In a morphogen gradient, different concentrations of a substance specify different cell fates. A morphogen can spread via: the extracellular matrix (ie, Dpp or BMP-4); vesicular transport; cytonemes (ﬁlopodia); or diffusion within a syncitial embryo (ie, bicoid or nanos).
In Drosophila the anterior and posterior systems establish morphogen gradients of bicoid and nanos protein in the syncytial Drosophila embryo. In Anterior system, morphogen gradient is Bicoid, a protein transcription factor (only possible because Drosophila embryo is a syncytium). In terminal and dorsal-ventral systems, morphogen gradient is (presumably) gradient of active ligand, then gradient of active transcription factor in nucleus (i.e., Dorsal protein).
The terminal and dorsal-ventral systems in Drosophila are the genetically and molecularly best characterized cell signaling pathways with gradient effects. For example, the anterior domain is established by a gradient of the transcription factor Bicoid (a protein); Bicoid binds to a DNA sequence in the hunchback promoter. Also, the posterior domain is established by a gradient of the protein Hunchback, which is controlled a gradient of the translation factor Nanos.
Dpp or BMP-4