Anatomy & Physiology II – Human Development (Ch. 28)

Learning Objectives

• Trace the entire sequence from sperm deposition ➔ fertilization ➔ implantation ➔ embryogenesis ➔ fetal growth ➔ birth.
• Identify every anatomical obstacle sperm encounter on their way to the oocyte.
• Distinguish events that happen before vs. after implantation.
• List the three germ layers and give representative derivatives of each.
• Explain all pregnancy-related endocrine changes (ovarian, placental, pituitary, parathyroid, adrenal).

Fertilization

• Definition
  • Fusion of haploid nuclei of sperm (1n) and secondary oocyte (1n) to form a diploid zygote (2n).
• Sperm Journey & Obstacles
  • Ejaculated into vagina; millions die in acidic pH.
  • Cervical mucus blocks many; only few thousand enter cervix ➔ uterus ➔ uterine tubes.
  • Survival window inside female: 3{-}5 days.
• Oocyte Availability
  • Secondary oocyte swept into uterine tube by ciliary action.
  • Travel to uterus takes \approx 72\ \text{h}; oocyte viable only \approx 24\ \text{h}.
• Capacitation ("priming")
  • Series of biochemical changes that destabilize the sperm plasma membrane & acrosome.
  • Enables motility pattern & acrosomal reaction.

Acrosomal Reaction & Prevention of Polyspermy

• Penetration sequence
  1. 100s of capacitated sperm tunnel through corona radiata.
  2. Binding to zona pellucida triggers release of acrosomal enzymes (hyaluronidase, acrosin) → local digestion of glycoproteins.
  3. FIRST sperm contacting oocyte plasma membrane fuses and releases nucleus.
• Blocks to polyspermy
  • Fast block (\textit{electrical})
  • Instant \mathrm{Na^+} influx depolarizes oolemma, preventing additional fusion.
  • Slow block (cortical reaction)
  • \mathrm{Ca^{2+}} wave triggers exocytosis of cortical granules →
    • Zonal-inhibiting proteins destroy remaining sperm receptors.
    • Mucopolysaccharides form physical barrier.

Completion of Meiosis & Zygote Formation

• Secondary oocyte (arrested at metaphase II) completes meiosis upon sperm entry →
  • Produces true ovum + second polar body.
• Sperm & ovum pronuclei merge → single-cell, totipotent zygote.
• Twinning
  • Monozygotic (identical): single zygote splits during totipotent stages.
  • Dizygotic (fraternal): two separate oocytes fertilized by two sperm.

Early Cleavage & Totipotency

• Mitoses without growth (cleavage) while traveling toward uterus.
  • 2-cell → 4-cell → 8-cell → Morula (≈16 cells).
• Cells remain totipotent through morula; differentiation not yet specified.

Blastocyst Structure (Day 4–6)

• Hollow sphere ≈70–100 cells.
• Components
  • Trophoblast (a.k.a. trophectoderm) → future extra-embryonic tissues & placenta.
  • Inner cell mass (embryoblast) → embryo proper.
  • Blastocoel: fluid-filled cavity.

Gastrulation & Germ Layers (≈Day 10–16)

• Inner cell mass forms embryonic disc → primitive streak.
• Germ layers
  • Ectoderm (“outside”)
  • Epidermis, hair, nails, nervous system (neural plate/brain, spinal cord).
  • Mesoderm (“middle”)
  • Skeletal muscle, bone, heart, blood vessels, blood cells, connective tissues.
  • Endoderm (“inside”)
  • Lining of GI tract, liver, pancreas, lungs.
• Memory aid:
  • Ectoderm = exterior (skin, brain)
  • Mesoderm = middle (everything else)
  • Endoderm = internal tubes (gut & lungs)

Implantation (Day 6–8)

• Blastocyst “hatches” by shedding zona pellucida.
• Trophoblast secretes proteolytic enzymes → digests endometrial epithelium, embeds in posterior uterine wall (most common site).
• Endometrium overgrows, surrounding conceptus.
• Signals generated = positive pregnancy test.

hCG (Human Chorionic Gonadotropin)

• Secreted by trophoblast post-implantation.
• Maintains corpus luteum → sustained secretion of progesterone & estrogen → prevents menses.
• Detected by urine pregnancy kits.

Abnormal Implantation & Assisted Reproduction

• Ectopic pregnancy
  • Implantation outside uterus (e.g., uterine tube ➔ tubal pregnancy, abdominal cavity).
  • Placenta previa: placenta covers cervix → hemorrhage risk.
• In Vitro Fertilization (IVF)
  1. Retrieve oocytes post-ovulation or aspirate follicles.
  2. Collect & concentrate sperm (remove seminal fluid).
  3. Combine gametes in petri dish → fertilization.
  4. Transfer resultant zygote/early embryo into uterine endometrium.

Embryogenesis Timeline

• Week 3
  • Potency switch: totipotent → multipotent.
  • Primitive streak, neural groove, pharyngeal arches appear.
  • Somite formation (days