Eye of the beholder
The iris of the eye is the only internal organ normally visible from outside the body: In this image, taken with a scanning electron microscope (which can magnify objects up to 300,000 times), parts of the iris have been re-coloured to highlight the different structures. From looking at the structure of this organ in such great detail, we can get a pretty good idea of how it works. The iris contracts and expands to vary the size of the pupil, the aperture through which light reaches the rtina at the back of the eye.
It does this using two sets of muscles, both coloured pink in this image.
The sphincter muscles are closest to the pupil and are arranged like a drawstring around the top of a bag. When they contract, the pupil shrinks.
The second set of muscles, the dilator muscles (also coloured pink), are arranged like spokes in a wheel, and when they contract the pupil opens.
The yellow-coloured fibres attached to the dilator muscles are like tension cables, providing an anchor for the dilator muscles to pull against the sphincter muscles and open the pupil. This mechanism can change the amount of light passing through the pupil by up to 30 times, which enables us to have good vision in such a wide range of light levels. The other fibres in this image, coloured green and blue, are part of a structure called the iris stroma. This gives each of us eyes which are unique in texture and colour, ranging from pink in albinos, through greys, blues, greens, hazels to the darkest browns.
The fascinating thing is that eyes make only one pigment, melanin - the same one that also colours our skin and our hair. Most iris colours result from the different number and distribution of melanin-producing cells caught in the stromal fibres you see in the picture.
Albino eyes look pink because they don't produce any melanin. The pink colour is from red blood cells. Blue-eyed people have normal amounts of melanin in the cells at the back of the iris, but very little in the stroma at the front. This means that long wavelengths of light (red-orange) are absorbed at the back of the iris and short wavelengths (blue-purple) are scattered and reflected by the stroma, giving the iris a blue appearance.
People with brown eyes have much more melanin in the stroma absorbing more light and giving the iris its brown colour. Green eyes are different again, with an additional brownish-yellow pigment called lipofuscin present in the stroma.
We now know that eye colour is controlled by at least three pairs of genes, which play different roles in the cellular reactions that produce melanin.
But although genes may regulate how much melanin is produced in a cell, the way the cells are distributed in the iris stroma is random. The arrangement of the fibres in the stroma is also not under direct genetic control. This means that not only does every individual have a unique iris pattern of fibres and pigment, but also each eye is unique.
Scientists have taken advantage of this feature to develop a technology that enables people to be recognised on the basis of their iris pattern. A scanner locates the iris and takes a picture which can be stored as a unique iris code which can later be used to identify you. The machine can't be fooled by contact lenses or even by false eyes with irides painted on them. And the technology is not something of the future - you can see it now in at airports.
Philippa Bayley is a graduate student on the Wellcome PhD in Neuroscience program at University College London