Modification of the phalangeal pattern of the digits in the chick embryo leg bud by local microinjection of RA, staurosporin and TGF beta's.

Many experimental studies show that in the avian chick limb the digits are specified at early stages of development by characteristic concentrations within the limb mesoderm of a still unidentified morphogen diffusing from the posterior margin of the bud, linked with a specific pattern of homeobox gene expression. In all these studies, digits are distinguished by their size, morphology and phalangeal pattern rather than by their position within the autopodium. In this work we report the induction of digits that have otherwise normal morphology but lack an interphalangeal joint. This suggests that the patterning of these joints is not necessarily linked to the control of the outgrowth of the digital rays. Missing interphalangeal joints were induced by microinjection into the third interdigital space of the leg bud of stage 28 to 31 chick embryos of retinoic Acid (RA), staurosporine and TGF beta 1 and beta 2, but not by microinjection of FGF or EGF. Our results also suggest that the pattern of insertion of the long tendons and the formation of the flexor cutaneous pad at the plantar surface of the digits are both linked to the establishment of the interphalangeal joints.