TILIR #18: Ota, Ito, and blue milk

Blue flour-water
from Wikipedia

TIL: Nevus of Ota and nevus of Ito are two lesser know types of dermal melanocytosis, or gray-blue patches of skin. The more common form is the gray birthmark often found on the lower back or butt called congenital dermal melanocytosis, but more commonly known as Mongolian spots. The medical field is trying to shy away from this latter terminology... in case it offends Mongolians? It's not good or bad so not sure why it would offend. I think it might be more that the name is just arbitrary and inaccurate, with the condition having nothing to do with Mongolia or its people.

Anyways, nevi of Ota and Ito are gray-blue patched of skin in the distribution of the first/second branches of the trigeminal nerve (top half of the face) and the posterior supraclavicular and lateral brachiocutaneous nerves (neck, upper arm, upper back).

The Tyndall effect is what gives these lesions a bluish hue. This is the same effect that gives rise to blue eyes and turns milk with flour in it blue. In the milk, flour particles scatter light with the energetic blue light preferentially scattering over the red wavelengths.

In both eye color and the above skin lesions, the flour-like light-scattering particle is melanin, the dark pigment that colors skin. The bluish gray skin color with any of the dermal melanocytoses happens just like the blue milk, with melanin scattering the blue light and absorbing the red. Eye color is far more interesting. Irides with lots of melanin absorb almost all the light coming in because any blue scattered light from all but the surface-most layers of the iris just get caught by the melanin in front of it when trying to bounce out. These eyes tend to look dark brown.

Iris color diagram (made by me!)

The less melanin in the iris, the more blue light is able to escape back out of the eye after scattering. Bright blue eyes have the perfect goldilocks amount of melanin to scatter light but not get in its own way. Greens and hazels are somewhere in between. Albino persons have no melanin in their iris and so neither absorb nor scatter light coming in. The iris is basically see-through so the red of the retina behind can shine through giving the appearance of red eyes.