silas.edwin.johnson
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CHAMELEON COLOR CHANGE:
As I’m sure you know, Chameleons are capable of color change. And, when I first was interested in how they did this, I looked it up and was provided with the simple answer “Chameleons can change color due to their special chromatophores”. This is something most people interested in chameleons know, but I still feel as though using the word chromatophore as an explanation leaves you just as much in the dark as you were before. So, what I want to do here today is explain on a deep scientific level how chromatophores work.
VISIBLE LIGHT:
However, to understand how chromatophores work, you need to understand two things: how light works, and how human skin works. We’ll start with how light works. Light is a type of electromagnetic radiation, which is to say a wave that carries energy through space. Electromagnetic radiation can have different frequencies and thus, different energies. When electromagnetic radiation has any wavelength between 400 and 700 nm (or 1.63 eV – 3.26 eV of energy), it is called visible light because it can be picked up by the human eye. To show you how this works, let’s say we have a brick which appears red to us. The brick itself doesn’t have a color, but a molecular structure. When the sun emits its white light (white light is a combination of all visible light), and that light hits the brick, all colors aside from red are absorbed by the brick due to the brick’s chemical structure. When the red is reflected back to your eye, you find the brick to be red.
HUMAN SKIN:
Now, let’s look at this light in a more complex biological system. I, myself am Caucasian so my skin appears a kind of peach. A cell’s color is determined by the proteins they produce. Some proteins when hit by light reflect certain colors, making the protein appear those colors. When a cell has a sufficient amount of a given protein, it appears to be that color. “Skin cells” are keratinocytes, and they contain keratin which doesn’t reflect light at all. Therefore, keratinocytes are clear. However, within your mesh of keratinocytes, you also have some melanocytes (melanin-producing cells). Melanin reflects brown, black or yellow light and so, from a glance, my skin looks peach because of the small but not negligible amount of melanin in my skin compared to keratin. There are also other things that dictate your skin color such as carotene and hemoglobin, but I’m not going to talk about that. If you want to look into that, it’s very interesting stuff.
Now, if you want to talk about the organization of human skin, we have a top layer of keratinocytes, and after a few layers of this, you’ll find the bottom layer of skin – called the stratum basale which has keratinocytes in addition to the melanocytes.
CHROMATOPHORES ARE NOT THE SAME AS MELANOCYTES:
Let’s talk about chameleon skin, starting from the outermost layer. Chameleons, like people have keratinocytes on the outermost layers. As you go deeper past the outer epidermis (epidermis just means “protective layer of skin”), you’ll find the upper dermis which contains the superficial iridophores (We’ll talk about these later). Then, in the deeper dermis are the deep iridophores and xanthophores, erythrophores and melanophores. You may notice that never once in that explanation did, I use the word chromatophore, and that’s because the word chromatophore just describes all color-producing cells in amphibians, fish, reptiles, crustaceans and cephalopods (things like squids). You are a human and so you do not have chromatophores but instead the melanocytes we discussed earlier. Birds also have melanocytes.
So, let’s talk about the difference between the two. Melanocytes are one kind of cell, while chromatophores are a whole class of cells. The aforementioned iridophores, xanthophores, erythrophores and melanophores are all chromatophores. There are also other chromatophores not found in chameleons called cyanophores and leucophores. Chromatophores are very closely associated with the vertebrate lineage. They probably evolved around 450 to 480 MYA in the jawed fish – the Gnathostomata. Cephalopods and crustaceans evolved chromatophores independently, so we’ll ignore them, but all vertebrates at one point or another had chromatophores. Both birds and mammals evolved to also have the melanocytes and then eventually lost their chromatophores.
The biggest difference between the melanocytes and the chromatophores are in how the color is actually produced. We already talked about how melanocytes produce the pigment melanin which is how they appear colored. Chromatophores will also produce some pigments, but they also sometimes contain guanine nanocrystals arranged in lattices. When the light hits these crystals, the dispersed light is shown to be blue or turquoise. These crystals can also be dispersed or aggregated, and depending on their level of concentration, when light hits the crystals, the skin will appear to be different colors.
