Layers of the Retina

The retina is a delicate, semi-transparent, multilayered sheet of neural tissue that lines the inner surface of the posterior two-thirds of the globe's wall. With a surface area of approximately 266 mm², it extends from the optic disc to the ora serrata.

1.0Visible Landmarks of Human Retina

• Optic Disc 
• Retinal Blood Vessels 
• Area centralized with fovea and foveola 
• Peripheral retina and ora serrata 
• Thickest near the optic disc 
• Thin towards the peripheral

Optic Disc

• Circular or slightly oval app. 1.5mn 
• The central contains a depression known as the Physiological Cup

Area centralis with fovea and foveola 

• It is demarcated by upper and lower arcuate and temporal retinal vessels. 
• It corresponds to an app. 150 of the visual field and is adopted for accurate diurnal vision and colour discrimination. 
• It is divisible into fovea and foveola. 

Fovea

• The centre of the area is 4mm temporal to the centre of the optic disc.
• In this layer, there are no rods. 
• Cones are larger and more abundant; their central part consists of cones and their nuclei covered by a thin internal limiting membrane. 
• All other layers are absent in this region. In the centre of the foveola is a tiny depression known as an umbo. It corresponds to the foveolar reflex.

Equator

• The equator of the fundus is located about 14 to 15 mm away from the limbus. A simple way to find the fundus equator is to locate the vortex veins. 
• Choroidal veins drain into a vortex vein in each quadrant of the eye. Typically, there are four vortex veins (one in each quadrant), but the number can range from 8 to 10. 
• These veins pass through the eyeball obliquely via scleral canals for approximately 4 mm before exiting the globe behind the equator. 
• Some of these vortex veins may have dilatations of different sizes and shapes, known as vortex vein ampullae. 
• Essentially, ampulla refers to the dilated sac of the vortex vein.
• The equator can be visualized as an imaginary circle drawn through the ampullae of the vortex veins. 
• A pigmented crescent or choroidal pigmentation next to a vortex vein ampulla is not uncommon. 
• There are reports of vortex vein varices being mistaken for a potential choroidal mass.

Peripheral Retina

• 4 Regions 
• Near periphery: Circumscribed region of about 1.5mm around the area centralis. 
• Mid periphery: Occupies a 3mm wide zone around the near periphery. 
• Far periphery: Extends from the optic disc 9-10mm on the temporal side and 16mm on the nasal side in the horizontal meridian. 
• Ora serrata: The peripheral margin of the retina consists of a dentate fringe. The retina ends here, and the ciliary body starts. Here, the sensory retina is firmly attached to the vitreous and RPE. 
• Servations are less developed temporally, where cystic degeneration is most common. 
• Ora serrata marks the transition between the attenuated Retina and Inner Columnar Non-Pigment cells of Pars ciliarines as retinae.  
• The RPE continues anteriorly as the outer cuboidal cell layer of the ciliary body. Cystoid degeneration starts at the outer plexiform layer at a younger age and is more marked on the nasal side.  
• They extend between the inner and the outer limiting membrane in elders and communicate with the vitreous, leading to Retinal Detachment. 

2.0Microscopic Structure Of Retina

It has 10 layers: 

1. Retinal pigment epithelium 

2. Layer of rods and cones 

3. External limiting membrane 

4. Outer nuclear layer 

5. Outer molecular (plexiform) layer 

6. Inner nuclear layer 

7. Inner molecular (plexiform) layer 

8. Ganglion cell layer 

9. Nerve fibre layer 

10.Internal limiting membrane

Retinal pigment epithelium

• The outermost layer comprises a single layer of hexagonal-shaped cells containing pigment. 
• It is firmly attached to the underlying Bruch's membrane and loosely attached to a layer of rods and cones. 
• Space bet. RPE and sensory retina are called sub-retinal space. Separation of RPE from the sensory retina is called retinal detachment. 
• On electron microscopy, the adjacent RPE cells are connected by tight junctions and constitute the outer blood-retinal barrier. –Terminal bars.
• Terminal bars- ( Gap junctions, zonula occludens and zonula adherens. 
• Zonula occludens forms the external component of the Blood-retinal Barrier. 
• Extracellular matrix-VERHOEFF'S MEMBRANE fills the rest of the intercellular space in the light microscope.

Functions

• Important role in photoreceptor renewal and recycling of Vit. A. 
• Absorption of scattered light by Melanin Granules. 
• Transport of nutrients and metabolites through the extraretinal blood barrier. 
• The interphotoreceptor matrix participates in the retina's attachment to the RPE and facilitates the Phagocytosis of the shed discs of the outer cone segments. 
• On the basal surface, RPE cells produce type 4 collagen, heparin sulphate, and laminin, which become incorporated into the lamina vitrea of Bruch's membrane.

Layer of Photo Receptors

• There are about 120 million rods and 6.5mm cones. 
• End organs of vision transform light energy into visual impulses. 
• Rods contain photosensitive rhodopsin, responsible for peripheral vision and vision under low illumination. 
• Cones also contain a photo-sensitive substance responsible for central vision and colour vision. 
• The highest density of cones is at the fovea. 
• Rods are absent at the fovea and maximum below the optic disc.

