Avian scale development: IX. Scale formation by scaleless ( [formula omitted]) epidermis under the influence of normal scale dermis

Unlike normal scutate scales whose outer and inner epidermal surfaces elaborate β (β-keratins) and α (α-keratins) strata, respectively, the scaleless mutant's anterior metatarsal epidermis remains flat and elaborates only an α stratum. Reciprocal epidermal-dermal recombinations of presumptive scale tissues from normal and mutant embryos have demonstrated that the scaleless defect is expressed only by the epidermis. In fact, the scaleless anterior metatarsal epidermis is unable to undergo placode formation. More recently, it has been determined that the absence of epidermal placode morphogenesis into a definitive scale ridge actually results in the establishment of a scale dermis which is incapable of inducing the outer and inner epidermal surfaces of scutate scales. Can the initial genetic defect in the scaleless anterior metatarsal epidermis be overcome by replacing the defective dermis with a normal scutate scale dermis, i.e., a dermis with scale ridges already present? Or, are the genes involved in the production of a β stratum regulated by events directly associated with morphogenesis of the epidermal placode? In the present study, we combined scaleless anterior metatarsal epidermis (stages 36 to 42) with normal scutate scale dermis (stage 40, 41, or 42) old enough to have acquired its scutate scale-inducing ability. After 7 days of growth as chorioallantoic membrane grafts, we observed grossly and histologically, typical scutate scales in these recombinant grafts. Electron microscopic and electrophoretic analyses have verified that these recombinant scales are true scutate scales. The scaleless mutation, known to be expressed initially by the anterior metatarsal epidermis, can be overcome by exposing this epidermis to appropriate inductive cues, i.e., cues that direct the differentiation of the outer and inner epidermal surfaces of the scutate scales and the production of specific structural proteins. We have determined that the time between stages 38 and 39 is the critical period during which the normal scutate scale dermis acquires these inductive abilities.