Prenatal Development

Fertilization & First Fortnight - Tiny Beginnings

• Fertilization: Ampulla of fallopian tube.
  • 📌 Penetrate Corona, Attack Zona, Fuse Membrane.
  • Sperm capacitation & acrosome reaction (enzymes: hyaluronidase, acrosin).
  • Cortical reaction prevents polyspermy. Oocyte completes meiosis II. Zygote formed.
• Cleavage & Blastocyst Formation:
  • Morula: 16-cell stage (day 3-4).
  • Blastocyst (day 4-5): Trophoblast (→ placenta) & Embryoblast (→ embryo).
  • Hatching from zona pellucida (day 5-6).
• Implantation: Starts day 6-7, completes day 10-12.
  • Trophoblast → Cytotrophoblast & Syncytiotrophoblast (produces hCG).
• Week 2 - Bilaminar Disc:
  • Embryoblast → Epiblast & Hypoblast.
  • Cavities: Amniotic & Yolk Sac.

⭐ Rule of 2s for 2nd week: 2 germ layers (epiblast, hypoblast), 2 cavities (amniotic, yolk sac), 2 trophoblast layers (cytotrophoblast, syncytiotrophoblast).

Germ Layers & Organogenesis - Body's Blueprint

Gastrulation (Week 3) forms three primary germ layers, the foundation for all tissues and organs. Organogenesis, organ development, occurs mainly Weeks 3-8; a critical period highly sensitive to teratogens.

• Ectoderm (Outer layer):
  • Nervous system (brain, spinal cord), epidermis (skin, hair, nails), sense organs.
  • Neural crest cells: Peripheral Nervous System (PNS), melanocytes, adrenal medulla.
  • 📌 Mnemonic: "Ecto = Attracto & Smarto" (external features & nervous system).
• Mesoderm (Middle layer):
  • Muscles (skeletal, smooth, cardiac), connective tissue (bone, cartilage, blood), cardiovascular system, urogenital system, notochord.
  • 📌 Mnemonic: "Meso = Muscle, Movement, & More" (bones, blood).
• Endoderm (Inner layer):
  • Epithelial linings of Gastrointestinal (GI) & respiratory tracts, bladder, urethra.
  • Parenchyma of liver, pancreas, thyroid, parathyroids.
  • 📌 Mnemonic: "Endo = Enternal linings & Endocrine glands."

⭐ Neural tube defects (e.g., spina bifida, anencephaly) arise from failure of neural tube closure (derived from ectoderm) by Week 4; folic acid supplementation significantly reduces risk.

Fetal Period & Placenta - Life Support System

• Fetal Period (Week 9 - Birth): Rapid body growth; tissue/organ differentiation.
  • Viability: ~24 weeks.
  • Milestones: Quickening (16-20 wks); lanugo, vernix caseosa; surfactant (~24 wks, adequate ~34 wks).
• Placenta: Life Support
  • Structure: Fetal (Chorion frondosum) & Maternal (Decidua basalis).
  • Functions: 📌 N-HERB+P: Nutrition, Hormone (hCG, hPL, P, E), Excretion, Respiration ($O_2$/$CO_2$ exchange), Barrier, Protection.

  ⭐ hCG is produced by syncytiotrophoblast; basis of pregnancy tests.

  • Circulation: 2 Umbilical Arteries (deoxygenated blood from fetus); 1 Umbilical Vein (oxygenated blood to fetus).
• Amniotic Fluid: Fetal urine/lung secretions. Cushions, allows movement, lung dev. Volume ~1L at term.
• Umbilical Cord: 2 arteries, 1 vein, Wharton's jelly. Length ~55 cm.

Teratology & Anomalies - Developmental Detours

• Teratogen: Agent causing congenital anomalies (birth defects).
• Key Principles of Teratology:
  • Susceptibility: Genotype & developmental stage dependent.
  • Critical Period: Organogenesis (Weeks 3-8) is most sensitive.
  • Dose-response relationship exists.
  • Specific mechanisms lead to defined patterns of anomalies.
• Types of Anomalies:
  • Malformation: Primary structural defect (e.g., neural tube defect).
  • Disruption: Secondary breakdown of normally formed tissue (e.g., amniotic band syndrome).
  • Deformation: Abnormal form/position due to mechanical forces (e.g., clubfoot).
  • Dysplasia: Abnormal cellular organization within a tissue (e.g., achondroplasia).
• Common Teratogens: 📌 TORCH infections (Toxoplasmosis, Other, Rubella, CMV, Herpes), alcohol, thalidomide, valproic acid, isotretinoin.

⭐ The most sensitive period for teratogenic effects, leading to major congenital anomalies, is during organogenesis, from weeks 3 to 8 of embryonic development.

High‑Yield Points - ⚡ Biggest Takeaways

• Fertilization in ampulla; implantation by day 10-12, syncytiotrophoblast invades.
• Gastrulation (week 3): primitive streak initiates formation of three germ layers.
• Neurulation (weeks 3-4): neural tube formation; folic acid prevents defects.
• Organogenesis (weeks 3-8): peak teratogen sensitivity; major organs form.
• Notochord: induces neural plate, defines primitive embryonic axis.
• Pharyngeal arches: key for head and neck structures; know their derivatives.
• Fetal period (week 9 to birth): primarily growth and functional maturation.