Abnormalities of skin or coat color are of great concern to pet owners, especially when the abnormality is an obvious fault in a show animal. Of greater importance to veterinarians is the realization that these disorders may reflect a serious underlying disease. An understanding of the normal pigmenting process is necessary in order to correctly diagnose these disorders.

Three basic pigments and the optical effect called scattering are responsible for normal skin color. Scattering is the rearrangement of light as it passes through a turbid medium (e.g., skin, hair, and mucosa). The degree of pigment and the density of the medium containing the pigment combine to give the various colors.

Skin and Hair Pigments

Melanin. Melanin is formed by specialized cells (melanocytes) in the basal cell layer of the epidermis, hair follicle, and mucous membranes. Melanogenesis involves the oxidation of tyrosine to melanin. This reaction is catalyzed by a copper-containing enzyme called tyrosinase. Melanin is injected through dendritic processes from the melanocytes into epithelial cells. Some free melanin pigment is engulfed by dermal cells called melanophages. Melano-genesis is increased by ultraviolet light, increased temperature, and friction. Sulfhydryl compounds inhibit tyrosinase activity and thus decrease melanin production.

The intensity of melanin pigment is not determined by the number of melanocytes but by the size and number of melanin granules contained in epithelial cells. Dark-skinned animals have larger, more numerous melanin granules in several layers of epithelial cells.Albino individuals have sufficient melanocytes but cannot form melanin because of a defect in tyrosinase activity.

The colors produced by melanin range from brown when diluted to yellow, orange, or orange–red when concentrated. Melanin is a strong pigment and usually obscures the other pigments in dark-skinned animals.

Melanin is the primary black–brown pigment of hair and pheomelanin is the yellow–red pigment. Pigmentation may be uniform throughout the shaft or have alternating bands of varying degrees of pigment. White hair is largely devoid of melanin. Pigment cells within the hair root (bulb) deposit the pigment in or between the cortical and medullary hair cells. The amount of pigment placed in the hair is genetically determined and creates the optical effects characteristic of particular breeds. The distribution of melanocytes within hair follicles is also genetically determined. The distribution of melanocytes within hair follicles is also genetically determined. This accounts for spotting, ticking, and other color patterns.

Hemoglobin. Blood vessels of the mucous membranes, sparsely pigmented skin, and white nails are penetrated by light and thus contribute to the color of these tissues. The primary pigment is hemoglobin. Oxyhemoglobin is redder, and reduced hemoglobin is bluer. Thus, the overall hue is determined by the ratio of oxyhemoglobin to reduced hemoglobin. Rapid changes in skin, mucous membrane, or nail color result from changes in vessel diameter, blood flow, and degree of hemoglobin oxidation. Increased reddening of the skin, mucous membrane, or nails is caused by vasodilation, whereas vasoconstriction or hypotension has the opposite effect. Cyanosis is apparent when reduced hemoglobin is present in concentrations of 5 g or more per deciliter of blood. Extreme cold may also cause increased skin redness because the lower temperature may decrease oxygen utilization in local tissue. Thus, more oxyhemoglobin is present to give the red color.

Carotene. Carotene and its related pigments impart a yellowish color to the skin. It is relatively unimportant in small animals.

This process may involve melanin, hemoglobin, carotene, and pigments of endogenous (bilirubin) or exogenous origin. Melanin and bilirubin are the most clinically important pigments.

Melanosis