The effect of the ectoderm on the dorsoventral pattern of epidermis, muscles and joints in the developing chick leg: a new model.

In order to investigate the effect of the ectoderm on the pattern of mesodermal components in the chick leg, the ectoderm of the early limb bud was dorsoventrally reversed with respect to the mesoderm. The dorsoventral pattern was assessed by examination of the muscles, skeleton and epidermal differentiation. The earlier the stage at which the recombinants were made, the more complete was the reversal of the dorsoventral pattern of the limb mesoderm to conform with the ectoderm, and distal regions showed more complete reversals than proximal ones. Analysis of dorsoventral pattern along the leg shows that the dorsalization starts from the dorsal ectoderm and affects mesodermal structures in its progress to the midline and then to the ventral ectoderm. There was no strict congruence between the dorsoventral patterns of cartilage, muscle and epidermis. The results suggest that a signal from the dorsal ectoderm modifies ventral mesoderm before ventral ectoderm affects dorsal mesoderm. These observations about the way in which the mesoderm responds to ectoderm reversal suggest a new model. The model is based on two distinct signals, both initiated within the progress zone: (1) a factor made by dorsal ectoderm which specifies a dorsal state as distinct from a ventral state, and (2) a symmetrical positional signal from the apical ectodermal ridge which patterns the tissues along the dorsoventral axis.