Studies of pigment transfer between Xenopus laevis melanophores and fibroblasts in vitro and in vivo¹

Abstract

Frog melanophores rapidly change colour by dispersion or aggregation of melanosomes. A long-term colour change exists where melanosomes are released from melanophores and transferred to surrounding skin cells. No in vitro model for pigment transfer exists for lower vertebrates. Frog melanophores of different morphology exist both in epidermis where keratinocytes are present and in dermis where fibroblasts dominate. We have examined whether release and transfer of melanosomes can be studied in a melanophore-fibroblast co-culture, as no frog keratinocyte cell line exists. Xenopus laevis melanophores are normally cultured in conditioned medium from fibroblasts and fibroblast-derived factors may be important for melanophore morphology. Melanin was exocytosed as membrane-enclosed melanosomes in a process that was upregulated by alpha-melanocyte-stimulating hormone (alpha-MSH), and melanosomes where taken up by fibroblasts. Melanosome membrane-proteins seemed to be of importance, as the cluster-like uptake pattern of pigment granules was distinct from that of latex beads. In vivo results confirmed the ability of dermal fibroblasts to engulf melanosomes. Our results show that cultured frog melanophores can not only be used for studies of rapid colour change, but also as a model system for long-term colour changes and for studies of factors that affect pigmentation.

Melanosome transfer to keratinocyte in the chicken embryonic skin is mediated by vesicle release associated with Rho-regulated membrane blebbing

“…Melanosome-containing vesicles are discharged from a melanocyte into extracellular spaces, and subsequently engulfed by neighboring keratinocytes. Our findings are consistent with one of the three prevailing models proposed by in vitro studies6789101112131415161718192021 (See also Introduction). Of course, our study does not exclude the other models of cytophagocytosis- and exocytosis-mediated transfer67810111521, and it is indeed underway to see if melanocytes in different locations of the body send their melanosomes in a different way.…”

“…Such phenomena were shown using Xenopus cells12, human melanoma cells9, and human foreskin cells1617. This model was corroborated by Wu et al 18,.…”

“…Studies using these techniques have proposed three prominent models to explain how the melanosome transfer is achieved6789101112131415161718192021. First model is that the melanosome-containing tip of a dendrite or filopodium protruding from melanocytes is directly cytophagocytosed by adjacent keratinocytes.…”

Pheomelanin in the skin of Hymenochirus boettgeri (Amphibia: Anura: Pipidae)

“…(vi) Melanoma, where the process of melanin synthesis is deregulated, tends to reveal increased ratio of pheomelanin to eumelanin (21), contained in round melanosomes with amorphous or vesicular rather than lamellar or fibrous ultrastructure (22). Their transfer to the target cells resembles the amphibia (the 'engulfed vesicles') type (22)(23)(24). It raises suspicion that pheomelanin must contribute to the skin pigment of amphibians, which demands confirmation.…”

Effects of Hydroquinone on Cytoskeletal Organization and Intracellular Transport in Cultured Xenopus laevis Melanophores and Fibroblasts

“…Chavin [24] also reported a release of melanin aggregates as a result of cell lysis induced by HQ. In mammalian and amphibian skin, melanosomes are released and taken up by surrounding skin cells to darken the skin [26][27][28][29][30][31]. One mechanism for bleaching of the skin could be if melanosome transfer to surrounding skin cells is affected by HQ.…”