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 m esodermal components in the chick leg, the ectoderm of the early limb bud was dorsoventrally reversed with respect to the mesoderm. The dors oventral pattern was assessed by examination of the muscles, skeleton and epidermal differentiation. The earlier the stage at which the reco mbinants were made, the more complete was the reversal of the dorsoven tral pattern of the limb mesoderm to conform with the ectoderm, and di stal regions showed more complete reversals than proximal ones. Analys is of dorsoventral pattern along the leg shows that the dorsalization starts from the dorsal ectoderm and affects mesodermal structures in i ts 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 dorsa l ectoderm modifies ventral mesoderm before ventral ectoderm affects d orsal 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 di s tinct from a ventral state, and (2) a symmetrical positional signal from the apical ectodermal ridge which patterns the tissues along the dorsoventral axis.