Topography of cutaneous mechanoreceptive neurones in dorsal root ganglia of skin-grafted frogs.

1. Topographical distribution patterns of dorsal root ganglion (d.r.g.) cutaneous neurones providing innervation to various body surfaces were examined in doublypithed Discoglossus pictus frogs.2. Using electrophysiological methods, sensory neurones innervating back skin were observed to predominate on the dorsal ganglionic surfaces, while belly skin neurones were most prevalent in the caudal half of the d.r.g. ventral surfaces. Flank neurones clustered in the medial half of d.r.g. ventral surfaces.3. Cutaneous neuronal distribution patterns within the d.r.g. of 180 degrees skin grafted animals were indistinguishable from those observed in control frogs.4. Sensory neurone distribution patterns were found to be altered in all animals with single skin type grafts. Projection patterns were always heaviest towards the autograft, which was on the opposite body surface (i.e. dorsally located d.r.g. sensory neurones projected to back skin on the frog's ventrum, while ventrally located neurones projected to belly skin on the frog's dorsum).5. The results are discussed in the light of several selective outgrowth mechanisms which have been proposed as a possible underlying basis for the development of misdirected wiping reflex behaviour in skin grafted anurans. The findings indicate that none of the proposed selective growth mechanisms can account adequately for the development of misdirected wiping reflexes, at least in the species studied.6. It is suggested that selective cell replacement, based upon competition among prespecified cutaneous neurones, might best account for the development of normal and misdirected wiping reflexes in frogs.