Retina of the eye: structure and function + retinal diseases

The retina plays a central role in vision – it converts light into electrical signals that are transmitted to the brain. But how exactly does the retina work and what happens when it becomes diseased? In this blog, you will learn all about the structure of the retina, the function of cones and rods and why the macula is so important as the site of sharpest vision.

We also shed light on how diseases such as retinal detachment, macular degeneration, diabetic retinopathy and retinitis pigmentosa can affect vision – and what treatment options are available. Stay tuned and immerse yourself in the fascinating world of the retina!

Structure of the eye – an overview

The eye is a complex organ that converts light into visual information in an impressive way. Various structures of the eye work together to produce clear images and transmit them to the brain. The most important components are the cornea, the lens, the vitreous body and the retina.

• Cornea: The transparent, outer layer of the eye that focuses light and transmits it to the lens. It acts like the window of the eye.
• Lens: Located directly behind the pupil, the lens serves to focus the light by changing its shape. In this way, the light is projected onto the retina to create a sharp image.
• Vitreous humor: A gel-like, transparent substance that fills the eyeball. The vitreous body supports the eye and transmits light to the retina.
• Retina: The retina is the innermost layer of the eyeball. It contains millions of light-sensitive sensory cells (rods and cones) that convert the incoming light into electrical signals.

How the eye processes light:

1. The light passes through the cornea and reaches the lens through the pupil.
2. The lens focuses the light, which then passes through the vitreous body to the retina at the back of the eye.
3. On the retina, especially in the macula (the center of the retina), light stimuli from rods and cones are converted into electrical signals.
4. These signals are transmitted through the optic nerve to the brain, where the image is processed.

This finely tuned process enables us to see in both dim light and bright light.

Structure of the retina of the eye: rods and cones

The retina of the eye is the central element for vision and consists of several layers of cells, including the light-sensitive photoreceptors – rods and cones. These two cell types are crucial for the perception of light and color.

• Rods: There are around 120 million rods in the retina. They are highly sensitive to light and enable us to see in dim light and distinguish between light and dark contrasts. Rods are mainly located in the peripheral areas of the retina and are less responsible for sharp vision.
• Cones: In contrast to the rods, there are around 6 million cones, which are responsible for sharp vision and color recognition. They are most densely arranged in the center of the retina, particularly in the macula (also known as the yellow spot ). The fovea, the center of the macula, is the area of sharpest vision and contains almost exclusively cones.

Function of the rods and cones:

• Rods: Serve to perceive light stimuli in low light conditions (twilight, night) and are specialized for light-dark vision.
• Cones: Enable sharp vision in daylight and are responsible for color perception. They react to different wavelengths of light, which allows us to distinguish colors in shades of red, green and blue.

Macula: the point of sharpest vision

The macula, also known as the yellow spot, is a small but extremely important area of the retina. It is located in the center of the retina and is responsible for sharp vision and the recognition of fine details and colors. The macula contains a particularly high density of cones, the light-sensitive cells responsible for color vision and detail recognition.

Function of the macula in the visual process:

• Enables fine details to be recognized.
• Responsible for color recognition thanks to the high density of cones.
• Central for sharp vision during activities such as reading, face recognition or focusing on objects.

The 10 layers of the retina:

The retina is a complex, multi-layered structure that converts light into electrical signals and transmits them to the brain via the optic nerve. Its layers play a crucial role in the visual process by processing light stimuli, converting them into nerve impulses and contributing to visual perception. The retina consists of a total of ten layers, each of which has a specific function.

1. Pigment epithelium:
   • The outer layer of the retina that borders on the choroid. It is crucial for the exchange ofnutrients and oxygen between the retina and the blood vessels. It also ensures the regeneration of the photoreceptors (rods and cones).
2. Photoreceptors (rods and cones):
   • This layer contains the light-sensitive sensory cells – rods (for seeing in dim light) and cones (for colors and sharp vision). They convert light stimuli into electrical signals.
3. Outer limit diaphragm:
   • Separates the photoreceptors from the subsequent layers and supports the structural integrity of the retina.
4. Outer grain layer:
   • This layer contains the cell bodies of the photoreceptors in which the signals are processed.
5. Outer plexiform layer:
   • Here the photoreceptors connect with the bipolar cells and transmit their signals further into the retina.
6. Inner grain layer:
   • This layer contains the cell bodies of bipolar cells, horizontal cells and amacrine cells. Bipolar cells are important for the transmission of signals from the photoreceptors to the ganglion cells.
7. Inner plexiform layer:
   • This layer is the place where bipolar cells come into contact with ganglion cells in order to transmit visual information.
8. Ganglion cell layer:
   • Contains the cell bodies of the ganglion cells, which bundle the electrical signals and transmit them to the optic nerve.
9. Nerve fiber layer:
   • The axons of the ganglion cells run through this layer and bundle together at the blind spot to form the optic nerve. This layer transmits the information to the brain.
10. Inner boundary membrane:
    • The innermost layer of the retina, which forms the transition to the vitreous body.

Diseases and effects on the function of the retina: Causes, symptoms and treatments

Retinal detachment

Retinal detachment (ablatio retinae) is a serious eye disease in which the retina of the eye detaches from its underlying layer, the pigment epithelium. This layer supplies the retina with oxygen and nutrients, so detachment can lead to loss of vision if not treated quickly.

