Patterning in the regeneration of type I cutaneous receptors.

1. Type I sensory fibres in cat hairy skin innervate structures characterized by twenty to fifty specialized epithelial (Merkel) cells aggregated in a small dome-shaped elevation. Only one fibre enters each dome and it branches repeatedly to supply at least one terminal to each Merkel cell. After the nerve is cut, the Merkel cells and the dome ultimately disappear.2. The distribution of domes on the posterior thigh was mapped before interruption of the femoral cutaneous nerve and after its regeneration. Regeneration after nerve crush was apparently complete, producing a coincidence pattern similar to those seen in control studies where the nerve was not damaged. After cutting the nerve fewer domes returned, but coincidence of regenerated femoral cutaneous domes with old sites generally was significantly greater than would be expected by chance alone. Non-femoral cutaneous fibres sprouting into the denervated femoral cutaneous field tended to form domes at old sites. Domes were also reformed on scars where domes had been excised.3. Domes appearing at new locations and on excision scars were often small and close together (clustered). Individual domes in a cluster could be innervated by different Type I fibres.4. Type I fibres are directed by some mechanism to sites formerly occupied by domes and to sites where domes are being induced.