In the January issue, PCMR published a paper that asked the seemingly simple question of ‘What is a vertebrate pigment cell?’ (Schartl et al., PCMR 29, 8–14, 2016). An interesting question for a journal that uses ‘pigment cell’ in its title! When I was a young student, I loved definitions, the more universal they were the better. Nevertheless, my thesis advisor told me that there is really no downside to making up one's own private definition as long as one rigorously adheres to it during a particular discourse. Later, I realized just how hard it is to come up with a precise definition. What is a human organism, for instance? Roughy 1014 cells sharing a common genome, or an assembly of perhaps one thousand species whose non-human cells collectively outnumber the human cells by orders of magnitude? Or what is a gene? A coding sequence?—too restrictive. Evolutionarily selectable?—so are intragenic regions, etc. The authors of the above PCMR paper restrict their definition to cells that make pigment. But what are the criteria that allow us to lump them together in this way? Function, such as generation of color? Well, pigment cells can also be found in internal organs, and the function still eludes us for many. Lineage and development, cell biology, or biochemical pathways? All too diverse and often unknown. And so no definition of a pigment cell will easily please everyone. Indeed, another publication in these pages implied that the provenance of pigment should perhaps not be part of the definition, at least not of the definition of pigment cell differentiation, as recipient cells nonetheless differentiate into pigmented cells using a specialized genetic program to acquire pigment, just not to produce it themselves (Weiner et al., PCMR 27, 1014–31, 2014). In any event, whatever is your preferred definition, there will always be exceptions: you will probably not want to exclude albino melanocytes from among pigment cells, nor count erythrocytes or ‘melanophages’ among them. So what are definitions good for? Above all, I believe, they help students who yearn for order in the seemingly chaotic world that is biology and who have difficulties understanding why two scientists can use the same term for different things. But once students learn to live with the complexity in biology, the usefulness of all-encompassing definitions may fade. In fact, universally acceptable definitions are often quite toothless as they have to accommodate every possible exception. Nevertheless, some authors argue for their value as conceptual guides. As pointed out by Schartl et al., a definition of pigment cells guides us in classifying these cells according to chosen criteria and may point to specific gaps in our knowledge. This in itself is a worthwhile exercise, and so I hope, as do the authors, that their definition paper ‘will evoke lively discussion in the community’. The current issue has many interesting subjects to offer as well. In an extensive review article, Dorothy Bennett discusses the important question of senescence in melanoma. Original articles cover aspects from genotyping to methods in phenotyping and short letters areas from genetics of pigmentation to correlations between pigmentation and pain perception. You will also realize that the term ‘clinical’ now seems more frequent in PCMR, and this trend may continue. In any event, I hope you enjoy our publication as much as I enjoy serving as Editor, and I wish you all much enthusiasm and success.
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