Figure 1.

Two morpho-regulatory signaling centers control vertebrate limb-bud development. (A) Skeletal preparation of a mouse forelimb at birth. (B) Skeletal preparation of a fetal chicken wing at day 15 of embryonic development. Red and blue histological stains mark ossified bone and cartilage, respectively. Despite morphological differences, the basic bauplan along both axes is conserved. (Prox-dist) Proximodistal axis, (ant-post) anteroposterior axis, (Sc) scapula, (Cl) clavicle, (Hu) humerus, (Ra) radius, (Ul) ulna. Digit identities are indicated by numbers. (C) Visualization of the AER by in situ detection of Fgf8 transcripts in a mouse limb bud. (D) The ZPA expresses the Shh morphogen. (E) Wolpert's French flag model: A concentration gradient forms by diffusion of a morphogen from a source and positional information is determined in groups of cells by inducing distinct responses to specific concentration thresholds (indicated by blue, white, and red).