Dorsal versus ventral scales and the dorsoventral patterning of chick foot epidermis.

The dorsal and ventral scales of the chick foot can be distinguished morphologically and molecularly: the dorsal oblong overlapping scuta expressing both alpha and beta keratins, and the ventral roundish nonprotruding reticula expressing only alpha keratins. The question arises how En-1 and Lmx1, whose role in dorsoventral limb patterning has been well established, can affect skin morphogenesis, which occurs 8 to 12 days later. Forced expression of En-1 or of Lmx1 in the hindlimb have, respectively, as expected, a ventralizing or a dorsalizing effect on skin, leading to the formation of either reticula-type or scuta-type scales on both faces. In both cases, however, the scales are abnormal and even glabrous skin without any scales at all may form. The normal inductive interactions between dermis and epidermis are disturbed after En-1 or Lmx1 misexpression. Effectively, while Lmx1 endows the dermal precursors of the ventral region with scuta inducing ability, En-1 blocks the competence of the dorsal epidermis to build scuta.