Developmental Biology and Embryology
Core concepts
Follow development from gametogenesis through fertilization, cleavage, gastrulation, neurulation, and organogenesis. Know cleavage patterns (holoblastic versus meroblastic, radial, spiral, rotational), blastula and blastocoel formation, and gastrulation movements (invagination, involution, ingression, delamination, epiboly). Understand the fate of the three germ layers: ectoderm gives epidermis and nervous system, mesoderm gives muscle, skeleton, blood, and kidneys, endoderm gives the gut lining and associated organs. Be familiar with the organizer concept (Spemann-Mangold organizer), induction, competence, determination versus differentiation, and the molecular players (BMP, Wnt, Hox genes). Know extraembryonic membranes in amniotes (amnion, chorion, allantois, yolk sac) and placental types.
Common traps
Do not confuse determination (commitment) with differentiation (specialization). Cleavage does not increase embryo mass. The notochord induces the neural tube, but the neural tube itself forms from ectoderm. In mammals, cleavage is rotational and holoblastic; in birds, meroblastic discoidal. Germ cells arise from the yolk sac wall and migrate to the gonads.
Worked example
Rule of thumb: ask which germ layer supplies the structure. The neural tube and epidermis come from ectoderm; the notochord and somites from mesoderm; the gut tube lining from endoderm. A question about the origin of the notochord is answered by mesoderm, not ectoderm.