Describe the anatomy and physiology of the retina and the vitreous.

Structure of the Retina

• The retina is the innermost, neurosensory coat of the eye.
• It develops from the optic cup: the inner layer forms the neurosensory retina and the outer layer the retinal pigment epithelium.
• The two layers are apposed but never fused, and the potential space between them — the remnant of the embryonic optic vesicle cavity — is the subretinal space. This is the plane of retinal detachment, and it explains why a detachment separates the photoreceptors from the pigment epithelium rather than lifting the whole retina off the choroid
• The ten layers, from outside inward:
• 1. Retinal pigment epithelium — a single layer of hexagonal pigmented cells
• 2. Photoreceptor outer and inner segments — the rods and cones
• 3. External limiting membrane — junctions between Muller cells and photoreceptors

Blood Supply and the Blood-retinal Barrier

• The retina has A dual blood supply, and the division explains most retinal signs:
• The inner retina, from the internal limiting membrane to the inner nuclear layer, is supplied by the central retinal artery — an end artery with no anastomoses
• The outer retina — the photoreceptors and outer nuclear layer — is supplied by diffusion from the choriocapillaris
• This is why a central retinal artery occlusion produces a cherry-red spot. The inner retina becomes oedematous and opaque, but at the foveola the retina is so thin and has no inner layers.
• A cilioretinal artery is present in 15 to 30% of eyes, arising from the ciliary circulation; where it supplies the fovea it spares central vision in a central retinal artery occlusion
• The inner blood-retinal barrier is formed by tight junctions between retinal capillary endothelial cells; its breakdown causes the leakage of diabetic retinopathy and vein occlusion
• The outer blood-retinal barrier is formed by tight junctions of the retinal pigment epithelium; its breakdown causes central serous chorioretinopathy

Photoreceptors and Phototransduction

• Rods — about 120 million; absent at the foveola, densest at 20 degrees eccentricity; contain rhodopsin; responsible for scotopic (dim light) vision, with high sensitivity but low resolution and no colour discrimination
• Cones — about 6 to 7 million; concentrated at the fovea; three types with different opsins for short, medium and long wavelengths; responsible for photopic vision, acuity and colour
• Phototransduction — rhodopsin is opsin plus 11-cis retinal, a derivative of vitamin A. Light isomerises 11-cis to all-trans retinal, activating transducin, which activates phosphodiesterase, which hydrolyses cyclic GMP; the fall in cGMP closes sodium channels

The Vitreous

• The vitreous is a transparent gel of 4 mL, occupying 80% of the volume of the globe. It is 99% water, given its structure by a fine network of type II collagen holding hyaluronan.
• It has NO blood supply, NO nerves and NO capacity for regeneration; once removed or liquefied it is replaced by aqueous
• Attachments, in order of strength — the vitreous base, straddling the ora serrata (2 mm anterior and 4 mm posterior), which is so firm that it cannot be separated even surgically and which is why the retina tears at its posterior margin; the disc margin; the macula; along the retinal vessels; and at areas of lattice degeneration and old scars — which is precisely why lattice degeneration predisposes to retinal tears