HORMONAL INFLUENCES ON THE DEVELOPMENT OF AMPHIBIAN PIGMENTATION PATTERNS

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.