How Do Blue and Green Eyes Exist When Those Pigments Are Not Found in the Human Eye?

How Do Blue and Green Eyes Exist When Those Pigments Are Not Found in the Human Eye?


Eye color is a function of pigmentation both
at the back of the iris (iris pigment epithelium) and in its stroma (the front of the iris),
as well as the density of the cells in the stroma. In most cases these factors, and hence eye
color, are determined by genetics with potentially as many as 15 different genes identified to
date seeming to affect the ultimate coloring in some way. However, contrary to what you might think,
blue and green pigment do not exist in the human ocular fluid or iris. In fact, only a couple of pigments are involved
in eye color: melanin (brown) and lipchrome (yellow). Aside from potential extreme albinism cases,
all eyes have some amount of melanin in the back of the iris, although not all eyes have
lipochrome. For brown eyes, there is high concentration
of melanin in the back of the iris as well as in its stroma. The result is the melanin absorbing both short
and long wavelengths of light from across the visible spectrum, with the density of
melanin in these two layers ultimately determine the shade of brown, which in extreme cases
can appear near black. On top of the melanin, amber eyes get their
color from the added presence of lipochrome in the stroma. As we said earlier, blue and green pigments
do not play a role in human eye color. So how does anyone have blue or green eyes? Well, the remaining human eye colors, including
blue and green, are in part a trick of the light. With blue eyes, there is a fair amount of
melanin in the back of the iris, but relatively little melanin within the stroma, making it
translucent, with light getting scattered around when it encounters this layer. This scattering occurs more at the shorter
wavelengths of light, most relevant here being blue, while longer wavelengths are typically
passed through and ultimately absorbed by the melanin at the back of the iris. The net result is the eye appearing blue,
even though if you were to dissect the eye in question, you’d see the person physically
had brown eyes. This same type of thing is more or less what’s
happening in the sky when sunlight is scattered in the atmosphere, making it appear blue,
even though if the light from the Sun was not being scattered or absorbed in the atmosphere,
the sky would look similar to the way it does at night in the day time. Grey eyes work almost exactly like blue eyes,
with a good amount of melanin at the back of the iris absorbing the longer wavelengths,
but little within the stroma. The reason these people’s eyes don’t appear
blue is thought to be due to larger deposits of collagen that cause a different kind of
scattering that isn’t as frequency dependent. This type of scattering is basically what
is happening when water droplets in clouds cause them to look some shade of grey. In contrast to blue and grey, green eyes have
far less melanin at the back of the iris, so slightly less absorption; they also have
some lipochrome in the stroma. As with blue and grey eyes, there is little
melanin in the stroma and so a bit of light scattering helps impart a blue hue, the net
result of all of this being the eye appearing some shade of green. Hazel eyes, which can shift from brown to
green, have more melanin in the anterior (front) border layer of the eye, and, like green,
grey and blue eyes, scatter certain wavelengths of light resulting in a somewhat shifting
eye color. On that note, the fact that these latter hues
are largely dependent on a trick of the eye, rather than, for instance, blue pigmentation,
is why people with these eye colors sometimes appear to have a different eye color than
they normally do. Nothing has actually changed physically in
their eyes, but changing the lighting may end up changing the wavelengths and concentration
of light getting scattered and absorbed, resulting in the appearance of, for instance, a normally
slightly green eye looking slightly blue. As to why some people have two eye colors,
multiple eye colors can be caused by a variety of things from genetics to various medical
conditions, such as Horner’s syndrome and Waardenburg syndrome. Regardless of the underlying cause, the individual
in questions ends up with a non-uniform distribution of melanin in one or both eyes. In some cases, the individual might have two
completely different colored eyes (complete heterochromia). Perhaps the most common cause of this is genetic. However, other phenomena can cause a single
eye to completely change color, such as inflammation like that suffered by actress Mila Kunis,
which changed the color of one of her eyes. External causes include topical medications
used to treat glaucoma, potentially turning a blue eye to brown. Another variation of heterochromia occurs
where at least one eye displays two different colors throughout, sometimes with an inner
ring differing from the outer coloration (central heterochromia). Another type occurs where only a portion of
one eye has a different color. As with more uniform eye coloring, in all
cases the resulting eye color is largely a function of brown and yellow pigmentation
and its distribution and density in the back of the iris and in its stroma. Speaking of eyes, Warby Parker! Bonus Facts:
Ever wonder why you see random flashes and the like when you rub your eyes? Well, wonder no more: Called phosphenes, these
are characterized by perceiving some form of light when there is little or no light
actually entering the eye, making it an entopic phenomenon (meaning the source of the phenomenon
is within the eye itself). Phosphenes are most commonly introduced by
simply closing your eyes and rubbing them or squeezing them shut, tightly; generally
the harder you rub or squeeze, the more phosphenes you’ll see. This pressure stimulates the cells of the
retina and, thus, makes your brain think you are seeing light. Specifically, Grüsser et al. demonstrated
that pressure in the eye results in activation of retinal ganglion cells in a very similar
way to how they activate as a response to light. There are also several other ways phosphenes
can be generated. These include through: electrical stimulation;
intense magnetic fields; hallucinogenic drugs (phosphenes not to be confused with hallucinations,
which are generated in the brain, not the eye); a blow to the head or a particularly
severe sneeze where extra pressure is created in the eye on the retina and possible stimulation
of the visual cortex; low blood pressure, which can be experienced if you stand up really
fast after you’ve been relaxing; exposure to radiation; meditation or extended visual
sensory deprivation; various side effects from medical drugs; and certain diseases of
the retina and nerves. And if you’re wondering, the word “phosphene”
was coined by surgeon JBH Savigny deriving from the Greek “phos”, meaning “light”, and
“phainein”, meaning “to show”. Moving on, have you ever wondered what that
stuff in your eyes, commonly called “sleep” or “eye crusties” is when you wake up
in the morning? Well, wonder no more: “Sleep” is a type of
“rheum”, which is the name for discharge from your nose, mouth or eyes during sleep. More specifically, eye rheum is known as “gound”. Gound is made up of a mixture of dust, blood
cells, skin cells, etc. mixed with mucus secreted by the conjunctiva, as well as an oily substance
from the meibomian glands. The meibomian glands are a type of sebaceous
gland that line the rim of the eyelids with about fifty on the top and twenty five on
the bottom of each eye. They secrete an oily substance called meibum
that performs a variety of functions including: helps seal your eyes in an air tight fashion
when they are closed; prevents tears from spilling onto your cheeks; and helps keep
tears that coat your eyes from evaporating. It is this oily substance that is one of the
primary components in gound, mixed with mucin from the conjunctiva and various foreign particles
in your eye. Normally, when you are awake, the gound is
naturally washed away via tears and the blinking motion. However, as you sleep, you obviously don’t
blink so the meibomian secretions and other components of the gound tend to gather in
the corners of your eyes, as well as along your eye lines and dries out, creating hard
yellow-ish “eye boogers”.

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