A microtubule-independent component may be involved in granule transport in pigment cells

Abstract

COLOUR change in lower vertebrates is based on the movement of thousands of pigment granules within integu-mental chromatophores. In some types of pigment cells, particularly those of teleost fish, pigment aggregation in the cell centre and redispersion into the cell processes may take place within fractions of a minute1,2. These cells may therefore be a suitable model system for the study of intra-cellular vesicle transport. There is considerable information about the structure of chromatophores, in particular their prominent microtubule system2–5, but the mechanism of pigment movements remains obscure. Pigment granule transport is thought to be microtubule-dependent6 and microtubules definitely play a part in this process1,2,6. However, it remains to be determined whether these organelles also directly provide the driving force for the movements. The crucial question is: what happens when the microtubule frame is withdrawn experimentally? We therefore tested the ability of chromatophores to move pigment granules in conditions where microtubules were absent, and further investigated the nature of the physical relationship between microtubules and pigment granules. We now report the presence of a second component involved in the transport process.