What is the macula?
The macula is the central part of the retina. It is about 5mm across and is the most sensitive part of the retina. It is the part of the retina used when looking directly at objects, such as when reading or recognising faces at a distance. At its centre is the fovea, where there is the highest concentration of cone type of photoreceptor (light-sensing cells). These are the cells that see fine detail in good light and also see colour.
Unfortunately, many retinal diseases affect the macula more than other parts and become more common with age.
The relationship between the macula and the retinal pigment epithelium (RPE) and choroid is the same as for the rest of the retina. However, to understand macular diseases better, including age-related macular degeneration (AMD), a little more detail is needed. The centre of the fovea is much thinner than the rest of the macula and is densely packed with cones. Under the fovea is the RPE, which is a layer of cells that acts as a caretaker for the macula; the photoreceptors and RPE cells interlock. The RPE cells lie in a single layer on a very thin sheet of material called Bruch’s membrane, under which is the choroid.
Under normal circumstances Bruch’s membrane acts as a barrier to blood vessels from the choroid growing into and under the macula itself. At the same time, oxygen and nutrients are free to pass from the choroid to the macula and waste products from the macula to the choroid. The macula is very active and uses a lot of energy and oxygen. Surprisingly, it uses more oxygen when asleep than in the day when reading; this is why ‘resting your eyes’ by closing them is not necessary for patients with macular disease.