Homology of patterns, homology of mechanisms

After a reminder of the important notions developed in the third lesson, concerning in particular the development of the zebrafish pectoral fin and the two types of skeleton that make it up (endo- and exo-), the use of marker genes appears necessary to try to deepen possible homologies with the tetrapod forelimb. As markers and determinants of the proximo-distal axis, Hox genes are discussed and their expression dynamics during chiridial limb development are examined in detail. The existence of these genes in fish is reviewed, and their expression in the fin bud at different stages is described.