Mark

I thought you might find this piece interesting.

Just a little paper I wrote on reptile pigment about 7-8 years ago. This is just a small bit of it. There's a little info here about eye pigment as well...



Almost all of the colors we see in snakes come from color bearing cells called Chromatophores. There are three classes of chromatophores in snakes. 1. Xanthophores, 2. Melanophores and 3. Iridophores. These three types of chromatophores have different pigment functions. The Xanthophores produce the yellow to red pigments. The Melanophores produce the brown and black pigments. And the iridophores react with xanthophores and melanophores to reflect other hues of color(like green and blue.) The xanthophores and iridophores reside in the dermis layer of the skin just beneath the epidermis. The melanophores can be in both the dermis and epidermis. The epidermis is a layer of about 30 (mostly dead) skin cells deep on top of the living dermis layer. The Chromatophores are arranged in layers. The top layer contains the Xanthophores, in the middle are the iridophores and the bottom layer is the Melanophores. The Melanophores also have finger like projections called dendrites that can extent upward between and around the iridophores and Xanthophores. These dendrite fingers can sometimes send melanin (the brown or black pigment) to the surface of the dermis and into the epidermis partially covering up the pigment produced by the Xanthophores. This is why some snakes like Boa constrictors can quickly change from light to dark.



OK. Now that you know about chromatophores let's switch gears for awhile..



If you've ever seen a baby snake that was born or hatched prematurely you may have noticed it had a lot less pigment compared to a fully developed baby. Thats because all of the chromophores have not yet migrated from the neural crest to the dermis of the skin. So, what's the neural crest? Let me explain.. Early in the embryonic development of snakes the cells that will eventually become the chromophores are being created in a region called the neural crest. I like to think of the neural crest as a cell nursery. At different points in development these newly formed cells leave their neural crest home in waves and seek out other parts of the developing

embryo. The waves of pigment cells leaving the neural crest and migrating to the dermis are called chromatoblasts. But sometimes things go wrong and there is a defective mutant gene that will not allow chromatophores to develop in the neural crest or the defective gene will prevent the migration of the chromatoblasts to the dermis. Either way no chromatophores ever reach the dermis. This condition is called leucism. Leucistic snakes have no (or very limited) melanophores, no xanthophores and only very limited amounts of iridophores and their skin appears almost completely white. However, eyes get most of their pigment from cells that migrate from a region called the neural tube and not the neural crest. So the eyes are affected very little by the leucistic mutant gene. Leucism can be classed as a genetic malfunction of the neural crest.

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