We wish to understand how limbs are positioned with respect to the dorso-ventral axis of the body in vertebrate embryos, and how different regions of limb bud ectoderm, i.e. dorsal ectoderm, apical ridge and ventral ectoderm, originate. Signals from dorsal and ventral ectoderm control dorso-ventral patterning while the apical ectodermal ridge (AER) controls bud outgrowth and patterning along the proximo-distal axis. We show, using cell-fate tracers, the existence of two distinct ectodermal compartments, dorsal versus ventral, in both presumptive limb and flank of early chick embryos. This organisation of limb ectoderm is the first direct evidence, in vertebrates, of compartments in non-neural ectoderm. Since the apical ridge appears to be confined to this compartment boundary, this positions the limb. The mesoderm, unlike the ectoderm, does not contain two separate dorsal and ventral cell lineages, suggesting that dorsal and ventral ectoderm compartments may be important to ensure appropriate control of mesodermal cell fate. Surprisingly, we also show that cells which form the apical ridge are initially scattered in a wide region of early ectoderm and that both dorsal and ventral ectoderm cells contribute to the apical ridge, intermingling to some extent within it.

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