IGF-I AND INSULIN IN THE ACQUISITION OF LIMB-FORMING ABILITY BY THE EMBRYONIC LATERAL PLATE

Acquisition of limb-forming ability by discrete legions of the lateral plate of the chick embryo is dependent on a medial-lateral inductive signaling cascade moving sequentially from the area of Hensen's node t o the semitic mesoderm, the intermediate mesoderm, and then to the pro spective limb-forming regions of the lateral plate. IGF-I and insulin are expressed by medial tissues as they are influencing the prospectiv e limb-forming regions of the lateral plate. Here we report that IGF-I and insulin, but not FGF-2 or FGF-4, induce the formation of limb bud -like structures in vitro from prospective limb regions before they ha ve acquired the ability to form limbs independent of medial tissues, a nd also induce the formation of limb bud-like structures from the pros pective flank. The limb bud-like structures induced by IGF-I and insul in possess a thickened cap of ectoderm along their distal tips that re sembles the apical ectodermal ridge (AER) and this thickened distal ap ical ectoderm expresses the AER-characteristic homeobox-containing gen e Msx-2. Like in normal limb buds, a population of highly proliferatin g cells which express the homeobox-containing gene Msx-1 are localized in the mesoderm directly subjacent to the thickened AER-like structur es induced by IGF-I and insulin. However,the limb bud-like structures induced by IGF-I and insulin do not express sonic hedgehog, which enco des a secreted signaling molecule that has been implicated in regulati ng the anteroposterior patterning of the developing limb bud. IGE-I- a nd insulin-treated prospective limb explants give rise to rudimentary limbs containing identifiable skeletal elements when grafted into the coelom or to somites of host embryos. Overall, these results suggest t hat IGF-I and insulin may be endogenous signals produced by medial tis sues that are involved in conferring limb-forming ability to the later al plate and may promote the initial outgrowth of limb buds and possib ly induce the AER. However, other signals are necessary to promote the expression of genes such as sonic hedgehog that regulate the patterni ng of the developing limb. (C) 1996 Academic Press, Inc.