The Anatomy of the Retina

This nerve-filled issue layer lines the inner back wall of the eyeball

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The retina is a thin layer of tissue that lines the very back of the inside of the eyeball. The retina contains millions of cells that perceive light, color, and fine details in the things you see. A number of diseases can affect the retina, including cancer. If any part of the retina becomes damaged, your vision may be compromised.

Anatomy

The retina is a single layer of tissue that contains nerve cells that transmit images to the optic nerve. The parts of the retina include:

  • Macula: A small area at the very center of the retina. The macula provides the best focus for seeing small details on items directly in front of you, such as the text of a book.
  • Fovea: A tiny depression at the center of the macula. The fovea (also called the fovea centralis) is the point of sharpest focus.
  • Photoreceptor cells: These are the nerve cells that enable the eye to perceive light and color.
  • Cones: One type of photoreceptor cell, the cones sense and process the colors red, blue, and green to provide full-color vision. The retina holds approximately 6 million cones.
  • Rods: Another type of photoreceptor cell, responsible for sensing light levels and providing peripheral vision. The retina holds approximately 120 million rods.
  • Peripheral retina: the retinal tissue that extends beyond the macula. The nerves in the peripheral retina process peripheral vision.

Anatomical Variations

Retinal conditions may be present at birth and inherited (called inherited retinal disorders, or IRDs). These conditions include:

  • Achromatopsia: Complete color blindness
  • Choroideremia: Progressive loss of photoreceptor cells in the retina
  • Leber congenital amaurosis: A group of genetic mutations that affect nearly every nerve cell in the retina and cause severe impairment of vision
  • Retinitis pigmentosa: A group of genetic mutations that affect the retina’s photoreceptor cells
  • Stargardt’s disease: A genetic mutation that affects the macula

Function

As light enters the eye through the cornea, pupil, and lens, it projects onto the retina. The nerves of the retina process this light and the associated images, then transfer their signals to the optic nerve. The optic nerve transports these signals to the brain, where the perception of the images occurs.

The many nerve cells of the retina allow you to see in low-light conditions, perceive the sharp edges of delicate images like flower petals, perceive a full range of colors, and view a wide field of vision.

Associated Conditions

Despite the fact it’s somewhat protected inside the eye, the retina can be affected by a wide variety of conditions, including trauma. The most common conditions affecting the retina include:

  • Age-related macular degeneration (AMD): One of the most common types of retinal disease, AMD causes progressive central vision loss. It does not affect peripheral vision. AMD occurs in two types—dry and wet. Dry AMD is the more common type and occurs when the tissue layer of the macula becomes thinner with age. Wet AMD is rarer and occurs when new blood vessels grow abnormally in the retina, leak fluids, and cause scarring of the macula.
  • Cancers: Cancerous (malignant) tumors of the retina are rare but include diseases like retinoblastoma. Retinoblastoma occurs in children and results from an inherited gene mutation that causes retinal cells to multiply too quickly. Retinoblastoma may be curable, depending on whether or not the cancer has spread beyond the eyeball.
  • Detached or torn retina: A condition that results from the retinal tissue pulling away from the back of the eyeball. This can occur due to trauma (for example, a blow to the head) or due to a malfunction of the normal shrinking process of the fluid (the vitreous) that occupies the inside of the eye. The vitreous naturally shrinks a bit with age, but sometimes the shrinking vitreous sticks to the retina and pulls it away from the back of the eye.
  • Diabetic retinopathy: A deterioration of the retinal tissue due to excessive glucose (sugar) levels in the blood. Left untreated, diabetic retinopathy can lead to blindness.
  • Macular edema: A build-up of fluids in the retinal tissue that causes the macula to swell. This swelling distorts vision.
  • Retinitis pigmentosa (RP): Considered a rare disease, RP is thought to affect around one in 4,000 people in the U.S. This inherited disease causes mutations in any of 50 genes responsible for creating proteins that enable the photoreceptor cells of the retina to work. The signs and symptoms of retinitis pigmentosa usually arise in childhood, with sensitivity to bright light or poor low light vision. Most people with RP experience progressive vision loss to near-blindness.

Tests

Eye doctors (optometrists and ophthalmologists) use a number of tests to examine the retina directly and indirectly. These tests include:

  • Amsler grid: A simple printed grid made up of parallel lines running in two directions. If any of the lines look wavy, it could be a sign of macular degeneration.
  • Fluorescein angiography: A test that uses dye injected in a vein to illuminate the blood vessels of the retina. A special camera takes pictures of the retina after the dye has been injected.
  • Optical Coherence Tomography (OCT): A non-invasive imaging test of the retina. This test is similar to a computerized tomography (CT) scan and creates detailed, cross-sectional images of your retinal tissue.
  • Retinoscope: The bright flashlight-type instrument an eye doctor uses to look directly at the retinal tissue inside your eye.

Some of these tests require administering dilating eye drops that dilate (open) the pupil and make it easier to see the retina.

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Article Sources
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  • American Academy of Ophthalmology. Retina. Updated May 8, 2018.

  • American Academy of Ophthalmology. Retinal detachment. Updated March 1, 2016.

  • American Society of Gene & Cell Therapy. Inherited retinal diseases.

  • U.S. National Library of Medicine, MedlinePlus. Retinoblastoma. Updated March 4, 2020.

  • U.S. National Library of Medicine, MedlinePlus. Retina. Updated March 4, 2020.