Cornification in developing claws of the common Australian skink (Lampropholis guichenoti) (Squamata, Lacertidae)

An ultrastructural and immunocytochemical study on developing lizard claws has been done. Claws originate from an epidermal thickening covering the tip of digits under which mesenchymal cells aggregate. The outer side of the dorsal terminal scale of a digit gives origin to the unguis, and suggests that claws are modified scales sustained by the last phalanx. Beneath four-seven layers of embryonic alpha-keratin cells, beta-keratin cells are differentiated but no shedding complex is formed. Beta-keratin cells extends also over the apical part of the ventral side (sub-unguis) which is mainly formed by alpha-keratin cells. Initially beta-keratin filaments have a prevalent oblique orientation along the plane containing the proximal apical axis of the digit. Later bundles become more irregularly distributed. Beta-keratin cells of the unguis remain largely separated and do not merge into a syncitium like in the beta-layer of scales. The thickness of the horny layer is higher in the unguis than in the sub-unguis and decreases from the claw tip toward its basal most part. It is hypothesized that the evolution of claws occurred by the extension of the outer scale surface of the terminal scales beyond the digit tip under the influence of the apical mesenchyme in the terminal phalange.