Experimental evidence for the regeneration of nerve fibres controlling colour changes after anterior spinal section in the minnow (Phoxinux PhoxinusL)

Abstract

A method for quantitative recording of the general tint of the skin of the minnow is described. Using this method, the colour changes in response to black/white background reversal of normal and of equilibrated chromatically spinally operated minnows, previously black- or white-adapted for more than 9 months, were plotted to give standard curves. These enabled a clear distinction to be made between the rapid colour changes of the normal minnow with intact chromatic nervous system and the relatively very slow changes, only under hormonal control, of the chromatically spinally operated fish. Twenty minnows that had been white-adapted for more than 9 months and 19 that had been black-adapted for the same time were subjected to chromatic spinal section and replaced on the same backgrounds. At intervals their colour changes were recorded and plots of these records were compared with the standard curves. In the course of about 10 months 11 of these minnows showed a good recovery of rapid colour change, 9 showed medium recovery, 8 showed poor recovery and 11 continued to change colour at a rate typical of hormonal control. After another 9 months there was generally no further improvement. This varying degree of recovery of rapid colour change appears to have been the result of regeneration of chromatic fibres in the spinal cord, since a second section anterior to the level of the first and made 19 months after it was followed by darkening of the whole animal. Later it was able to change colour again in response to background reversal but these colour changes were of the slow hormonal type. Observations that recovery of nervously controlled colour change improved with time until some steady condition was reached and that there was great variation in this final degree of nervous control suggest that a number of chromatic fibres may run in the cord. Further,since the colour and pattern produced by the melanophores were affected equally over the whole body as the recovery of rapid colour change proceeded, it appears possible that each chromatic nerve fibre in the cord contributes to the state of excitation of a postganglionic system which is common to all the melanophores in the skin. Records of colour changes at certain stages during regeneration in the cord indicate that there was recovery of nervous control in one direction, i.e. for paling or darkening, while the change in the other direction was still only hormonal. Such observations suggest, in addition to the familiar concept of a nervous aggregating system, the existence of an active pigment-dispersing nervous mechanism.