What is the Blood Retinal Barrier?

The human body is very complex and there are some barriers within it that are more functional than structural. The blood-brain barrier is one example. This is a barrier that prevents the blood mixing with the cerebrospinal fluid, the fluid that bathes the spine. Partly formed by endothelial cells that stop bacteria or large molecules getting between the two sets of fluids, it can also prevent some drugs passing from the blood into the brain. This has been a problem for researchers trying to develop drugs to treat some types of brain cancer.

The blood retinal barrier is a similar type of barrier but it exists at the back of the eye. The inside of the eye has its own special microenvironment because of its unusual situation in the body. It is a highly sensitive organ and it contains delicate and intricate structures to enable us to see. Most specialised organs of this kind are well protected inside the body but the eyes are right at the front of the face and they are open to the world.

What is the Point of the Blood Retinal Barrier?

The front of the eyes are continually exposed to the outside world and they must be able to cope with debris, bacteria, fungi, viruses and parasites trying to get a foothold in the body through this apparently moist and soft entry point. The eye must maintain a high level of immunity to pathogens coming in from the outside but it must also protect its own delicate tissues in the retina. If the immune system was to be in overdrive most of the time, the retina would just become scar tissue and we would all be blind very early in life.

The blood retinal barrier prevents immune cells from the main blood supply getting into the eye to marshal a full blown immune response. It acts as a sentry point to keep the eye separate from the rest of the body so that the eyes can remain fully functioning, even though they are regularly exposed to foreign pathogens.

Where is the Blood Retinal Barrier?

The blood retinal barrier that prevents too much interchange between the main blood system of the body and the blood supply to the retina is actually at two different sites. Firstly, the endothelial cells that lie on the inside of the vast bed of blood capillaries that supply the retina, and secondly, the layer of cells between the blood system and the outer retina. These cells are the retinal pigment epithelial cells and they lie on top of a membrane called Bruch’s membrane between the blood vessels and the outer retina. This pigmented layer is often called the tapetum in cats – it’s what reflects from the back of the eye in photographs that have ‘white spots’ where the eyes should be.

What Happens When the Blood Retinal Barrier Breaks Down?

When the eyes are healthy and the two layers of cells that keep the blood retinal barrier in tact are strong and have tight junctions between the individual cells, there is no problem. Unfortunately, many eye conditions cause inflammation in the back of the retina, and this can even happen just because of the ageing process. When it does, the delicate balance is destroyed and things can deteriorate very quickly.

As the inflammation affects the blood vessels, this causes more inflammation and a build up of fluid between the layers of the retina. When this happens in the central part of the retinal, the result is macular oedema – and a loss of vision. Early signs can be blurring of vision, or a central hazy or blind spot, or floating cloud like sensations.

What Eye Conditions Are Responsible?

Age-related macular degeneration is one common cause of the complete loss of the blood retinal barrier. Other eye conditions include diabetic retinopathy – the damage to the retina that occurs in people who have had diabetes for many years, and retinal vein occlusion – when the vein taking blood away from the retinal gets blocked by a clot.

Is There Any Treatment?

There are efforts to develop steroid implants that can be placed into the eyeball at the first sign of inflammation due to one of these underlying diseases to see if that process can be stopped before irreversible damage is done to the blood retinal barrier. Once the delicate layers of cells have been disrupted, they cannot heal and loss of vision, although it may be halted by some treatments, can never be reversed. They may not look like anything special compared to the photoreceptor cells of the retina, but the cells that form the blood retinal barrier do help protect our sight.