Causes of retinal detachment:

• Age: With increasing age, the vitreous body that fills the eye can detach from the retina and cause tears or holes in the retina. These tears are the most common cause of retinal detachment.
• High myopia: People with high myopia have an increased risk because their eye structure is stretched, making the retina more susceptible to tears.
• Eye injuries: Injuries to the eye, for example due to accidents or blows, can damage the retina and trigger a detachment.
• Retinal diseases: Pre-existing conditions such as diabetic retinopathy or macular degeneration canalso lead to retinal detachment.
• Genetic predisposition: Some people have a hereditary predisposition that increases the risk of retinal detachment.

Symptoms of retinal detachment:

• Flashes in the eye: Sudden flashes of light that are often a sign of an incipient detachment.
• Moving shadows or veils: Affected people often describe that it feels as if a dark curtain is being drawn over part of their field of vision.
• Floating spots (mouches volantes): An increased occurrence of streaks or spots in the field of vision may indicate tears in the retina.
• Deteriorated vision: If the retinal detachment progresses, central or peripheral vision may be impaired.

Treatment of retinal detachment:

Retinal detachment requires immediate treatment to prevent permanent loss of vision. The treatment options depend on how far the detachment has progressed:

• Laser therapy: If a tear in the retina is discovered before a complete detachment occurs, it can be closed using a laser. The laser “welds” the retina back into position.
• Cryotherapy (cold treatment): For smaller tears or holes, cold can also be used to reattach the retina.
• Vitrectomy: In cases of advanced retinal detachment, the vitreous body is surgically removed and replaced with a liquid or gas to push the retina back into place.
• Sealing procedure: A small silicone band (seal) is placed around the eyeball to press the retina back against the pigment epithelium.
• Pneumatic retinopexy: A gas is injected into the vitreous cavity, which presses the retina against the eye wall so that it can heal.

Prognosis and aftercare:

The earlier the detachment is treated, the better the chances of recovery. If the optic nerve or the macula(the area of sharpest vision) is already affected, the quality of vision may remain impaired after the operation. Regular eye checks are important after a retinal detachment, as new detachments can occur.

A retinal detachment is a medical emergency that must be treated immediately to prevent permanent blindness.

Macular degeneration

Macular degeneration affects the macula, the area of the retina responsible for sharp vision. It usually occurs in old age and is a common cause of vision loss.

Causes of macular degeneration:

• Age: The main risk lies in age, especially from the age of 60.
• Smoking and diet: Smoking damages the blood vessels in the eye, and a poor diet can accelerate the progression.
• Genetic predisposition: There is an increased risk if there are known cases in the family.

Symptoms of macular degeneration:

• Blurred vision: The center of the field of vision becomes blurred.
• Distorted lines: Straight lines suddenly appear wavy or crooked.
• Dark spots: Blind spots may appear in the central field of vision.

Treatment of macular degeneration:

• Nutrition: Food supplements with lutein, zinc and vitamins can slow down the progression.
• Injections (wet AMD): In the wet form, anti-VEGF injections help to stop the formation of leaky blood vessels.
• Visual aids: Magnifying glasses and aids make everyday life easier for people with impaired vision.

Early diagnosis and regular check-ups are crucial to slow down the progression of the disease.

Diabetic retinopathy

Diabetic retinopathy is a complication of diabetes that affects the retina of the eye. The high blood sugar level damages the blood vessels of the retina, which can lead to loss of vision if left untreated. It is one of the most common causes of blindness in adults.

Causes of diabetic retinopathy:

• High blood sugar levels: Long-term elevated blood sugar levels damage the fine blood vessels in the retina.
• Duration of diabetes: The longer someone has diabetes, the higher the risk of developing retinopathy.
• High blood pressure and cholesterol: These factors aggravate vascular damage and accelerate the progression of the disease.

Symptoms of diabetic retinopathy:

• Blurred vision: One of the first symptoms is blurred vision, which often occurs gradually.
• Floating spots: Small, dark spots or streaks caused by bleeding into the retina.
• Deterioration of vision: Without treatment, vision loss progresses and can lead to blindness.

Treatment of diabetic retinopathy:

• Blood sugar control: The most important measure is strict control of blood sugar levels in order to slow down the damage to the retina.
• Laser therapy: This method can obliterate leaky blood vessels and thus slow down the progression of the disease.
• Injections: Similar to wet macular degeneration, anti-VEGF injections can be used to inhibit the growth of abnormal blood vessels.
• Vitrectomy: In advanced cases, surgical removal of the vitreous body (vitrectomy) may be necessary to eliminate bleeding.

Retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited retinal disease in which the photoreceptors (rods and cones) in the retina gradually deteriorate. The disease leads to a gradual deterioration in vision and can later lead to blindness.

Causes of retinitis pigmentosa:

• Genetic predisposition: RP is caused by genetic mutations inherited from the parents. It affects the sensory cells of the retina, which are responsible for the perception of light.
• Progressive degeneration: The rods, which are responsible for low-light vision and peripheral vision, are usually affected first.

Symptoms of retinitis pigmentosa:

• Night blindness: One of the earliest symptoms is the inability to see well in dim light or darkness.
• Tunnel vision: As the disease progresses, the field of vision narrows, leading to tunnel vision. Peripheral vision is severely restricted.
• Deterioration of central vision: In the later stages, central vision may also be impaired, making it more difficult to see clearly.

Treatment of retinitis pigmentosa:

• Currently no cure: There is no cure for RP, but the disease can be slowed down.
• Vitamin A therapy: Studies have shown that high doses of vitamin A can slow down the progression of the disease in some cases.
• Special visual aids: Magnifying visual aids and other aids can help those affected to cope better with everyday life.
• Gene therapy and research approaches: Various research projects are underway focusing on gene therapies or stem cell therapies to halt or reverse the progression of the disease.