Posts Tagged ‘Science’

Heterochromia in Anime.

October 10, 2011

Heterochromia iridis refers to a disparity in pigmentation either within the iris, or between the irides of the two eyes. Today I will be talking about complete heterochromia in humans, where each individual eye is a different colour. Personally, I find complete heterochromia to be an incredibly attractive trait, however I believe that it has turned into one of the worst possible cliché’s within anime, and I’d like to talk about it.

Heterochromia is both striking, and attractive. (Just like this girl, ba-dum)

The above image is clearly not true heterochromia in that the subject is in cosplay, using impossibly brightly coloured contacts (One of which is red, no less) but I hope you get my point… I consider heterochromia beautiful. In reality, things are a little bit different.

Ant’s Super Hyper Mega Biology Fun Time.
(Allow me to educate you.)

To keep things relatively simple, eye colour is determined by the concentration, location and type of pigments within the iris. These pigments or melanins are produced by a type of cell within the iris, called a melanocyte. The size and distribution of the melanocytes within the stroma and the iris epithelial tissue plays a large part in determining eye colour. It might surprise you to learn that most, if not all babies are born with blue eyes, as it takes time for these melanocytes to produce their pigments. Simply speaking, the more melanin in your eyes, the more brown your eyes are.

Little or no melanin within the stroma means blue eyes, a little means green, lots means brown. Other colours such as hazel, grey and amber are a result of the interaction of other pigments and proteins within the eye, whilst red is a complete lack of pigmentation in both the stroma and the iris epithelial tissue, which, unless someone has extreme albinism, is impossible as the epithelial tissue always has a thick layer of melanocytes. (Even most human albinos have blue eyes, I’ll have you know.)

Google, yaay.

Heterochromia can either be acquired, or have been present since birth. Acquired heterochromia is a result of one eye gaining or losing more pigment than the other, more “normal” eye. A brown eye may become green and then blue depending on severity, or vice versa. Acquired heterochromia is usually related to an hypoplasia (decreased growth) or hyperplasia (increased growth) of the iris tissue, or by damage to the melanocytes during development either through illness or physical damage. By killing off the melanocytes of one eye during development, that eye will stay blue, resulting in heterochromia. Genetic diseases leading to heterochromia in this manner can also be inherited however.

Chimeras and mosaics have two different sets of cells within their bodies, containing 2 different sets of genes. A chimera (yes, they do exist) is a result of the fusion of two fraternal twins, and the difference in DNA of the two sets of cells within the body can be as different as any sibling. A mosaic is a result of a mutation occurring during development resulting in a new set. If one eye receives one set of genes, and one eye the other set, and the different sets of genes code for different eye colours, the result will be heterochromia (eye colour is a polygenic phenotype so a change or difference in any number of genes could do  this.). This type of inherited heterochromia is much rarer than acquired heterochromia.

I would like to point out that complete heterochromia is listed as a rare disease by the office of rare diseases, meaning that less than 200,000 people in the USA have it at any one time. (or less than one in 1500 people) and whilst much of the time asymptomatic, it can be in itself a rare indicator of prevalent disease, such as a tumour of the eye, or a result of several serious illnesses, or physical injury. In anime, it seems fashionable to have a heterochromatic main character. You would think that this number is closer to about 1 in 10. Whilst real life heterochromia is rare, and possibly an indicator of health problems or defects, in fiction it allows for perhaps some decent character development or growth. In anime it is used little more than “because it looks cool”, or because they can use it as an excuse to give someone an eyepatch (A contact lens just isn’t enough) because it also “looks cool”. It is used so frequently, and without much impact on the character nowadays, that it just gets on my nerves. It used to feel unique, but after the Nth time seeing it, its gotten boring.

Next up, I’ll probably talk about how eye patches just plain tick me off, or how purple isn’t a real hair colour. But whatever. (more…)

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