Defective histogenesis and morphogenesis in the anterior shank skin of the scaleless mutant

Abstract

Abstract Timed‐sequence studies of scaleless anterior shank skin have shown that both histogenesis and morphogenesis are abnormal. An epidermal placode does not form in the mutant and thus the subsequent morphogenetic movements of placode into scale ridge are blocked as well. The dermis of the anterior shank remains flat rather than forming overlapping ridges. Two histologically distinct epidermal surfaces (outer and inner) are formed on either side of a normal scale ridge. In scaleless only a single epidermal surface develops and this is unlike either surface of a normal scale. An isolated dermal ridge of 16‐day normal embryonic scale can induce eight‐day chorionic epithelium to form both the outer and inner epidermal surfaces. Scaleless dermis of the same age when recombined with eight‐day CE will induce an epidermis typical of the normal outer scale surface, but no inner epidermal surface is formed. Previous studies have shown that the genetic defect is expressed in the epidermis and that at earlier stages scaleless dermis can, when associated with a normal epithelium, participate in placode formation and histogenesis of both epidermal surfaces. It seems reasonable to conclude that placode formation is initiated by the epidermis which thereby establishes heterogeneity in the scale dermis; thereafter, the scale dermis directs the histogenesis of the two epidermal surfaces.