The retina and retinal pigment epithelium (RPE)

The retina and the retinal pigment epithelium (RPE) are crucial for vision

The Retina听

The retina听is听the light sensitive听region听of the eye听and听can be easily viewed by听healthcare practitioners听using听light connected to a magnifying lens (an ophthalmoscope).听A听snapshot of the retina听can also be taken using a microscope attached to a camera. This听so called听fundus听image听allows eye doctors to examine the health of the retina and identify听any potential issues that may affect sight.听The retina is transparent, allowing eye doctors to see听the blood vessels supplying the retina. The eye is the only part of the body in which blood vessels can be directly viewed. Doctors can also use the fundus image to look at the health of the optic disc and the macular region.听

The听central area of the retina听is known as the听macula, (Latin for yellow spot).听Within the听macula听a听specialised听indentation, called the fovea, contains the听greatest concentration of cone photoreceptor cells, the cells听responsible听for detecting colour. This high concentration of cone cells听allows for听the听fine听vision needed to read, drive, watch TV and recognise faces. Next to the macula is a bright spot, the optic听disc.听The optic disc is the start of the optic nerve, where the听electrical听visual signal听produced by the retina is听carried to the brain to form an image. The optic disc creates our听鈥榖lind spot鈥櫶齛s no photoreceptor cells reside here.听听

A schematic showing the anatomy of a fundus image.

The retina is composed of 10 distinct听layers听composed of different听cell types, with听specialised听roles.听Light听passes听through all the layers of the retina before it reaches听the听layer containing the听photoreceptor听cells. The photoreceptor layer of the retina consists of light sensitive cells known as rods and cones. There are around 125 million rods and 7 million cones in our eyes. Rods听are needed听for vision in dim light/night time听conditions, whereas cones work best in bright light conditions and听are听essential for distinguishing between colours and forming a clear听accurate image.听Photoreceptor cells听are highly specialised听cells听that听absorb light and, through a cascade of chemical signals, convert it into electrical information needed to visualise an image.听听

A schematic showing the anatomy of the retina. Light entering through the pupil at the front of the eye, passing through the retina layers until reaching the retinal pigment epithelium (RPE)

The retinal pigment epithelium听听

Beneath the photoreceptors is the retinal pigment epithelium (RPE). The RPE is a single layer of cells tightly joined听so that they form a barrier between the retina and the听underlying choroid. The RPE has crucial functions听that are needed to keep the retina alive and听enable photoreceptor cells to detect听light.听听

Phagocytosis听of outer segments

In order to work effectively, the light absorbing parts of the photoreceptor cells need to be replaced daily.听The RPE acts a recycling station for听this waste material, collecting this debris in a process known as phagocytosis,听before听recycling the useful parts, which can then be reused. The RPE cells, ensure that debris does not build up underneath the retina.听

Absorption of stray light

The RPE听are pigmented because they听contain a dark pigment听molecule听called melanin. Melanin absorbs听any stray light听that may interfere with听the visual image.听

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The RPE听form a barrier between the听retina听and the choroid.听The RPE is a single layer of cells underneath the retina. Individual RPE听cells听are tightly joined to their neighbours, producing an effective barrier听that听regulates听the transport of听nutrients, water and听molecule听solutes听between the retina and choroid. This barrier also prevents the infiltration of new blood vessels from the choroid. When the RPE barrier is compromised during disease this can lead to choroid neovascularisation (growth of new blood vessels into the retina).听

A schematic showing the retinal pigment epithelium RPE acting as a barrier between the choroidal blood supply and the photoreceptor layer, along with the rest of the retina.

Recycling vision protein

The RPE acts as听recycle station, recovering key proteins from retinal waste听before returning them back to the听photoreceptor cells.听听

听

RPE cells are not light-sensitive, but they do help maintain the light sensitive photoreceptors in the retina. Without听functional RPE, the photoreceptor cells become damaged and vision deteriorates. This phenomenon is seen in many diseases including age-related macular degeneration and听inherited听forms of blindness, such as听bestrophinopathies.

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