Distribution and possible function of an adrenomedullin-like peptide in the developing chick limb bud

Adrenomedullin (AM) is a multifunctional peptide that exhibits discrete domains of expression during mouse embryogenesis consistent with a role in regulating growth and differentiation during morphogenesis. Here we report that AM immunoreactivity is present at high levels throughout the apical ectodermal ridge (AER) of the chick limb bud as the AER is directing the outgrowth and patterning of underlying limb mesoderm. Immunostaining is particularly strong along the surfaces of the contiguous cells of the AER. AM immunoreactivity attenuates as the AER regresses and is absent from the distal apical ectoderm of stage 20 limbless mutant limb buds which fail to develop an AER. To explore the possible role of AM in AER activity, we examined the effect of exogenous AM and an AM inhibitor on the in vitro morphogenesis of limb mesoderm, cultured in the presence and absence of the AER. Although exogenous AM cannot substitute for the AER in promoting outgrowth of limb mesoderm in vitro, a specific AM antagonist, AM(22-52), impairs the outgrowth and proliferation of limb mesoderm cultured in the presence of the AER. This is consistent with the possibility that inhibition of endogenous AM activity in the AER impairs the ability of the AER to promote limb morphogenesis. Taken together, these studies suggest that an AM-like molecule may function in an autocrine fashion to regulate some aspect of AER activity.