From Melanocytes to Melanoma Cells: Characterization of the Malignant Transformation by Four Distinctly Different Melanin Fluorescence Spectra (Review)

Abstract

The melanin fluorescence emitted by pigment cells of the human skin has been a central research topic for decades, because melanin, on the one hand, protects against (solar) radiation in the near-UV range, whereas on the other hand, melanocytes are the starting point for the malignant transformation into melanoma. Until recently, however, melanin fluorescence was not accessible in the context of conventional spectroscopy, because it is ultraweak and is overshadowed by the more intense so-called autofluorescence of endogenous fluorophores. The advent of a new method of laser spectroscopy has made this melanin fluorescence measurable in vivo. A stepwise two-photon absorption with 800 nm photons is used, which more selectively excites melanin (dermatofluoroscopy). Our review summarizes the experimental results on melanin fluorescence of the four types of cutaneous pigment cells from healthy and malignant tissues. Outstanding is the finding that different types of melanocytes (i.e., melanocytes of common nevi, versus dysplastic nevi or versus melanoma cells) show characteristically different fluorescence spectra. The possibilities of using this melanin fluorescence for melanoma diagnosis are shown. Moreover, the uniform fluorescence spectra emitted by different melanoma subtypes are essential. Conclusions are drawn about the molecular processes in the melanosomes that determine fluorescence. Finally, experimental suggestions for further investigations are given.

Keywords: dermatofluoroscopy; dysplastic nevi; melanin fluorescence; melanoma subtypes.

Figure 1

Different modes of excitation of fluorescence. (a) Left: conventional fluorescence excitation by one photon (e.g., 400 nm). (b) Middle: excitation by stepwise absorption of two photons via a real intermediate energy level (e.g., 800 nm, preferably from a nanosecond laser). PRINCIPLE OF DERMATOFLUOROSCOPY. (c) Right: excitation by simultaneous absorption of two photons via virtual energy level (e.g., 800 nm, preferably from a femtosecond laser). This mode of excitation is used in femtosecond laser spectroscopy. Due to the small cross-section, a nanosecond (ns) pulse excitation with tolerable intensities gives rise to only an extremely weak fluorescence.

Figure 2

The four characteristic melanin fluorescence spectra of pigmented skin cells in the spectral range between 430 nm and 650 nm. Upper left: melanocytes; upper right: nevomelanocytes; lower left: dysplastic nevomelanocytes; lower right: melanoma cells. The fluorescence is excited by stepwise two-photon absorption with 800 nm/2 ns pulses (principle of dermatofluoroscopy). (The intensity above 650 nm results from another non-linear process not considered here. In any case, it is ensured that its intensity is zero below 650 nm.)

Figure 3

Representative measurements on different types of melanoma: (a) SSM, (b) NM, (c) ALM, and (d) choroidal melanoma, all measurements performed in histological specimen (FFPE).