R186 Current Biology, Vol 8 No 6

When a fish or a reptile changes
color, it is because of the movement
of tiny pigment granules in
specialized skin cells called
melanophores. These cells have
hundreds of membrane-bound
pigment granules, or melanosomes,
which can be aggregated at the
center of the cell or dispersed
throughout the cytoplasm. When the
melanosomes are aggregated the
animal appears lighter, and as they
disperse the animal’s skin becomes
darker.

The image above is of a frog
melanophore in culture. The
melanophore has been fixed and
indirectly labelled with fluorescent
antibodies against microtubules
(green) — the tracks along which the

pigment is moved. The
melanosomes (red) have been
imaged by collecting back-scattered
light on a laser-scanning confocal
microscope with the fluorescent filter
set removed. A DNA-specific dye,
chromomycin A3 (blue), labels the
nucleus. The three separate images
have then been pseudocolored and
combined electronically.

Melanophores are frequently
used as a model system for the study
of intracellular organelle transport
and the motor proteins that power it.
During pigment aggregation,
melanosomes are transported along
microtubules by the motor protein
cytoplasmic dynein. By contrast,
during pigment dispersal the
melanosomes are thought to be

moved along microtubules and along
actin microfilaments by the kinesin-
related protein kinesin-II and by
myosin-V, respectively.

The movement of pigment
therefore involves both the
regulation of the melanosome-
associated motors and the
coordination of organelle transport
along two separate cytoskeletal
systems. 

For more details, see: Rogers et
al., Proc Natl Acad Sci USA, 1997,
94:3720–3725; Rogers and Gelfand,
Curr Biol, 1998, 8:161–164.
(Photograph kindly provided by
Stephen Rogers, Department of Cell
and Structural Biology, University of
Illinois, 601 South Goodwin Avenue,
Urbana, Illinois 61801, USA.)

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