Retinoic Acid cannot be the Morphogen in Reaction-Diffusion Models for the Formation of the Chick Wing Bud

Abstract

In 1968 Saunders & Gasseling discovered that a group of cells at the posterior margin of the chick wing bud has a remarkable morphogenetic potency: if transplanted to the anterior site of a host limb bud, a mirror image duplication of the normal pattern of digits develops (cf. Figure 3.1). This group of cells was labeled the ‘zone of polarizing activity’ (ZPA) and ever since extensive studies have established many interesting features of the digit pattern formation. A simple model was put forward by Tickle et al. (1975) which could describe the experimental facts of pattern duplications. This model was based on the assumption that the ZPA is the source of a morphogen which diffuses along the anteroposterior axis. At the same time the morphogen is supposed to decompose via first order kinetics. Asymptotically, a stable morphogen gradient is established which, by appropriate threshold concentrations, can separate the morphogenetic field into bounded areas for digits 4, 3 and 2 of the wing bud (Tickle et al. 1975) (cf. Figures 3.2 and 3.3).