Fig. 6

Summary model for the molecular control of median fin formation. Epidermis specification by Tfap2 regulates the HGT-gene CesA and unidentified genes (Gene x) whose products are responsible for synthesis and secretion of cellulose and other components of the tunic in all epidermal cells. Epidermis patterning initially launched by Fgf9/16/20 (dorsal midline induction) and Admp (ventral midline induction) activates two separate networks: one that regulates peripheral sensory neuron specification, and one that regulates fin outgrowth. Although Msx and Klf1/2/4/17 are essential for fin formation, their downstream effector(s) are not known (Gene z). Medio-lateral fate is most likely regulated by midline cells (for example, in DMH1-treated embryos, midline fate is lost and Gh6 fails to get expressed in the ventral medio-lateral cells, see Additional file 14: Fig. S8). Medio-lateral cells express the HGT-gene Gh6 and possibly other genes (Gene y) that inhibit fin outgrowth