Reptile & Amphibian Forums

This is a little paper I wrote this morning on chromatophores, pigments, albinos and leucistics for one of my up-coming web pages. I thought a few of you guys might enjoy reading it so I'm posting it here. If any of you find any faults with it or disagree with any of the statements please feel free to let me know.

And Paul H. if you read it I would like to hear your opinion of it. I'm always happy to learn.

All of the colors we see in snakes come from color bearing cells called Chromatophores. There are three sub-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 produce the metallic shine that can be seen in some snakes like D'alberts pythons and rainbow boas. 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 dead skin cells on top of the living dermis layer. The chromatophores are also 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. The melanophores can send melanin (the brown or black pigment) through these dendrites 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.

So what's a albino?

As mentioned above the melanophores are the chromatophores responsible for the production of brown and black pigment. They do this by making melanin. However, to make melanin these melanophore cells must first contain tyrosinase. An albino is any animal that contains a mutant gene that either limits
or completely halts the formation of tyrosinase. There are other ways that melanin production can be halted or blocked but more study needs to be done before we have all the answers. What we do know is that some albinos have some tyrosinase and some
don't. Those with it are called T albinos and those without are called T- albinos. The phenotypes of different T albinos have a large range of expression. Some forms of T appear to have no melanin at all while others look almost like completely normal wild types. For example, one form of albino ratsnake has a phenotype more consistent with that of a T- albino (T- albino always = no black in the phenotype.) However, when
tested for tyrosinase it proved to be tyrosinase positive. To test for tyronsinase a cluster of chromatophores is biopsied from the dermis and incubated in a dopa solution. If the melanophores turn black we know those cells contain tyrosinase. If they remain clear they are T-.
I believe that only snakes with a MUTANT T gene should be called T albinos. If there is an apparent reduction of melanin in a natural geographical localized population (like Hog island boas for example) the term T albino does not apply. These natural populations should be termed "hypomelanistic" to distinguish them from other darker wild populations. And so that they are not confused with their mutant T albino couterparts.

What's a leucistic?

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 had not yet migrated from the neural crest to the dermis of the skin. 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 cell 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 leucisim. Leucistic snakes have no melanophores, no xanthophores and only very limited amounts of iridophores and their skin appears completely white. However, eyes get their pigment from cells that migrate from a region called the neural tube and not the neural crest. So the eyes are not affected by the leucistic mutant gene. Leucisim is a genetic malfunction of the neural crest only.

--Mark Dwight

I forgot that the kingsnake forums don't allow the plus sign to be used. So, where ever you see a T with no minus or negative sign it means T-positive. Arrrgggg.

I like the piece in many ways, but it would be the better for some corrections.

>All of the colors we see in snakes come from color bearing cells called Chromatophores. There are three sub-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 produce the metallic shine that can be seen in some snakes like D'alberts pythons and rainbow boas. 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 dead skin cells on top of the living dermis layer. The chromatophores are also 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. The melanophores can send melanin (the brown or black pigment) through these dendrites 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.

Better would be describing the epidermis as around 30 cells deep. As given, it could be interpreted as 30 cells scattered over the entire body.

The epidermis is not completely dead cells. The basal epidermal cells are living.

According to Bechtel's Reptile and Amphibian Variants, there are dermal and epidermal melanocyctes. Dermal melanophores and xanthophores are concentrated in a narrow zone immediately below the epidermis. Iridophores are immediately below the other two if present at all.

I need to check another source, but I do not think that iridophores produce the shine in D'Alberts pythons and rainbow boas.

>So what's a albino?

Might put in the dictionary definition of "albino". See below.

>As mentioned above the melanophores are the chromatophores responsible for the production of brown and black pigment. They do this by making melanin. However, to make melanin these melanophore cells must first contain tyrosinase. An albino is any animal that contains a mutant gene that either limits
or completely halts the formation of tyrosinase. There are other ways that melanin production can be halted or blocked but more study needs to be done before we have all the answers. What we do know is that some albinos have some tyrosinase and some
don't. Those with it are called T albinos and those without are called T- albinos. The phenotypes of different T albinos have a large range of expression. Some forms of T appear to have no melanin at all while others look almost like completely normal wild types. For example, one form of albino ratsnake has a phenotype more consistent with that of a T- albino (T- albino always = no black in the phenotype.) However, when
tested for tyrosinase it proved to be tyrosinase positive. To test for tyronsinase a cluster of chromatophores is biopsied from the dermis and incubated in a dopa solution. If the melanophores turn black we know those cells contain tyrosinase. If they remain clear they are T-.
I believe that only snakes with a MUTANT T gene should be called T albinos. If there is an apparent reduction of melanin in a natural geographical localized population (like Hog island boas for example) the term T albino does not apply. These natural populations should be termed "hypomelanistic" to distinguish them from other darker wild populations. And so that they are not confused with their mutant T albino couterparts.

