Microscopic analysis of lizard claw morphogenesis and hypothesis on its evolution

Morphogenesis of claws in the lizard Lampropholis guichenoti has been studied by light and electron microscopy. Claws originate from a thickening of the epidermis covering the tips of digits under which mesenchymal cells aggregate. Mesenchymal cells are in continuity with perichondrial cells of the last phalange, and are connected to the epidermis through numerous cell bridges that cross an incomplete basement membrane. The dense lamella is completed in non-apical regions of the digit where also collagen fibrils increase. The dorsal side of the developing claw derives from the growth of the outer scale surface of the last scale of the digit. The corneous layer, made of beta-keratin cells, curves downward by the tip of the growing claw. The epidermis of the ventral side of the claw contains keratohyaline-like granules and alpha-keratinocytes like an inner scale surface. The thickness of the horny layer increases in the elongating unguis while a thinner and softer corneous layer remains in the subunguis. These observations show that lizard claws derive from the modification of the last scale or scales of the digit, probably under the influence of the growing terminal phalanx. Some hypotheses on the evolution of claws in reptiles are presented.