Retina Anatomy

The retina is the light-sensitive tissue that lines the inside of the eye. The retina functions in a manner similar to film in a camera. The optical elements within the eye focus an image onto the retina of the eye, initiating a series of chemical and electrical events within the retina. Nerve fibers within the retina send electrical signals to the brain, which then interprets these signals as visual images. ^[1]

The human retina is located on the inner surface of the posterior two-thirds to three-quarters of the eye. The eye itself is a mostly hollow organ, roughly spherical in shape. In adults, the eye measures approximately 22 mm in diameter. The walls of the eye consist of the firm outermost coat, comprised of the white sclera in the posterior three-quarters of the eye and the clear cornea in the anterior one-quarter of the eye. The middle layer consists of the uveal tract made up of the choroid posteriorly and the ciliary body and iris anteriorly. The retina is the innermost layer. It lines the entire posterior portion of the eye with the exception of the area of the optic nerve and extends anteriorly to end 360 degrees circumferentially at the ora serrata.

The total area of the retina is approximately 1,100 mm² ­­. The circumferential diameter at the equator of the adult eye averages 69 mm. The retinal distance circumferentially passing posteriorly from a point on the ora serrata to a point on the ora serrata 180 degrees away is approximately 50 mm. The average healthy retina is 250-µm thick immediately adjacent to the temporal margin of the optic nerve.

The retina thickens to approximately 400 µm in the macular area around the fovea and thins to 150 µm in the fovea. This difference in thickness in the central and noncentral portions of the macula can be appreciated when the eye is examined with an ophthalmoscope. The retina thins as it approaches the equatorial region of the eye and further thins to 80 µm at the ora serrata.

Retinal nerve fibers exit the eye through the optic nerve, located nasally and on the same plane as the anatomical center of the retina. There is no retinal tissue overlying the optic nerve head. The optic nerve head or optic disc is oval in shape and measures approximately 1.75 mm. vertically and 1.5 mm horizontally. The center of the optic disc is located 4.5 mm to 5 mm nasal to the anatomical center of the retina.

The center of the retina provides the greatest resolving power of the eye. This area, responsible for central vision, is known as the macula. The center of the macula is called the fovea.

The inner surface of the retina is adjacent to the vitreous of the eye. The outermost layer of the retina, the retinal pigment epithelium, is tightly attached to the choroid.

Assuming that the ocular media (cornea, anterior chamber, lens, and vitreous) are not cloudy, the living retina can be examined using a direct or with a high diopter biconvex retinal lens at the slit lamp. In addition, the retina may be photographed using a retinal camera. The arterioles and venules of the retina are the only blood vessels whose wall can be directly examined in the living human without an incision.

The retina, with the exception of the blood vessels coursing through it, is transparent to the examiner up to its outer layer, the retinal pigment epithelium. The transparent portion of the retina is known as the neurosensory retina. The examiner sees the neurosensory retina against the background orange color of the melanin containing retinal pigment epithelium and blood-filled choroidal layer of the eye.

The neuroretina is tightly attached to the underlying retinal pigment only at the margins of the optic nerve and at the ora serrata. There is a potential space between the neurosensory retina and the retinal pigment epithelium. In a retinal detachment, this space fills with fluid and detaches the neurosensory retina from the underlying retinal pigment epithelium.

The living retina may be imaged using photography, fluorescein angiography, laser polarimetry, or optical coherence tomography.

Blood supply of the retina

There are two circulations to the retina, both supplied by the ophthalmic artery, the first branch of the internal carotid artery on each side. The outer and middle retinal layers, including the outer plexiform and outer nuclear layers, the photoreceptors, and the retinal pigment epithelium, are nourished by branches of the posterior ciliary arteries, which enter the back of the eye outside the optic nerve.

These vessels also supply the choroidal layer, external to the retina. The inner retina is supplied by the central retinal artery, the branch of the ophthalmic artery that enters the optic nerve 4 mm posterior to the eye. The central retinal artery has 4 main branches within the retina.

These vessels emerge from the optic nerve head and run radially away from the optic nerve.

The temporal vessels curve towards and around the fovea. The macular vessels arise from branches of the superior temporal and inferior temporal arteries. There is a blood vessel-free and capillary-free zone of approximately 500 µm in diameter in the area of the fovea.

In about 1 out of 5 people, the inner layer of the macula is dually supplied by cilioretinal arteries branching from the posterior ciliary arteries.

