Chromatophores

I have had several questions of late regarding pigmentation and chromatophores.  There is a lot of information out on the misinformation super highway about chromatophores, but it is highly confusing.  Part of the reason for this is many people take information from studies done in mammals and think that can be lumped into one big pot with studies done in reptiles.  I would even argue that lumping mainstem reptile studies with studies in archosaurs might be a mistake.  The fact is they do not function in the same way, they do not go through the same development and they do not even have the same cells.  Mammals lack xanthophores (and the subclass erythrophores) and iridophores.  Mammals also lack dermal melanophores.  Mammals (some argue) do not even have melanophores, but instead have melanocytes.  The point is that the misinformation super highway (MiSH - not to confuse with MSH which is melanophore (melanocyte) stimulating hormone) is full of people that do not do the proper research and do not fully understand the subject they are writing about.  Some in the misinformation super highway's drunk lane (abbreviated 'wikipedia') do much more than confuse the issues, they actually write things that are incorrect and when it is corrected, change it back the the incorrect information (see the wikipedia article on leucism that a colleague of mine at another college tried to correct and wound up getting his stuff changed back to the incorrect information and told that he did not offer credible citations when he used research papers, peer reviewed literature and expert's text books as references).

The result is that there is a mass of confusion and it stems from the MiSH and wikipedia.  For an entry level of understanding about reptile and amphibian chromatophores you should start with the following three resources:

Reptile and amphibian variants - Bechtel, 1995 (book).

Dermal Chromatophores - Taylor and Bagnara, Am. Zoologist, 122:43-62(1972)

The Dermal Chromatophore Unit - Bagnara, Taylor and Hadley, The J of Cell Bio, 38:67-79(1968)

I will have a more thorough discussion on this topic later in the semester.

In brief, mammalian melanocytes do not appear to be the same as the melanophores in reptiles and amphibians.  Indeed, they do not appear to be the same as the chromatophores of invertebrates or fish either.  The chromatophore is a neural crest cell in its typical origin, though chromatophores not from neural crest develop in the eye.  They start out as a protochromatophore or chromatoblast.  They then differentiate into one of three, or four, types.

Chromatophore Subtypes - xanthophores, iridophores and melanophores contain all elements of all the chromatophore types.  Thus, melanophores contain pterinosomes and the iridophore plates (called reflecting platelets), but what makes them distinctly one type or another is the degree to which they contain the other structures.  Melanophores are melanophores because they contain around 99.9% melanosomes and only a small percentage of the other structures.  This is important to note, because this fact is what gave rise to the single progenitor theory for chromatophres.

Melanophores - contain mostly melanosomes and are capable of two forms of pigment production.  Eumelanin is brown to black and pheomelanin is orange to rust or rusty brown.  Melanophores, unlike melanocytes in mammals, generally do not inject their melanosomes into keratinocytes.  They are also usually able to move their melanosomes into their dendrites or into the perikaryon depending on neurohormonal stimulation.  The melanins are contained within the melanosomes.

Xanthopores - contain two major pigment bodies the pterinosomes containing pteridines and vesicles that contain fats with stored carotenoids.  Another class of organelle may exist in which the pteridines are converted to drosopterins and some people have suggested the name drosopterinosome.  However, since drosopterins are made from pteridines, this may be a bit of a splitter attitude, and really may not be valid.  But it cannot be denied that yellow pteridine rich cells occur within microns of orange or red drosopterin rich cells, so there may be something to the separation.  At any rate, xanthophores can be divided into at least two subtypes.
          Yellow xanthophores - pterinosome and pterinidine rich.  Since they are yellow to yellow orange and the term xanthophore can apply to the red xanthophores as well, there is a good argument to refer to this subtype as luteophores, but that term has yet to catch on. 
          Red xanthophores (erythrophores) - pterinosomes (drosopterinosomes) are rich in drosopterins which range from orange to red and even violet.

 Iridophores -  while possessing all the organelles of the other chromatophores, the iridophores primarily use refractile platelets formed by crystals of the uric acid based DNA components called purines.  Specifically the purines hypoxanthine, guanine and possibly adenine.  Basically theses platelets act as prisms and refract light to form certain colors and interact with different pigment bearing chromatophores to vary the colors.