \xrightarrow[\text{PDE-5}]{\text{\small inhibited by sildenafil}} 5’\text{GMP}

Reproductive Cancers

• Breast Cancer: Leading female cancer mortality ages 35–45. \sim2\times10^{5} new U.S. cases; \sim4\times10^{4} deaths. Early detection (self-exam, mammography) critical. No proven link between oral contraceptives & incidence.
• Prostate Cancer: \sim1.9\times10^{5} new cases; \sim3\times10^{4} deaths annually. Screening: PSA blood test + digital rectal exam. Symptoms: weak urine stream, incomplete emptying.
• Testicular Cancer: Most common male cancer 15–35 yrs; \sim3/100{,}000 incidence; 95 % survival with orchiectomy + chemo.
• Ovarian Cancer: >50\% lethality (≈22{,}000 cases, 14{,}000 deaths). Vague symptoms (bloating, fullness, pelvic pain) → late diagnosis. Tx: surgery, chemo, radiation.

Infertility (10–15 % of Couples)

• Male factors: low sperm count, abnormal morphology, cryptorchidism.
• Female factors: impaired oocyte maturation, blocked tubes (endometriosis/PID), hostile uterine environment, immunological rejection of embryo.
• Modern interventions: hormonal induction, IVF, ICSI, surgical correction of tubal/uterine defects.

Connections & Real-World Relevance

• Integrates endocrine (HPG axis), immune privilege, and embryology from earlier chapters.
• Ethical layer: sex selection myths vs. evidence, contraception safety, STI public-health trends, early-diagnosis advocacy.
• Practical concerns: timing intercourse for conception, recognizing silent STDs, routine cancer screenings, understanding pharmacology (sildenafil).
• Philosophical angle: maternal–fetal symbiosis showcases immune tolerance; placenta as a blueprint for transplant medicine.

Study Tips

1. Pair hormone names with source & target effect (e.g., hCG → corpus luteum).

2. Re-draw the sperm attrition table until you can recall % values.

3. For cancers: memorize incidence vs mortality patterns—ovarian stands out for high lethality.

4. Practice explaining oxytocin’s dual role (labor + lactation) to a friend to cement positive-feedback logic.