Retinal blood vessels maintain the blood-retinal barrier due to nonfenestration of the vascular endothelium. Choroidal endothelial vascular cells are fenestrated.

The arterial intraretinal branches then supply capillary networks, which subsequently drain into venules and then into the central retinal vein. The arterial circulation emanating from the posterior ciliary arteries drain out of the eye through one or two vortex veins in each of the 4 quadrants of the eyeball. The vortex veins penetrate the sclera and merge into the ophthalmic vein.

Cellular anatomy of the retina

The retina consists of millions of cells packed together in a tightly knit network spread over the surface of the back of the eye. These cells can be divided into a three basic cell types, photoreceptor cells, neuronal cells, and glial cells. ^[2]

Photoreceptor Cells

Photoreceptor cells consist principally of cones and rods. Cones function best under illuminated conditions and provide color vision. Rods function primarily in dim light and provide black-and-white vision. Each human retina contains approximately 120 million rods and 6 million cone photoreceptors.

Each rod or cone cell contains photoreceptor elements and an axon. Cones have a projection called a pedicle at the termination of the axon, which contacts a dendrite. Rods have a similar projection called a spherule. There are three types of cones, varying in specific sensitivity to different wavelengths (colors) of light.

Each photoreceptor portion of a rod or cone is anatomically divided into an outer segment and an inner segment. In a rod, the outer segment contains the pigment rhodopsin within free floating discs. These discs are stacked within a sleeve, similar to a roll of coins. Iodopsin pigment is contained within cones in folded layers inside a continuous outer membrane. The photoreceptor inner segment contains the cell nucleus and other subcellular structures.

The central retina is cone dominated and the peripheral retina is rod dominated. The highest density of cones is at the center of the fovea. There are no rods in the center of the fovea. At the center of the macula is the fovea, a pit where the cones are smallest and arranged in a mosaic to provide highest and most efficient optical density.

There is a third class of photoreceptors within the retina (in addition to the rods and cones). These are the much rarer intrinsically photosensitive retinal ganglion cells, which are stimulated by light even when all rods and cones are blocked. The photosensitive ganglion cells contain the pigment melanopsin. Alteration in this pigment by light is involved in non – image-forming responses to light, such as synchronization of circadian rhythms to the light-dark cycle, contributing to regulation of pupil size and influencing release of melatonin from the pineal gland.

Neural Cells

Neural cells (nerve cells) include bipolar cells, ganglion cells, horizontal cells, and amacrine cells.

Bipolar cells are contained entirely within the retina, connecting the photoreceptors to the ganglion cells. These are vertically oriented (perpendicular to the retinal surface). There are nine types of bipolar cells. Bipolar cells are postsynaptic to rods and cones.

Ganglion cells have dendrites that synapse with bipolar cells. The axons of ganglion cells become the nerve fiber layer within the retina and then become optic nerve fibers terminating within the brain.

Horizontal cells connect bipolar cells with each other. Horizontal cells are the laterally interconnecting neurons in the outer plexiform layer of the retina. Horizontal cells are responsible for allowing eyes to adjust to see well under both bright-light and dim-light conditions. These are horizontally oriented (parallel to the retinal surface).

Amacrine cells connect bipolar and ganglion cells with each other. Amacrine cells function within the inner plexiform layer, the second synaptic retinal layer where bipolar cells and retinal ganglion cells form synapses. There are about 40 different types of amacrine cells, most lacking axons. Like horizontal cells, amacrine cells are horizontally oriented and work laterally, affecting the output from bipolar cells. Each type of amacrine cell connects with a particular type of bipolar cell, and generally has a particular type of neurotransmitter.

Supporting Cells of the Retina

Glial cells are interspersed between and among the axons of the ganglion cells within the retina and optic nerve. These supporting cells of the retina include Muller cells, astrocytes, and microglial cells.

Muller cells, the principal glial cells of the retina, form a supporting matrix radially across the thickness of the retina and also form the inner and outer limiting membranes of the retina. Muller cell bodies sit in the inner nuclear layer and project irregularly thick and thin processes in either direction to the outer limiting membrane and to the inner limiting membrane. Muller cell processes insinuate themselves between cell bodies of the neurons in the nuclear layers and envelope groups of neural processes in the plexiform layers. Retinal neural processes are only allowed direct contact at their synapses.

Astrocyte cell bodies and processes are almost entirely restricted to the nerve fiber layer of the retina.

Microglial cells are of mesodermal origin. Unlike the Muller cells and astrocytes, they are not neuroglial.

