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