Practice question

Conditional specification asserts that cell fate emerges primarily from extrinsic cues provided by neighboring cells rather than inherited determinants. Surrounding tissues secrete morphogens, contact-dependent ligands like Delta and extracellular matrix components that activate specific signal transduction pathways including Smad, MAPK, Wnt and Notch in responsive cells, reprogramming transcriptional networks. Examples include vertebrate mesoderm induction where vegetal Nodal instructs marginal ectoderm, and neural crest lineage diversification shaped by BMP and Wnt encountered during migration. Because environment instructs identity, cells maintain flexibility, can be redirected by transplantation, and collectively regulate missing parts ensuring robust patterning.