Electroreceptors and mechanosensory lateral line organs arise from single placodes in axolotls.

The lateral line system in salamanders consists of mechanoreceptive neuromasts and pit organs, distributed in lines on the head and trunk, and electroreceptive ampullary organs located adjacent to the cephalic lines of mechanoreceptors. Although numerous studies have documented that neuromast and pit organs and the cranial nerves that innervate these receptors arise from a dorsolateral series of placodes, there is no agreement concerning the number of these placodes, the specific groups of receptors that arise from them, or the embryonic origin of ampullary organs. A developmental model was recently proposed (Northcutt et al., 1994) in which all these placodes, except for the most posterior one, elongate to form sensory ridges whose central zones initially form neuromast and pit organ primordia and whose lateral zones subsequently form ampullary primordia. To test this model, individual placodes were unilaterally extirpated, or placodes from pigmented wild-type axolotl embryos were homotopically or heterotopically transplanted into albino hosts. Extirpation resulted in the loss of all three receptor classes, and both homotopic and heterotopic transplants produced pigmented receptors of all three classes in albino hosts. The receptors in the heterotopic transplants still formed lines which occasionally retained their normal orientation despite differentiating in an ectopic environment. These experiments demonstrated that, as previously postulated, specific lines of neuromasts and pit organs do arise from each placode, and ampullary organs also arise from many of the same placodes. The distribution of receptors that develop following incomplete extirpation or heterotopic transplantation also indicates that each placode is patterned regarding receptor classes and orientation prior to sensory ridge formation.