Stimulation of Cultured Iridophores by Amphibian Ventral Conditioned Medium

Abstract

That the ventral integument of adult frogs (Rana pipiens) contains factor(s) that stimulate iridophore expression (adhesion, morphologic appearance, proliferation) was demonstrated on iridophores derived from tadpoles of R. pipiens and Pachymedusa dacnicolor, and maintained in primary culture in a growth medium based upon Leibovitz's L-15. Experimental growth medium (VCM) conditioned by a one-hour exposure to pieces of ventral skin of adult R. pipiens induced iridophores to assume a broad and stellate appearance, to form confluent sheets, and to proliferate over a nine-day period. Iridophores in control medium assumed long thin profiles, detached easily, and exhibited no signs of proliferation. Unknown cells containing reflecting platelets and unusual other organelles appeared uniquely in chromatophore cultures of P. dacnicolor in VCM. The intense stimulation of iridophore expression in VCM is consistent with the known inhibitory effect of this medium on melanization and with its purported role in the determination of dorsal/ventral pigment patterns of amphibians. The results are discussed in terms of a prevailing theory about pigment cell origins and development.

Melanization stimulating factor (MSF) and melanization inhibiting factor (MIF) in the integument of fish

“…The major reason why the ventral surface of animals is white seems to be due to the presence of many light-coloured iridophores that totally reflect light. Thus, if the action of MIF is to account for the pale ventral surface, it follows that this factor must exert a trophic effect on iridophores that is opposite to its inhibitory action on melanophores (Bagnara and Fukuzawa, 1990).…”

“…Chromatophores were obtained from advanced tadpoles of R. pipiens at about stage XII (Taylor and Kollros, 1946) utilizing procedures described previously (Bagnara and Fukuzawa, 1990). In brief, the tadpoles were pithed and their skins were removed and placed in BSS.…”

Fish melanocortin system

“…In teleosts, this countershading is achieved by a patterned distribution of the pigment cells, with the light-absorbing and light reflecting chromatophores mostly distributed in the dorsal and ventral areas, respectively (Fuji, 1993). Early experiments in amphibian and fish species suggested that the dorsal-ventral pigment pattern is achieved through a putative diffusible melanization inhibition factor (MIF), mainly produced by cells in the ventral skin, which inhibits melanoblast differentiation and stimulates or supports iridophore proliferation in the ventrum (Fukuzawa and Ide, 1988, Bagnara and Fukuzawa, 1990, Zuasti et al, 1992Zuasti, 2002). Amphibian MIF was reported to block stimulation of the melanization provoked by α-MSH in neural explants of Xenopus (Fukuzaga and Bagnara, 1989).…”

Not just black and white: Pigment pattern development and evolution in vertebrates

“…In amphibians, a ventral specific factor, melanization inhibiting factor (MIF), represses differentiation and melanization of melanocytes in vitro [76,77]. MIF is highly expressed in unmelanized ventral skin and not only suppresses melanization, but also appears to promote iridophore localization in this region [78,79]. Similar to MIF, an agouti homolog expressed mainly in the golfish ventrum blocks melanization in cell culture [80].…”