Fine structural analysis of limb development in the wingless mutant chick embryo.

The fine structure of the normal and wingless chick limb bud was examined with scanning and transmission electron microscopy. The apical ectodermal ridge (AER) of the normal limb bud was composed of pseudostratined columnar cells. These cells contained gap junctions, electron-dense vesicles, and numerous microtubules and microfilaments that were oriented perpendicularly to the basal lamina. Microfilaments were also found coursing transversely in the basal cell cytoplasm. The ectoderm of the wingless mutant limb bud lacked a well-developed AER and resembled the dorsal and ventral ectoderm of the normal embryo. Gap junctions and electron-dense vesicles found in the AER of the normal limb bud were not apparent in the mutant ectoderm. The normal-limb bud mesoderm is composed of stellate cells that are oriented at right angles to the overlying ectoderm. There is a prominent subectodermal space that is traversed by numerous mesenchymal cell filopodia. The mesodermal cells of the mutant limb bud are compact and round and have short stubby filopodia, while the cells of the adjacent flank mesoderm are stellate. The subectodermal space is absent and the mesodermal cells are in intimate association with the basal lamina of the overlying ectoderm. Ruthenium red was employed as an extracellular marker for glycosaminoglycan$. No differences were found in the distribution of these substances in normal and mutant limb buds. In several cases the basal lamina of the mutant limb bud ectoderm was discontinuous aqd the lamina lucida was not apparent. The results indicate that the mutation has an effect on the limb buds' ability to maintain a well-developed AER and basal lamina. It also suggests that the wingless gene affects the shape and possibly the mobility of the limb-bud mesoderm cells.