Positional information in the amphibian limb.

(1) The concept of positional information is applied to a large amount of data obtained previously in experiments on developing and regenerating amphibian limbs. Only the proximo-distal axis of the limb is considered. It is shown that the concept provides a simple, unitary hypothesis which satisfactorily accounts for the experimental data, and may moreover suggest meaningful new approaches. (2) It is suggested that the boundaries of the bipolar limb system lie in the girdle skeleton and at the distal end of the limb, respectively, and that it is the apical epidermis of the growing or regenerating limb which defines the distal boundary conditions. A relatively stable gradient of positional information is assumed to be set up in the mesoderm (or mesenchyme). It is further shown that the differentiated limb retains its positional information and upon amputation imparts it to the base of the blastema. (3) To explain an apparent discrepancy between the developing and the regenerating limb, it is proposed that dedifferentiation of mesodermal limb tissues upon amputation entails a change os positional value in the mesenchyme. Consequently, the amputation level does not once and for all specify the positional value at the proximal end of a mass of blastemal mesenchyme, particularly when the mesenchyme is transplanted in such a way that its linear size decreases (regression) or increases (furion of several blastemas).