Figure 7. Making maxillary barbels with Wnt signals: location of a new dermal cell population, and alternative models of appendage outgrowth.

Diagrammatic section through a developing maxillary barbel bud, distal end to the right. An outer later of stratified epithelium (blue) covers a convex dermal core. Within the core is a mass of acellular matrix (yellow), in which is embedded arranged a single layer of Wnt-responsive cells (the TCF+ core cells, red). These cells contact each other circumferentially via overlapping cytoplasmic bridges; smaller cytoplasmic projections extend radially into the surrounding collagenous material. Activated TCF+ cells divide rarely, but increase secretory activity, depositing collagenous matrix to widen and lengthen the appendage. A) Paracrine activation: Wnt ligands secreted by the overlying epidermis activate the TCF+ core cells. B) Autocrine activation: TCF+ cells maintain their own activation by secreting Wnt ligand(s) that are bound to the surrounding matrix. For simplicity, other cell types within the dermal compartment (e.g., blood vessels, peripheral nerves, and melanophores) are not shown.