Formation of cutaneous appendages in dermo-epidermal recombinations between reptiles, birds and mammals.

1. Previous experiments on dermo-epidermal recombinations between birds and mammals have shown that the class-specific quality of the cutaneous appendages depends on intrinsic properties of the epidermis but that several steps of their morphogenesis are controlled by the dermis. This morphogenetic interplay has been tested further in new experiments with reptilian skin. 2. Reconstituted homo- and heterospecific skin explants, involving epidermis and dermis of lizard, chick and mouse, were cultured for 8 days on the chorioallantoic membrane of the chick embryo. 3. Homospecific recombinations of dorsal, caudal or ventral lizard epidermis and dorsal lizard dermis gave rise to small dorsal-type scales. Recombinants of dorsal, caudal or ventral lizard epidermis and ventral lizard dermis gave rise to large ventral-type scales. 4. Heterospecific recombinations of dorsal, caudal or ventral lizard epidermis and chick dermis from the glabrous comb region did not differentiate any scale structures. 5. Heterospecific recombinations of dorsal or caudal lizard epidermis and tarsometatarsal chick dermis formed large chick-type scales. 6. Heterospecific recombinations of dorsal, caudal or ventral lizard epidermis and chick feather-forming, or mouse hair-forming or whisker-forming dermis gave rise to tubercular scale primordia. The diameter and distribution of these primordia were in conformity with the feather, pelage hair and vibrissal patterns respectively. 7. Heterospecific association of lizard dermis and chick or mouse epidermis led to the formation of few epidermal placode-like pegs; those differentiated by the mouse epidermis were interpreted as hair bud structures. 8. The differentiation of reptilian scales is the result of dermo-epidermal interactions. Reptilian epidermis, when confronted with either reptilian, avian, or mammalian dermis, always responds to the dermal messages by forming scale buds. For final scale morphogenesis, however, reptilian dermis or avian scale-forming dermis is required. Reptilian dermis appears to be unable to induce extensive appendage formation in avian or mammalian epidermis. 9. A remarkable similarity exists in the mechanisms of skin differentiation in the three classes of amniotes. Indeed scales, feathers and hairs require two kinds of dermal messages for their complete morphogenesis: early ones, which can be transmitted from one class to another, and which are responsible for the initiation, site, size and distribution pattern of appendage primordia, whose class-specific quality (scale, feather or hair buds) is determined by the epidermis; and later specific ones which can only be understood within the class and which are necessary for the completion of the specific architecture of the cutaneous appendage.