The role of a reaction-diffusion system in the initiation of skin organ primordia. I. The first wave of initiation

Abstract

A general mechanism for the initiation of skin organ primordia is proposed. The main component of the mechanism is a reaction-diffusion (RD) system of epidermal origin, which acts as the primary source of positional information during initiation. The positional information is derived from spatial wave-like patterns in the concentration of the substances (morphogens) constituting the RD system. The RD system is switched on by a substance S, produced in the dermis at a rate which depends on the stage of development of the dermis. The region of epidermis where the RD system is switched on is separated from the region where it is switched off by a line referred to as the switch-boundary. It is demonstrated that a wide range of patterns are produced by the same RD system depending on the shape and rate of movement of the switch-boundary, the growth rate of the epidermis and the regional variation in the production rate of S. The calculated patterns are consistent with those seen in the first wave of hair follicle, feather follicle and reptilian scale initiation including the broad ventral scales of snakes. Despite the fact that the RD system resides in the epidermis, the dermis may still exert significant control over the size, density and macropattern of the skin organ primordia consistent with the observed behaviour of heterotopic and heterotypic recombinants (Dhouailly 1975; Sengel et al., 1980).