Heterochromia Iridis

In anatomy, heterochromia refers to a difference in colouration, usually of the iris but also of hair or skin.

Heterochromia eyes blue on the outside with or...

An eye with partial heterochromia

Heterochromia is a result of the relative excess or lack of melanin; a pigment. It may be inherited, or caused by genetic mosaicism –  the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg – other disease, or injury.

Heterochromia iridis is a heterochromia of the eye. There are two main kinds:

  • Complete heterochromia, one iris is a different colour from the other.
  • Partial heterochromia, part of one iris is a different colour from its remainder.

Eye colour, specifically the colour of the irises, is determined primarily by the concentration and distribution of melanin. The affected eye may be hyperpigmented (hyperchromic) or hypopigmented (hypochromic).

English: Heterochromia: Female cat with eyes o...

A cat with complete heterochromia

In humans, usually, an excess of melanin indicates hyperplasia (an increase in number of cells/proliferation of cells) of the iris tissues, whereas a lack of melanin indicates hypoplasia (a decrease in number of cells/proliferation of cells).

Although infrequently seen in humans, complete heterochromia is more frequently observed in other species, where it almost always involves one blue eye. The blue eye occurs within a white spot, where melanin is absent from the skin and hair.

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