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Department of Anatomy and Developmental Biology, University College London, London WC1E 6BT, United Kingdom
Pattern in the developing limb depends on
signaling by polarizing region mesenchyme cells, which
are located at the posterior margin of the bud tip. Here
we address the underlying cellular mechanisms. We
show in the intact bud that connexin 43 (Cx43) and
Cx32 gap junctions are at higher density between distal
posterior mesenchyme cells at the tip of the bud than
between either distal anterior or proximal mesenchyme
cells. These gradients disappear when the apical ectodermal ridge (AER) is removed. Fibroblast growth factor 4 (FGF4) produced by posterior AER cells controls signaling by polarizing cells. We find that FGF4 doubles gap junction density and substantially improves
functional coupling between cultured posterior mesenchyme cells. FGF4 has no effect on cultured anterior
mesenchyme, suggesting that any effects of FGF4 on
responding anterior mesenchyme cells are not mediated by a change in gap junction density or functional
communication through gap junctions. In condensing
mesenchyme cells, connexin expression is not affected by FGF4. We show that posterior mesenchyme cells
maintained in FGF4 under conditions that increase
functional coupling maintain polarizing activity at in
vivo levels. Without FGF4, polarizing activity is reduced and the signaling mechanism changes. We conclude that FGF4 regulation of cell-cell communication
and polarizing signaling are intimately connected.
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