The retina is the eye’s inner layer that is sensitive to light and the choroid is the eye’s vascular layer that has numerous blood vessels.
The macula is an oval spot on the retina that has few ganglion cells unlike other regions of the retina that have numerous ganglion cells.
Ganglion cells are a type of neuron (nerve cell) that form the optic nerve that leaves the eye on its way to the brain.
In some disorders of lipid (fat) metabolism which may be inherited, the enzymes, which are proteins responsible in this case for the breakdown of fats are deficient or defective resulting in the abnormal excessive accumulation of fats in retinal ganglion cells.
The ganglion cells become opaque because of this abnormal accumulation of lipids, however, the macula does not become as opaque as the rest of the retina since it contains few ganglion cells. This means the underlying red vascular choroid with numerous blood vessels can still be seen through the macula when the eye is viewed using a fundoscope (ophthalmoscope), which is an instrument used to view the eye’s interior.
The macula therefore compared to the rest of the opaque retina appears as a cherry-red spot due to the visible underlying red choroid.
For somewhat different reasons, the cherry-red spot can also be seen in central retinal artery occlusion and other conditions.
Reference:
Suvarna JC, Hajela SA: Cherry-red spot. J Postgrad Med, 2008 54(1):54-57. Go to reference
Human eye
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