Control of dorso-ventral patterns of differentiation in the developing chick wing

The influence of limb ectoderm on dorso-ventral pattern in the chick wing was studied by recombining stage 14-21 limb mesoderm with the same stage ectoderm in dorso-ventrally reversed orientation. Fully-developed wings obtained from stage 15-21 donor embryos have at their distal half dorso-ventral polarity conforming to the reversed ectoderm. The ectodermal effect is generally observed as a bidorsal feather pattern at the autopod and an almost complete dorso-ventral reversal of muscle and skeletal patterns. At the prelimb stages (younger than stage 15), however, control of dorso-ventral patterns in the wing resides in the mesoderm. Recombination experiments at stage 14, with dorso-ventrally reoriented ectoderm, result in wings with mesodermal dorso-ventral polarity. At the onset of limb development at stage 15, some of the control shifts to the ectoderm. By stage 16, the ectoderm has acquired dorso-ventral information and can impose this polarity on the patterns of mesodermal differentiation in the distal regions of the wing.

The dorso-ventral information in the ectoderm comes from the mesoderm which transfers this information to the overlying ectoderm between stage 14-16. The initial dorso-ventral information in the ectoderm is not stable and can be "reprogrammed" by stage 14 mesoderm. Subsequently, there is a gradual stabilization of the ectodermal information. At the same time the mesoderm loses its capacity to "reprogram" the dorso-ventral information in the ectoderm.

Dorso-ventral differentials in presumptive limb mesoderm were eliminated by superimpostion of two limb fields with opposing dorso-ventral axes. This interferred with formation of the ectodermal ridge and subsequent limb outgrowth. The dorso-ventral differences, however, are not necessary for ridge induction since dorsal and ventral limb mesoderms can promote formation of the ridge and fully-developed wings after transplantation to the dorsal limb or flank regions. The dorso-ventral differentials in the limb field are expressed as limb-promoting activity in the dorsal region and an inhibitory influence on limb initiation coming from the ventral limb region. The boundary between these two opposing influences may be significant in determining the site of ectodermal ridge formation.