The Pathway of Polarizing Activity from Hensen’s Node To the Wing Bud in the Chick Embryo

Abstract

The skeletal elements of the normal wing of a chick embryo are humerus, radius and ulna, the wrist and in antero-posterior order the digits 2,3,4. The traditional experiment to demonstrate that cells from the posterior edge of a stage 20 limb bud transmit a signal which lays down the antero-posterior axis of the digits is to transplant a cube about 100 µm³ of these cells to the anterior edge of another similarly staged limb bud. The result is widely known as the mirror-image reduplication with a digit combination of 432234 (Fig.la & lb) along the proximo-distal axis of the wing. (Saunders and Gasseling, 1968). This signal was interpreted in terms of a diffusible morphogen produced by the cells at the posterior part of the developing bud (Tickle et al., 1975). Increasingly retinoic acid and many of its analogues which produce similar reduplications in the 10 day old wing when applied to the early limb bud (Summerbell, 1983; Tickle et al., 1985) are thought to be the signal itself or at least are instrumental in its transmission or reception (Summerbell and Maden, 1990).