Control of the Melanophores of the Minnow (Phoxinus Phoxinus (L.))

Abstract

ABSTRACT The effects of sectioning the nerve fibres supplying a tail area of melanophores in the minnow under various experimental conditions were studied by a method of continuous observation employing visual and photomicrographic recordings. In the white-adapted fish, nerve section causes a band of melanophores to disperse. The initial dispersion later subsides, the band melanophores aggregating first at the band margins (primary fading). Subsequently the band melanophores react to background reversals in a similar way to normal ones. Hypophysectomy does not interfere with the initial dispersion or primary fading of band melanophores. Furthermore, these melanophores still react tobackground reversals, but in this case they react first at the margins of the band. Prolonged white-adaptation, favouring maximal activities of aggregating pituitary hormones, does not prevent the initial dispersion resulting from nerve section. Band melanophores with fibres sectioned 5 days previously react to diffusing aggregating neurohumours liberated by electrical stimulation. Melanophores with fibres newly sectioned remain dispersed, refractory to this aggregating agent. When the chromatic fibres at the level of the spinal cord are sectioned in a white-adapted fish with a faded tail band the normal melanophores disperse and later the band melanophores disperse first at the margins. A possible interpretation is that dispersing neurohumours are liberated by spinal section. Asymmetric responses of melanophores indicate that neurohumours can activate located regions of the individual melanophore. The mechanism of the melanophore dispersion resulting from nerve section is discussed together with the question of single or double innervation and the related activities of aggregating and dispersing neurohumours.