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Agouti Hair

The cellular environment is also an important factor in determining the type of pigment produced by the melanocyte. A good example is the formation of agouti hair that can be seen in this animation. Agouti is the grayish brown color seen in many wild animals, such as mice and rabbits. To produce this type of hair, the melanocytes have to be able to produce both forms of melanin pigment. The mechanism is relatively simple.

When hair first begins to grow in an agouti animal, eumelanin is produced. However, after the hair has grown a defined amount, the surrounding epithelial cells get activated and start producing agouti protein. This protein binds to the MSH receptor and blocks the MSH hormone. Consequently, phaeomelanin is produced. The hair continues to grow until it reaches a certain point at which time the surrounding epithelial cells stop producing agouti protein. Thus, MSH hormone can bind to its receptors again, and eumelanin is produced. This alternating of eumelanin and phaeomelanin production produces a striped agouti hair that contains a band of phaeomelanin in the middle.

We should also mention that the presence of melanocytes in normal regions throughout the body depends on their successful migration to those regions and on the normal development of the neural crest from which they stem. Thus, mutations that affect coat color may act by interfering with these two critical developmental processes. This fact explains why some animals with coat color defects can also have hearing loss or other developmental disorders.


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