Embryology & Development

From Zygote to Gastrula

• Timeline: fertilization (cortical reaction blocks polyspermy) to cleavage (mitosis with no net growth, so the nuclear-to-cytoplasm ratio rises) to morula to blastocyst to implantation to gastrulation (week 3) to neurulation.
• The blastocyst splits into the inner cell mass (the embryo) and the trophoblast (placenta; secretes hCG and mediates implantation). Indeterminate cleavage keeps early cells totipotent, allowing identical twins.

Germ Layers and Neural Crest

• Ectoderm: nervous system, epidermis, lens, inner ear. Mesoderm: muscle, bone, blood, cardiovascular system, kidneys, gonads. Endoderm: GI and respiratory linings, liver, pancreas, thyroid.
• The adrenal gland is split: cortex from mesoderm, medulla from neural crest (ectoderm). Neural crest cells also form PNS ganglia, Schwann cells, melanocytes, and craniofacial cartilage.

Induction and Fetal Circulation

• The notochord induces the neural plate (Sonic hedgehog); removing the inducer eliminates the neural tube, and transplanting one duplicates it — determination precedes visible differentiation.
• Fetal shunts bypass unused organs: $\text{ductus venosus (liver)}, \quad \text{foramen ovale and ductus arteriosus (lungs)}$ all closing at birth. Neural tube defects are folate-preventable; teratogens hit hardest during organogenesis (weeks 3-8).