Anatomy of the Retina
The retina is a thin, transparent, delicate layer of neural tissue lining the inner surface of the posterior segment of the eye. It is the most highly developed tissue of the eye. It appears purple red due to the visual purple of rods and the underlying vascular choroid. It plays a crucial role in vision by converting light into neural signals, which are then transmitted to the brain for image processing. Understanding the anatomy of the retina is fundamental for comprehending how vision works and for diagnosing and treating various ocular diseases.
Gross Anatomy
Location and Structure
Located between the choroid (outside) and the vitreous body (inside).
Extends from the optic disc (where the optic nerve exits the eye) to the ora serrata (the anterior margin).
Approximately 0.1–0.5 mm thick, thickest near the optic disc and thinnest at the ora serrata.
The posterior pole of retina is best examined by slit lamp, indirect ophthalmoscopy using +78D and +90D lens, and direct ophthalmoscopy.
Regions of the Retina
Optic Disc
Pink colored, well-defined circular area of 1.5 mm diameter.
All retinal layers terminate here except the nerve fiber layer, which passes through the lamina cribrosa to form the optic nerve.
A depression called the physiological cup is present.
Central retinal artery and vein enter through the center of this cup.
Lacks photoreceptors.
Macula Lutea
Also called the yellow spot.
Slightly deeper red than surrounding fundus, located temporal to the optic disc.
Central area of the retina responsible for high-acuity vision.
About 5.5 mm in diameter.
Contains a central depression known as the fovea centralis.
Fovea Centralis
Small depression in the macula.
Contains only cone photoreceptors → responsible for sharp central vision.
About 1.5 mm in diameter, most sensitive part of retina.
Center contains a shiny pit called foveola (~0.35 mm).
Located ~2 disc diameters temporal to optic disc and 1 mm below horizontal meridian.
No rods present.
Cones are tightly packed; other retinal layers very thin.
Only the internal limiting membrane is present.
The area (~0.8 mm diameter) around the foveola without retinal capillaries is called the foveal avascular zone.
Surroundings: parafoveal and perifoveal regions.
Peripheral Retina
Outside the macula.
Contains more rods → peripheral & night vision.
Best examined with indirect ophthalmoscopy and Goldmann’s three-mirror contact lens.
Ora Serrata
Serrated anterior margin where retina ends.
Firmly attached to vitreous and choroid.
Pars plana extends anteriorly from ora serrata.
Microscopic Anatomy
Layers of the Retina (10 layers, outer → inner)
1. Retinal Pigment Epithelium (RPE)
Outermost layer, single layer of pigmented cells.
Absorbs stray light, supports photoreceptors, visual cycle.
Firmly attached to basal lamina of choroid.
2. Photoreceptor Layer (rods & cones)
End-organs of vision (120 million rods, 6.5 million cones).
Rods: contain rhodopsin (visual purple), for peripheral & scotopic vision (low light).
Cones: contain photopsins, for central & color (photopic) vision.
3. External Limiting Membrane
Formed by junctions between photoreceptors & Müller cells.
4. Outer Nuclear Layer
Cell bodies (nuclei) of rods & cones.
5. Outer Plexiform Layer
Synapses between photoreceptors, bipolar cells, and horizontal cells.
6. Inner Nuclear Layer
Contains cell bodies of bipolar, horizontal, amacrine, and Müller cells.
Bipolar cells = first-order neurons.
7. Inner Plexiform Layer
Synapses between bipolar, amacrine, and ganglion cells.
8. Ganglion Cell Layer
Cell bodies of ganglion cells.
Ganglion cells = second-order neurons of visual pathway.
9. Nerve Fiber Layer
Axons of ganglion cells → converge to form optic nerve.
10. Internal Limiting Membrane
Innermost layer.
Formed by footplates of Müller cells.
Types of Cells in the Retina
Photoreceptors
Rods → low light, night vision.
Cones → color vision, high acuity.
Bipolar Cells
Relay from photoreceptors → ganglion cells.
Ganglion Cells
Axons form optic nerve → transmit visual signals to brain.
Horizontal Cells
Integrate/regulate input from multiple photoreceptors.
Amacrine Cells
Modulate transmission between bipolar & ganglion cells.
Müller Glial Cells
Provide structural & metabolic support.
Blood Supply
Central Retinal Artery → inner 2/3 of retina.
Choroidal Circulation → outer 1/3 including photoreceptors.
Clinical Relevance
Retinal Detachment → separation from RPE, vision loss.
Macular Degeneration → affects central vision.
Diabetic Retinopathy → vascular damage due to diabetes.
Retinitis Pigmentosa → genetic, progressive photoreceptor loss.
Summary
The retina is a complex multilayered structure crucial for vision. Specialized cells and layers convert light into neural signals, processed by the brain to form visual images. Knowledge of retinal anatomy is essential for diagnosing and treating ocular diseases.
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