If T-negative albino means no black whether or not there is tyrosinase activity, it destroys the connection of T-negative albino to the tyrosinase activity. As nobody has tested tyrosinase activity in any boa albino, there actually are no known T-negative albino boas.

I have a problem with the last four sentences here. The normal tyrosinase gene produces normal tyrosinase. A T-negative mutant tyrosinase gene either does not produce tyrosinase or produces a version of tyrosinase that cannot catalyse dopa. There can be other mutant tyrosinase genes that can produce partly functional tyrosinase that produces melanin at a less than normal rate. These would be among the T-positive albinos. There are other mutant genes that have no relation to the tyrosinase gene or tyrosinase but can also lighten up a snake. The mutant gene might affect the number of melanin granules produced or their size or their shape or the number of processed in the melanophore. These would also be among the T-positive albinos. So T-positive albino is really a grabbag category. It would be much better to give each unique mutant gene its own unique name.

I would also like to distinguish light snakes like the Hog Island boas from mutant forms. But I am against using hypomelanistic for snakes like the Hog Island boas. Hypo is entrenched as a synonym for salmon.

I'll get back with more comments later.

Paul Hollander

Main Entry: al·bi·no
: an organism exhibiting deficient pigmentation; especially : a human being who is congenitally deficient in pigment and usually has a milky or translucent skin, white or colorless hair, and eyes with pink or blue iris and deep-red pupil —compare MELANO —al·bin·ic /-'bin-ik/ adjective

Merriam-Webster's Medical Dictionary, © 2002 Merriam-Webster, Inc.

There is a section about rainbow boas in Mehrtens' Living Snakes of the World in Color. It says that the rainbow effect is produced by sunlight hitting microscopic ridges on the surface of the skin. Which to me indicates that the rainbow effect is not produced by any of the pigment cells.

>What's a leucistic?

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 had not yet migrated from the neural crest to the dermis of the skin. 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 cell 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 leucisim. Leucistic snakes have no melanophores, no xanthophores and only very limited amounts of iridophores and their skin appears completely white. However, eyes get their pigment from cells that migrate from a region called the neural tube and not the neural crest. So the eyes are not affected by the leucistic mutant gene. Leucisim is a genetic malfunction of the neural crest only.

I'd delete the last sentence entirely because I do not think that it is advisable to limit the malfunction to the neural crest. There may be malfunction of the cellular environment in the dermis, too.

The eyes are affected by the leucistic mutant(s), though less than the skin. There is less pigment than normal in both the retina and the iris.

Spelling -- leucisim should be leucism.

By the way, the leucistic mutant gene in the Texas rat snake is recessive to its normal allele. On the other hand, there are several codominant mutant genes in the ball python that produce a leucistic phenotype when homozygous (Mojave, yellowbelly, etc.). And in the cat, dominant white is a fully dominant mutant gene that produces white fur but colored (blue, green, orange) eyes. To me, this indicates that there could be more than one way to produce a leucistic phenotype. The jury is still out on the question whether these mutants in the ball python are alleles.

It might be worthwhile putting in a section on axanthic/anerythristic. Caramel in the corn snake seems to be a dilution of the red pigment to yellow.

Smooth green snakes live in my back yard. Over the years I have caught several that were brown instead of green. When I pickled one in formalin, the yellow pigment dissoved out of the skin leaving a coal black specimen. My best explanation for the brown phenotype is that the snake had malfunctioning iridophores.

Paul Hollander

Wow. I've spent 100's of hours over the last few months seeking out this info and yet if what you say is correct all of it is completly wrong. If you don't mind I'd like to discuss this further with you. However, if you don't mind I'd like you to take a look at this..
http://en.wikipedia.org/wiki/Chromatophore

and this.

http://www.petplace.com/reptiles/why-are-frogs-green/page1.aspx

After you read these we can discuss this in more detail.

These are only two of many dozens of references I've found that all agree with my post.

Dosn't IRIdophore get its name from the word IRIdecsent?

Dosn't Bechtel agree with the DCU(Dermal Chromatophore Unit) standard? Which places the xanthopores on top, the iridophores in the middle and then the melanophores "hanging" from the other chromatophores?

Either Bechtel was right and everyone else is wrong or visa versa. I don't know the answer however.

I have not been on line over the weekend, so this is the first chance I've had to respond.

>Wow. I've spent 100's of hours over the last few months seeking out this info and yet if what you say is correct all of it is completely wrong.

It is not completely wrong. For example, you got the dopa test 100%. And that is only one of many things. But the piece could use some tweaking in places.

>If you don't mind I'd like to discuss this further with you.

Okay. Drop me e-mail. I took a quick look at the frog piece and was not particularly impressed. I will check the Wikipedia piece later.

>Doesn't Bechtel agree with the DCU(Dermal Chromatophore Unit) standard? Which places the xanthopores on top, the iridophores in the middle and then the melanophores "hanging" from the other chromatophores?

I do not recall Bechtel mentioning the DCU. He had a picture of a section through the upper dermis, though. But your best bet is to read what he wrote rather than having it filtered through me.

Paul Hollander