Practice question

Morphogens such as retinoic acid act not simply as mitogens but as graded informational signals establishing concentration thresholds that differentially regulate gene expression programs. RA diffusing from proximal source forms gradient shaped by binding proteins CRABP, synthesis enzymes Raldh2, and degradative CYP26, activating retinoic acid response elements at distinct thresholds. High RA induces proximal Meis genes, low RA permits distal HoxA13, creating French flag pattern of transcriptional domains. This spatial transcriptional landscape confers positional identity, allowing blastema cells to know location and regenerate appropriate structures consistent with gradient interpretation model.