A proximo-distal gradient of FGF-like activity in the embryonic chick limb bud

Abstract.

In a microassay for anchorage-independent growth in soft agar, NR6 cells form colonies in a dose-dependent manner in the presence of fibroblast growth factor (FGF). Using this assay system, the ability of thin sequential slices of embryonic chick limb bud to promote colony formation was investigated. A functional gradient of colony-promoting ability along the proximo-distal axis of the developing chick limb bud (stages 22–26) was observed. The highest number of colonies was observed in the presence of the most distal slices, and colony number decreased progressively at proximal levels. This gradient was specifically eliminated by the addition of anti-FGF antibody to the assay, indicating that it was caused by a functional gradient of an FGF-like molecule. Limbs of stages 21–26 were assayed: before this time limb buds are too small to slice in the proximo-distal axis in the required manner. The FGF-like gradient was observed at stages 22 to 26.