The eye in brief
The eye is a remarkably refined optical instrument: the cornea, lens and retina work together to turn light into images, and the optic nerve carries them to the brain. Understanding its structure — and in particular the fragility of the retinal vascular network — is the first step towards understanding how diabetes can damage it.
- Cornea
- Transparent surface that refracts light — the eye's camera lens.
- Lens
- Lens that focuses images on the retina.
- Retina
- Light-sensitive inner membrane — the eye's film.
- Retinal capillaries
- Dense network of blood vessels that nourishes the retina; the first target of diabetes.
The eye as a camera
The eye works like a camera: it has lenses, an aperture and film. Light passes through four key structures before the brain turns it into an image.
CorneaLens
Light enters from outside through the cornea — the transparent surface that acts as the lens, bending the light rays.
Iris · Pupil · LensAperture + lens
Behind the cornea is the iris (the coloured filter), with the pupil at its centre — the aperture that controls how much light gets in. The lens is the part that focuses the image on the retina.
RetinaFilm
The image comes into sharp focus on the retina, which lines the whole back of the eye. Photoreceptors convert light into visual signals. The eye is filled with the vitreous body, a jelly-like substance.
Optic nerveTransmission cable
Visual signals are converted into electrical impulses and carried along the optic nerve to the brain, which interprets them as images. Any alteration in any of these structures leads to imperfect perception of images.
The retina and photoreceptors
The retina is the innermost membrane of the eye. Using its light-sensitive cells — the photoreceptors — it sends the information to be interpreted to the brain, through the optic nerve.
Among the cells that make up the retina are the cones, which are responsible for colour vision but are sensitive only to fairly bright light, and the rods, which are sensitive to low light levels but not to colour. Cones come in three types, responsible respectively for seeing the three primary colours (red, green and blue).
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Direction of the photoreceptors
Coloured histological section The retina is 'inverted': the photoreceptors sit in the outermost layer, resting on the retinal pigment epithelium, so light has to pass through all the other layers of retinal cells before reaching them. The pigment epithelium is rich in melanin: it absorbs light that has not been captured, preventing reflections, and nourishes the photoreceptors.
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The macula and sharp vision
Optic nerve — actual image Cones are concentrated in a central area of the retina called the macula, at the centre of which lies the fovea. This difference in photoreceptor density is responsible for the sharp vision at the point of fixation and for the blurred, less defined vision in the peripheral part of the visual field. The macula is the area most vulnerable to diabetic complications.
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Layers of retinal cells
Layering diagram The nerve cells of the retina are arranged in layers and classified as horizontal, bipolar, amacrine and ganglion cells. The axons of the ganglion cells form the optic nerve, which starts at an area called the optic disc — an area without photoreceptors, the so-called 'blind spot' or physiological scotoma.
In the fovea, where there are only cones, each cone effectively has its own ganglion cell, which is why vision there is so sharp; in the peripheral retina, by contrast, a single ganglion cell gathers signals from many photoreceptors. The test that measures the thickness and shape of the retina is optical coherence tomography (OCT) — an essential tool for diagnosing diabetic retinopathy and macular oedema.
Retinal circulation
The retina has a very high metabolic demand: to function it constantly needs oxygen and nutrients, supplied by a dense network of blood capillaries. This very vascular network is the target of diabetic retinopathy.
A fragile network
Retinal capillaries are among the smallest and most delicate in the body. Chronic hyperglycaemia from diabetes alters their wall, making them fragile and leaky. Microaneurysms form, fluid and proteins leak out, and some vessels become blocked, depriving areas of the retina of the oxygen they need.
The dual blood supply of the retina
The retina is nourished by two separate vascular systems: the retinal circulation proper — fed by the central retinal artery and its branches — and the choroidal circulation, which lies deeper and supplies the outer layers and the photoreceptors.
The macula has a central zone called the foveola, which has no blood vessels of its own: it depends entirely on the choroid for its nourishment. This particular anatomy makes the macula extremely vulnerable when the retinal circulation is altered, as happens in diabetic retinopathy. Diabetic macular oedema is the main cause of declining vision in patients with diabetes.