As I’m sure you know, Chameleons are capable of color change. And, when I first was interested in how they did this, I looked it up and was provided with the simple answer “Chameleons can change color due to their special chromatophores”. This is something most people interested in chameleons know, but I still feel as though using the word chromatophore as an explanation leaves you just as much in the dark as you were before. So, what I want to do here today is explain on a deep scientific level how chromatophores work.
VISIBLE LIGHT:
However, to understand how chromatophores work, you need to understand two things: how light works, and how human skin works. We’ll start with how light works. Light is a type of electromagnetic radiation, which is to say a wave that carries energy through space. Electromagnetic radiation can have different frequencies and thus, different energies. When electromagnetic radiation has any wavelength between 400 and 700 nm (or 1.63 eV – 3.26 eV of energy), it is called visible light because it can be picked up by the human eye. To show you how this works, let’s say we have a brick which appears red to us. The brick itself doesn’t have a color, but a molecular structure. When the sun emits its white light (white light is a combination of all visible light), and that light hits the brick, all colors aside from red are absorbed by the brick due to the brick’s chemical structure. When the red is reflected back to your eye, you find the brick to be red.
HUMAN SKIN:
Now, let’s look at this light in a more complex biological system. I, myself am Caucasian so my skin appears a kind of peach. A cell’s color is determined by the proteins they produce. Some proteins when hit by light reflect certain colors, making the protein appear those colors. When a cell has a sufficient amount of a given protein, it appears to be that color. “Skin cells” are keratinocytes, and they contain keratin which doesn’t reflect light at all. Therefore, keratinocytes are clear. However, within your mesh of keratinocytes, you also have some melanocytes (melanin-producing cells). Melanin reflects brown, black or yellow light and so, from a glance, my skin looks peach because of the small but not negligible amount of melanin in my skin compared to keratin. There are also other things that dictate your skin color such as carotene and hemoglobin, but I’m not going to talk about that. If you want to look into that, it’s very interesting stuff.
Now, if you want to talk about the organization of human skin, we have a top layer of keratinocytes, and after a few layers of this, you’ll find the bottom layer of skin – called the stratum basale which has keratinocytes in addition to the melanocytes.
CHROMATOPHORES ARE NOT THE SAME AS MELANOCYTES:
Let’s talk about chameleon skin, starting from the outermost layer. Chameleons, like people have keratinocytes on the outermost layers. As you go deeper past the outer epidermis (epidermis just means “protective layer of skin”), you’ll find the upper dermis which contains the superficial iridophores (We’ll talk about these later). Then, in the deeper dermis are the deep iridophores and xanthophores, erythrophores and melanophores. You may notice that never once in that explanation did, I use the word chromatophore, and that’s because the word chromatophore just describes all color-producing cells in amphibians, fish, reptiles, crustaceans and cephalopods (things like squids). You are a human and so you do not have chromatophores but instead the melanocytes we discussed earlier. Birds also have melanocytes.
So, let’s talk about the difference between the two. Melanocytes are one kind of cell, while chromatophores are a whole class of cells. The aforementioned iridophores, xanthophores, erythrophores and melanophores are all chromatophores. There are also other chromatophores not found in chameleons called cyanophores and leucophores. Chromatophores are very closely associated with the vertebrate lineage. They probably evolved around 450 to 480 MYA in the jawed fish – the Gnathostomata. Cephalopods and crustaceans evolved chromatophores independently, so we’ll ignore them, but all vertebrates at one point or another had chromatophores. Both birds and mammals evolved to also have the melanocytes and then eventually lost their chromatophores.
The biggest difference between the melanocytes and the chromatophores are in how the color is actually produced. We already talked about how melanocytes produce the pigment melanin which is how they appear colored. Chromatophores will also produce some pigments, but they also sometimes contain guanine nanocrystals arranged in lattices. When the light hits these crystals, the dispersed light is shown to be blue or turquoise. These crystals can also be dispersed or aggregated, and depending on their level of concentration, when light hits the crystals, the skin will appear to be different colors.
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