Effect of colcemid on the centrosome and microtubules in dermal melanophores of Xenopus laevis larvae in vivo

An electron microscopy study showed that in melanophores with dispersed and aggregated pigment the sensitivity df the centrosome and the stability of microtubules were different and depended on the colcemid concentration. The structure of the centrosome didn't change upon exposure to colcemid in dis persed melanophores. In aggregated melanophores, on exposure to 10(-6) M co lcemid, the centrosome retained its structure; colcemid at 10(-5)-10(-3) M caused a dramatic collapse of the centrosome, Treatment of aggregated melan ophores with colcemid resulted in the complete disassembly of the microtubu les; though microtubules in dispersed melanophores appear to be colcemid re sistant. Light microscopy studies indicated that in Xenopus melanophores wi th aggregated or dispersed pigment melanosomes didn't change their location after exposure to 10(-3)-10(-6) M colcemid. Subsequent incubation in colce mid-free medium revealed that the cells retained their ability to transloca te melanosomes in response to hormone stimulation. Electron microscopy data revealed the inactivation of the centrosome as MTOC (microtubule-organizin g center) in dispersed melanophores with melatonin substituted for MSH in t he presence of colcemid. In contrast, with melanocyte-stimulating hormone ( MSH) substituted for melatonin, we observed the activation of the centrosom e in aggregated cells. We showed that in aggregated melanophores pigment mo vement proceeded in the complete absence of microtubules, suggesting the in volvement of a microtubule-independent component in the hormone-induced mel anosome dispersion. However, we observed abnormal aggregation along colcemi d-resistent microtubules in dispersed melanophores, suggesting the involvem ent of not only stable but also labile microtubules in the centripetal move ment of melanosomes. The results raise the intriguing questions about the m echanism of the hormone and colcemid action on the centrosome structure and microtubule network in melanophores with dispersed and aggregated pigment.