In vivo experimental induction of interdigital tissue chondrogenesis in the avian limb bud results in the formation of extradigits. Effects of local microinjection of staurosporine, zinc chloride and growth factors.

In previous studies we had found that at late stages of development, when the early patterning control mechanism have ceased to act, the chick limb bud is able to form fully differentiated extradigits by subjecting the interdigital spaces to ectoderm removal. In this study we attempted to mimic this phenomenon by using local microinjections of substances which presumably have a biological action on the interdigital mesenchyme. Microinjection of staurosporine results in the formation of fully differentiated extradigits. The action of this drug appears to be due to the induction of chondrogenesis after the inhibition of the protein kinase C. Zinc chloride administration also causes ectopic chondrogenesis but it seems to act by arresting the interdigital cell death program through endonuclease inhibition. A clear differentiation of the zinc-induced cartilages into extradigits was no detected. This can be explained by the accompanying damage caused by zinc in the growing limb mesenchyme as deduced by the high incidence of hypophalangy in the normal digits. Both TGF beta 1 and TGF beta 2 have a weak effect as inducers of interdigital chondrogenesis; presumably they act by inducing chondrogenetic differentiation. Neither FGF nor EGF has any effect when administered by local microinjection. These results show that ectopic interdigital chondrogenesis induced by drug administration results in the differentiation of extradigits. This suggests that once a cartilage is formed in the autopodium it triggers a new signalling stage which leads to the morphogenesis of a digit. This morphogenetic process involves the patterning of skeleton, joints and tendons. In accordance with these observations, it can be proposed that early patterning of the limb results in the establishment of an autopodium with a defined but still plastic skeletal distribution pattern, while morphogenesis of each autopodial element would take place at a second stage by the activation of new signalling processes.