retinas

The retina is a light-sensitive tissue at the back of the eye. It converts light into neural signals that are processed by the brain. Humans have two retinas, one in each eye; together they enable stereoscopic and color vision. The retina is part of the central nervous system and develops from neural ectoderm.

Anatomy and organization. The retina consists of multiple layers of neurons and supporting cells. The photoreceptor layer contains rods and cones, whose cell bodies reside in the outer nuclear layer. The outer plexiform layer houses connections between photoreceptors and second-order neurons; the inner nuclear layer contains bipolar, horizontal, amacrine, and Müller cells. The inner plexiform layer contains connections to ganglion cells, whose axons form the optic nerve. The macula, especially the fovea, provides high-acuity vision. The retinal pigment epithelium and choroid beneath support nutrition and waste removal.

Physiology and special cells. Phototransduction in rods and cones converts light into electrical signals. Cones mediate color and fine detail; rods dominate in dim light and motion. Intrinsically photosensitive retinal ganglion cells regulate circadian rhythms and pupil size. The retina's blood supply comes from the central retinal artery, with the choroid supplying the outer retina.

Clinical relevance. Common retinal diseases include retinal detachment, diabetic retinopathy, age-related macular degeneration, and inherited dystrophies such as retinitis pigmentosa. Diagnostic tools include ophthalmoscopy, optical coherence tomography (OCT), and fluorescein angiography. Treatments range from laser therapy and anti-VEGF injections to vitreoretinal surgery; research continues into retinal implants and stem-cell therapies.