Practice question

Retinoic acid concentration gradient along embryo axis established by posterior Raldh2 synthesis and anterior Cyp26 degradation acts as morphogen instructing anterior-posterior identity via direct regulation of Hox clusters. Retinoic acid receptors heterodimerize with retinoid X receptor binding retinoic acid response elements upstream of Hox genes determining rhombomere boundaries in hindbrain and vertebral identities. Deficiency anteriorizes, excess posteriorizes development. While RA also influences limb patterning, its primary embryonic role is specifying anterior-posterior axis positioning of tissues through Hox gene collinear activation establishing body plan organization and segmental fate.