Arrangement of Nerve Fibres in the Retina

• Fibers from the nasal half of the retina travel directly to the optic disc as superior and inferior radiating fibers (SRF and IRF).  
• Fibers from the macular region form the papillomacular bundle (PMB), running straight through the temporal part of the retina.  
• Fibers from the temporal retina curve above and below the macula and the papillomacular bundle as superior and inferior arcuate fibers (SAF and IAF), separated by a horizontal raphe.

Arrangement of Nerve Fibres in the of The Optic Nerve Head

• Fibers from the peripheral retina are located deeper within the retina and occupy the outermost (superficial) region of the optic disc.  
• In contrast, fibers originating nearer to the optic nerve head are positioned superficially in the retina and occupy the central (deeper) portion of the optic disc.

Thickness of Nerve Fibre Layer at the Disc

• The thickness of the nerve fiber layer around the optic disc varies across quadrants, increasing in the following order:  
• Most lateral quadrant (thinnest)  
• Upper temporal and lower temporal quadrants  
• Most medial quadrant  
• Upper nasal and lower nasal quadrants (thickest).

Clinical Significance of Distribution and Thickness of Nerve Fibres at the Optimistic Disc Margin

• Papilloedema appears first in the thickest quadrant (upper nasal and lower nasal) and last in the thinnest quadrant (most lateral). 
• Arcuate nerve fibres occupying the superior temporal and inferior temporal quadrants of the optic nerve head are most sensitive to glaucomatous damage, accounting for an early loss in corresponding visual field regions. 
• Macular fibres occupying the lateral quadrant are most resistant to glaucomatous damage and explain the retention of the central vision till the end.
  

3.0Blood Supply of the Retina

• The outer four layers of the retina are supplied by (till outer nuclear layer) choriocapillaries. 
• The inner six layers get their supply from the central retinal artery, a branch of the ophthalmic artery. 
• Both of the above arteries partly cover the outer plexiform layer. 
• The fovea is avascular and is mainly supplied by choriocapillaries. 
• Branches of the central retinal artery perfuse the inner portion of the retina. (cherry-red spot in CRAO occurs due to choroidal circulation visible at fovea) 
• In 30% of eyes, a cilioretinal artery branching from the ciliary circulation supplies part of the inner retina, mainly the Macula Region. 
• The retinal blood vessels maintain the inner blood-retinal barrier. This physiological barrier is due to a single layer of non-fenestrated endothelial cells, whose tight junctions are impervious to tracer substances such as fluorescein.

• Retinal blood vessels lack an internal elastic lamina & a continuous layer of smooth muscle cells. 
• The retinal arteries are end arteries and have no anastomoses. The only place where the retinal system anastomoses is in the vicinity of lamina cribrosa. 
• The retina's veins unite to form the Central retinal vein at the disc, which follows the corresponding artery. 
• A capillary free zone of 500miceo metres in diameter in the foveal zone- FAZ.

Blood Retinal Barrier

• The outer blood-retinal barrier (BRB) is formed by tight junctions between the pigment epithelial cells in the retinal pigment epithelium (RPE).  
• The inner BRB is created by endothelial cells, which are tightly bound around the lumen by zonula occludens-type intercellular junctions.  
• These junctions typically restrict the free flow of fluids and solutes.  
• The endothelial cells are encased by a basement membrane, surrounded by a layer of pericytes, which is further enveloped by another basement membrane.

4.0Anatomy of Vitreous 

Vitreous Humor

• It is an inert, transparent, jelly-like structure that fills the posterior 4/5 of the cavity of the eyeball with normal volume – 4 mL  
• Hydrophilic gel with optical functions  
• Mechanically stabilizes the volume of the globe  
• Pathway for nutrients to reach the lens and retina

Structure Of The Vitreous

• Composed of a network of randomly oriented collagen 
• Fibrils interspersed with numerous spheroidal macromolecules of hyaluronic acid  collapse = conversion of gel into sol  
• Can be divided into cortex and nucleus (main vitreous body) 

Cortical Vitreous

• It is positioned adjacent to the retina posteriorly and to the lens, ciliary body, and zonules anteriorly.  
• The density of collagen fibrils is higher in the peripheral regions, where their condensation forms false anatomical membranes: the anterior hyaloid membrane and the posterior hyaloid membrane.

Cortical Vitreous

• The anterior hyaloid membrane is firmly attached to the posterior surface of the lens, while the posterior hyaloid membrane is loosely connected to the internal limiting membrane of the retina.

Main Vitreous Body (Nucleus)

• It has a less dense fibrillar structure in a true biological gel  site where liquefaction of the vitreous gel starts first 
• Hyaloid canal (Cloquet's Canal) – Hyaloid artery of the fetus

Attachments of vitreous

• Vitreous Base – part of the vitreous area is about 4 mm across the ora serrata, where the attachment is strongest.  
• Other firm attachments – around the margins of the optic disc, foveal region and back of the crystalline lens (ligament of Wieger)