Cell communication during vertebrate limb morphogenesis

Kelley and Fallon(3) first reported development of gap junctions which distinguish the inductively active apical ectoderm from adjacent dorsal and ventral ectoderm. They predicted that cells of the apical ectodermal ridge would be coupled to each other but not to cells in the adjacent dorsal and ventral ectoderm. To test this hypothesis, dyes with varying charge and molecular weight were injected into ridge and non-ridge ectoderm and subjacent mesoderm of 3.0-3.5 day Japanese Quail embryos. All dyes migrated immediately following injection. Intracellular injection of a single cell in the ectodermal ridge resulted in transfer of dye to neighboring cells but not to peridermal cells or to cells of the adjacent dol sal and ventral ectoderm. Furthermore, dye was not released from ridge cells into the subjacent mesoderm or extracellular matrix. Extracellular injection of dye between ridge cells filled spaces between epithelial cells brit stopped at the basal lamina. This observation was the same when dyes with positive, negative and neutral charges were injected. From these re suits, it is concluded that cells of the apical ridge are extensively coupled to each other but not to periderm nor adjacent dorsal and ventral ectoderm. Thus, the ridge is a functional syncytium. There is no evidence of dye coupling between ectodermal cells and subjacent mesoderm. The inability of dyes greater than 332 molecular weight to cross the basal lamina suggests that tissue interactions which are required for normal limb development may be mediated either through ions or other molecules with less than 332 MW, or through the molecular nature of the lamina and its associated extracellular matrix.