The spinal nerves innervate putative chemosensory cells in the ventral skin of desert toads, Bufo alvarius

Toads normally obtain water by absorption across their skin from osmoticall y dilute sources. When hyperosmotic salt solutions are presented as a hydra tion source to dehydrated desert toads, they place the ventral skin onto th e source but soon afterwards escape to avoid dehydration. The escape behavi or coincides with neural excitation of the spinal nerves that innervate put ative chemosensory cells in the ventral skin. In the present study, fluores cent dye translocated through the spinal nerves to those receptor cells in the epidermis was photoconverted in the presence of 3,3'-diaminobenzidine t etrahydrochloride for electron-microscopic observation of the cells and ass ociated nerve terminals. Most of the photoconverted cells were located in t he deepest layer of the epidermis. with some being in more intermediate lay ers. No labeled cell was seen in the outermost layer of living cells. In de sert toads, flask cells and Merkel cells are occasionally seen in the epide rmis. An association of nerve fibers with these epidermal cells has been re ported in some species of the anurans, In the present study. however, the c ytological features of the photoconverted cells are neither reminiscent of flask cells nor Merkel cells, but are similar to those of surrounding epith elial cells in each layer of the epidermis. We hypothesize a sensory functi on for these cells, because they have a close association with nerve fibers and participate in the transepithelial transport of salts that must pass t hrough all cell layers of the skin.