The Mechanisms responsible for pigment pattern formation in amphibians
and other vertebrates are little understood, although it is known tha
t a fundamental process is the migration of uncommitted chromoblasts f
rom the neural crest to the integument. Here, in response to appropria
te cues present in the integumental milieu, the various pigment patter
ns such as dorsal/ ventral, spots, stripes, etc., are expressed. Becau
se of its importance in color change, melanocyte stimulating hormone (
MSH) has been suggested to play a role in pigment pattern formation, b
ut current knowledge suggests that it functions only as an enhancer or
modifier of already determined patterns through the recruitment of la
tent melanoblasts or the stimulation of melanophore proliferation. Sim
ilarly, the profound changes in pigment pattern at metamorphosis resul
t from the permissive action of thyroxine that allows for the expressi
on of already pre-determined pigmentation patterns. Sex steroids may a
lso play a role in pigmentation pattern changes during sexual maturati
on. At present, the most promising candidates as causal agents in the
determination of pigmentation patterns are some large protein molecule
s present in the skin where they exert their effects in situ. These in
clude a melanization inhibiting factor (MIF). prevalent in the ventral
skin and presumably responsible for the pale ventrum of frogs and a m
elanization stimulating factor (MSF), most prevalent in dark dorsal sk
in and especially in dark spots of the dorsum. A possible interaction
between MIF and MSF and an interplay of these factors with hormones su
ch as MSH and thyroxine may very much influence the expression of pigm
entation patterns.