One Eye Is Blue, The Other Is Brown: A Mysterious Disease - Heterochromia iridis
By: Stefan Anitei, Science Editor - Softpedia
Human heterochromia
Do you know what David Bowie and many Border Collie dogs have in common? It is heterochromia, a condition in which the eyes are colored differently.
As heterochromia can also be encountered for hair or skin, the ocular one is called heterochromia iridis or heterochromia iridium. The iris can have a different hue
from the other (complete heterochromia), or only a part of it differs from the remainder (partial heterochromia or sectoral heterochromia).
The color differences are determined by variations of the melanin pigment within an iris, due to genetic causes (like Horner's syndrome), mosaicism (mutation occurred during the development), disease or injury. When hereditary, heterochromia is a dominant trait. The affected iris can be hyperpigmented (hyperchromic) or hypopigmented (hypochromic), usually connected with hyperplasia (overgrowth) of the iris tissues or hypoplasia (less tissue).
The melanocytes (melanin contains cells of the iris) do not synthesize in the absence of innervations, and unilateral congenital or birth injury of these nerves induces heterochromia.
Iris colobomata (gaps in the iris) produces heterochromia, the darker iris being the one with coloboma.
Microphthalmia (small eyes), determined by mutation or fetal alcohol syndrome, can induce heterochromia. Neoplasms (benign tumors) in the iris can change the color of just one eye (e.g. Lisch nodules). Melanosis (the abnormal increase of melanocytes) and port-wine stains make one eye darker.
Heterochromia in puppies of Border Collie
Individuals suffering from albinism, piebaldism (partial albinism), incontinentia pigmenti (hypopigmentation) are prone to heterochromia. Accidental iron deposits in the eye, iritis (iris infection) and certain eyedrops can induce heterochromia.
Heterochromia is much more common in mammals than in humans, particularly in some breeds, like cats (with one copper/orange eye and one blue eye), dogs, horses ("wall-eyed" with one brown and one white/blue eye), and water buffalo. In cats, the most prone are the Japanese Bobtails, Turkish Vans and the Turkish Angora while in dogs the Siberian Huskies, Australian Shepherds, and Border Collies.
Partial or sectoral heterochromia is a lot less frequent than total heterochromia and it’s usually associated with autosomally genetic diseases like Hirschsprung's and the Waardenburg syndrome.
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