The Retina

Retinal Layers

The retina consists of 10 layers with 6 different types of retinal cells. These play a key role in visual processing. Overall, they can be categorized into retinal pigmented epithelium (RPE) and neural retina. 

Retinal Pigmented Epithelium

The RPE is a single layer of cuboidal epithelial cells located in the outermost layer of the retina. It nourishes and supports the neural retina with two major functions:

• Form the blood-retinal barrier – tight junctions between the RPE cells form a barrier which prevents molecules passing from the choroid into the retina.
• Replenishes retinal – regenerates the photosensitive pigment following its activation as part of the visual cycle

Neural Retina

The neural retina (layers 2-10) consists of many types of cells but information flows through three primary neurons: photoreceptors → bipolar cells → ganglion cells.

The cells in the neural retina include:

• Photoreceptor cells – involved in phototransduction (a process which converts light photons to an electrical impulse, hyperpolarizing the receptor)
• Bipolar cells – stimulated on hyperpolarization of the photoreceptor, activating the ganglion cells
• Retinal ganglion cell – axons form nerve fibre layer of the retina and exit the eye at the optic nerve
• Horizontal cells – help the eye adjust to see well under both bright and dim light conditions through mechanisms such as lateral inhibition.
• Amacrine cells – modulate the output of ganglion cells to help with motion detection.

Structure of Photoreceptors

Photoreceptors are neuroepithelial cells that absorb light and convert it into an electrical signal via phototransduction. Photoreceptors are tightly packed together, allowing many photons to be transduced across a small area of the retina.

The structure of photoreceptors is adapted to their function with rods and cones being structurally compartmentalised into five distinct regions:

• Outer segment – captures and converts light to an electrical stimulus. It consists of tightly stacked membrane discs which contain proteins (rhodopsin in rods and photopsin in cones) required for phototransduction.
• Cilium – connects outer and inner segments
• Inner segment – houses metabolic organelles (e.g. lysosomes, mitochondria, endoplasmic reticulum) and provides energy for phototransduction
• Cell body – contains the cell nucleus
• Synaptic region – allows communication between the photoreceptor and bipolar cells

By SiliconProphet CC 3.0 via wikimedia commons

Figure 1
Structure of the cone cell.

Organisation of Cells in the Neural Retina

The cells of the neural retina are organised into 9 layers as follows:

Created in BioDigital Human

Figure 2
Overview of the cell layers in the retina

Types of Photoreceptor

There are three main types of photoreceptors in the human eye:

• Rods
• Cones
• Intrinsically photosensitive retinal ganglion cells

This article focuses mainly on the rod and cones. There are 120 million rod cells and 6 million cone cells in each eye.

Rods

Rods are much more sensitive to light than cones. They can transduce even a single photon and so are mainly responsible for scotopic vision (in low-light levels). However, as light levels increase their phototransduction cascades become saturated and they can no longer reflect changes in light intensity.

Rods are located on the outside of the fovea and contribute to peripheral vision. Thus, patients with degenerative changes of rod cells, such as retinitis pigmentosa, may present with night-blindness (known as nyctalopia) and/or peripheral vision loss.

Cones

Cones are concentrated in the fovea – the central part of this contains no rods. This is also the part of the retina with the highest acuity of vision. Cones are much less sensitive to light and so only contribute to daytime vision.

Cones are also responsible for colour vision, mediated by three different types of cones. Each type is sensitive to light of different wavelengths.

Red and green cones are much more abundant than blue cones but they act together to distinguish coloured visual input. Although cones are named after a specific colour, each one is sensitive to a variety of colours and wavelengths.

Red and green cones are much more abundant than blue cones but they all act together to distinguish coloured visual input. Although cones are named after a specific colour, they are actually sensitive to a variety of colours and wavelengths.

By OpenStax via wikimedia commons, CC 3.0

Figure 3
Colour sensitivity at different wavelengths

Fig 2 – Colour sensitivity at different wavelengths

Rods vs. Cones

Rods and cones are distinguished by the shape and morphology of their outer segments.

Clinical Relevance

Retinitis Pigmentosa

Retinitis Pigmentosa (RP) is a genetic disease leading to the degeneration of photoreceptors.

It initially affects the rods in the mid-peripheries and later progresses towards the macula and fovea, affecting both cones and rods, with symptoms reflecting its pathophysiology.

Initially, patients experience night blindness and patchy loss of peripheral vision as peripheral rods are affected. Subsequently, colour perception and visual acuity deteriorate as cones slowly atrophy. In later stages, patients may experience ‘tunnel vision’ – a loss of peripheral vision with the retention of the central visual field, producing a constricted circular, tunnel-like view.

Direct visualisation of the retina via ophthalmoscopy reveals a bone-spicule pigmentation, arteriolar attenuation and ‘waxy’ disc pallor.

By Christina Hamel [CC 2.0 via wikimedia commons]

Figure 3
A retina with the features of retinitis pigmentosa

Whilst there are several diagnostic tests for RP, the most important is the electroretinogram (ERG). This measures the electrical activity of various cell types in the retina in response to light. Other useful investigations include dark adaptometry, the electrooculogram and visual field testing.

Treatment of retinitis pigmentosa includes:

• Supportive measures – e.g. genetic counselling, helping to register visual impairment
• Pharmacological measures – carbonic anhydrase inhibitors (e.g. acetazolamide) to relieve macular oedema
• Cataract surgery – patients with RP may have a posterior subcapsular cataract and surgical correction can improve vision