Practice question

Morphogen activity typically requires localized source and short to medium range dissemination to generate gradient positional fields, involving paracrine diffusion through extracellular matrix, juxtacrine contact-dependent relay like Notch for boundary sharpening, and autocrine amplification sustaining competence and feedback. Endocrine mode involves secretion into bloodstream for systemic distribution to distant organs, resulting in uniform circulating levels that cannot maintain steep spatial gradient or precise threshold boundaries within a single tissue field. Hormones like insulin, estrogen and thyroxine act systemically regulating metabolism, not providing fine spatial coordinates. Hence endocrine signaling is generally not considered morphogenetic gradient mechanism for patterning.