How Color Change Works
 

Fig. 1 Shows distended melanophore fingers with melanosomes between iridophores and xanthophores. Used with permission by RUP.

 

 

Fig. 2 Shows from left to right: skin adapted to white background, melanosomes surrounding iridophores, yellow spots over iridophores, same as before except activated melanosomes, melanophores that are widespread, adapted to light background -- green combined effec of xanthophore and iridophores, and skin with xanthophores removed leaving iridophores. Used by permission of RUP.
     
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Reptiles primarily use pigmented containing cells, chromatophores, in a specific orientation for color change -- sometimes referred to as the dermal chromatophore unit (Fig. 3).  Depending on the species, there are different kinds of chromatophores, which all work together to reflect a characteristic color to the viewer.  These cells are located in the dermis. Xanthophores (yellow), melanophores (black/brown), and erythophores (red) are the most common color cells found in reptiles (Fig. 2). Each contains different pterinsomes (i.e. pigmentary organelles). Iridophores are also used to reflect light into the colored cells.  Light enters the skin and is reflect when makes contact with iridophores. Therefore, there is usually a particular structure used to produce a color. For example, if a reptile is prompted to change to a darker color, melanophores will move to cover other chromatophores and absorb light from iridopohores.  Histologically, melanophores are found underneath (between) other colored cells, such as xanthophores, which are above iridophores (Fig. 1). (Kuriyama et al. 2006) 
Melanophores are generally more active than the other chromatophores. While color-producing cells are stationary, melanophores, once stimulated, will secrete melanosomes, which move to "cover" the color-producing cells above (Fig. 1).  A gradient of melansomes occurs, showing different intensities of color. The secretion of melanosomes also causes the melanophore to expand, blocking more light (Taylor and Goldman 1969; Bagnara et al. 1967).

 

Fig. 3 The dermal chromatophore unit. This structure is found in anoles. Used by permission of RUP.

Fig. 4 Example of zebrafish melanophore dilating. Used Permission from Wikipedia under the GNU Free Documentation License.

 

There are different pathways of color change stimulation.  Two main physiological pathways include a hormonal or neural response to a stimulation (though, there are some species that use a combination of both).  For example, in the lizard, Anolis carolinensis, a stimulated pituitary gland will secret melanoctyte-stimulating hormone (MSH) which will then activate the melanophore.  In other species, such as chameleons, melanophores are directly stimulated by the nervous system, usually incorporating the neurotransmitter, norepinephrine (Taylor and Hadley 1970; Taylor and Goldman 1969). The fact that neural response is faster than hormonal means that color change will occur faster in reptiles that use such a pathway (Stuart-Fox et al. 2008)

The release of MSH or neurotransmitters is the result of many factors, which includes stress, illumination, and temperature. In the case of illumination, some reptiles, such as anoles, detect light via their eyes but also with photoreceptors on their skin. Generally speaking, the warmer it is, the lighter the reptile will be in order to absorb less light and therefore heat(Taylor and Hadley 1970; Macedonia 2001).

 

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