Friday, December 14, 2007

My eyes... my eyes... my eye color fascination

I admit, I am totally fascinated by eye color. I think it is pretty neat that the differing colors ultimately depend on the amount of melanin (pigment of skin and hair too) in the iris of the eye. Recall that the iris is the round structure in the front of your eye that forms and adjusts the opening called the pupil. Since melanin pigment ranges from black to brown, you might predict that eye (actually iris) colors should simply be shades of brown or light brown or even yellow... basically variations of brown. Yet, look around and you see eyes of blue, grey, green, and all shades of brown. Students are often surprised that there is no blue pigment in the iris or green pigment... just varying amounts of brown melanin. So how do we get all the amazing shades and colors of the iris? I was curious too...

Iris collage: digital images taken in BIO 25, Fall 2006/2007

Using Google, I came across the following wikipedia-ish website with a very nice description of the structural determinants of eye color... you should check it out.

According to this website, the overall determinants of eye color are the combination in the amount of melanin in the iris and the scattering of light (light bouncing back from an object) from the iris' structural, connective tissue layer (i.e., the stromal layer) made with lots of collagen. An iris with high amounts of melanin absorbs more light and appears as brown and there is very little scattering/reflection from the collagen layers of the iris so it appears smooth or velvety (see image above). Now, what about blue and green... in these cases there is less melanin pigment to absorb light and thus the light hits the collagen layer and is scattered/reflected as blue or grey-ish. With green/hazel eyes, there are moderate amounts of melanin which abrorbs some light and this would appear yellow or light brown but there is also some blue color from the remaining light hitting the collagen layers... yellow + blue =? Green. The greater amount of scattered/reflected light from the stromal layer in green or blue eyes gives the iris the weblike or stringy appearance as compared to the smooth brown eyes. But besides the differences in melanin, structurally the brown eyes would seemingly have the same stringy, weblike structure but you don't see it since the light is absorbed and not scattered. Get it? Freckles on the iris seem to be clusters of melanocytes with melanin pigment... oh yeah, the pupil is black since it is a hole and like a doorway into an unlit room, it looks dark or black. Shine enough light into the pupil and you get a reflection back from the retina that appears? Red due to blood vessels.... red eye in your pictures.

4 comments:

Unknown said...

I wonder how long it is going to be until the day people will pay to have eye color surgery to change their eye color to what they desire... I'll stick with my Super Dark Brown, although green would be nice.

Angelica said...

Have you checked out the HUMAN BODY: PUSHING THE LIMITS on the Discovery Channel? They had a episode on THE EYE and it was SUPER INTERESTING!

Unknown said...

What's it mean when you have freckles in your eyes that are black, like the pupil? My eyes are green with brown overlaying and there are tons of black, jagged freckles. My mom's are blue and one has a brown pigment spot. My freckles don't look like pigment spots.

Jessica Forester said...

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