Anatomic layers of the retina

Each of the microscopic layers of the retina has a name and contains various structures. ^[3]

Beginning with the innermost layer (closest to the vitreous) and proceeding outwards towards the choroid and sclera, these layers are as follows:

1. The inner limiting membrane

2. The nerve fiber layer

3. The ganglion cells layer

4. The inner plexiform layer

5. The inner nuclear layer

6. The outer plexiform layer

7. The outer nuclear layer

8. The outer limiting membrane

9. The rod and cone layer

10. The pigment epithelium

The inner limiting membrane is the boundary between the retina and the vitreous body. It is formed by astrocytes and the footplates of Muller cells together with a basal lamina. The nerve fiber layer is the layer of optic nerve fibers consisting of ganglion cell axon fibers, which course towards the optic nerve head. The ganglion cells layer contains the nuclei of ganglion cells, the axons of which become the optic nerve fibers for messages. There are also some displaced amacrine cells within this layer. Additionally, this layer also contains the non-rod and non-cone photoreceptors, the photosensitive ganglion cells, which are important for reflexive responses to bright daylight.

The inner plexiform layer contains the synapses between dendrites of ganglion cells and amacrine cells and the axons of bipolar cells. The inner nuclear layer contains the nuclei of horizontal, bipolar and amacrine cells. The inner nuclear layer is thicker in the central area of the retina compared with peripheral retina because of a greater density of cone-connecting second-order neurons (cone bipolar cells) and smaller and more closely spaced horizontal cells and amacrine cells concerned with the cone pathways. There are also nuclei of the supporting Muller cells.

The outer plexiform layer contains the rod and cone axons (projections of rods and cones ending in the rod spherule and cone pedicle), horizontal cell dendrites, and bipolar cells dendrites. Synapses among these structures occur within this layer. In the macular region, this layer is termed the fiber layer of Henle. The outer plexiform layer is also known as the outer synaptic layer.

The outer nuclear layer consists of the cell bodies of the retinal rods and cones. In the peripheral retina, the rod cell bodies outnumber the cone cell bodies, whereas the reverse is true for the central retina.

The outer limiting membrane (external limiting membrane) is the layer that separates the inner segment portions of the photoreceptors from their cell nuclei. The rod and cone layer (bacillary layer) contains the inner and outer segments of the rod and cone photoreceptors cells.

The pigment epithelium is the most external layer of the retina. It abuts on the choroidal layer of the eye. It contains a single layer of cuboidal-supporting cells for the neural portion of the retina. These cells contain melanin, which absorbs light and decreases light scatter within the eye.

The conversion of light into neural signals involves four basic processes: photoreception, transmission to bipolar cells through synapses, transmission to ganglion cells, and transmission along the optic nerve.

Photoreception

Focused (or unfocused) light passes through the inner layers of the retina to reach the photoreceptors (rods and cones). Because the photoreceptive cells lie outermost within the retina, light must first pass through and around the ganglion cells and through the thickness of the retina before reaching the rods and cones. The light does not pass through the pigment epithelium or the choroid, which are opaque.

The outer segments of the rods and cones contain photopigment, which captures individual photons of light and initiates neural signaling. The photoreceptor inner segments contain the axon terminal, where neurotransmitter is released to the bipolar cells. The inner segments also function efficiently to funnel light to the outer segments.

Rods function mainly in dim light and provide black-and-white vision, while cones support daytime vision and the perception of color. Rods and cones are primarily vertically oriented within the retina. Photoreceptors are not distributed evenly throughout the retina. Most cones lie in the fovea, whereas peripheral vision is dominated by rods. There are 6-7 million cones in the retina. There are 125,000,000 rods in the entire retina.

The macula is the central 3 mm of the retina. It has intense pigment supplied by the retinal pigment epithelium. The fovea is the central part of the macula. It is located 3-3.5 mm temporal to the temporal edge of the optic nerve head. The fovea contains only cones within a rod free area measuring 500 µm in diameter. Foveal cones are 2.3 µm in diameter and are packed very closely together. The retina is thinnest at the center of the fovea since there are no bipolar and ganglion cell here. Light strikes the receptors directly, allowing for best visual acuity. The bipolar cells connecting to these cones are displaced concentrically away from the fovea.

Transmission to bipolar cells through synapses

The outer segments of the rods and cones transduce the light and send the signal through the cell bodies of the outer nuclear layer and out to their axons. In the outer plexiform layer, photoreceptor axons contact the dendrites of both bipolar cells and horizontal cells. Horizontal cells are horizontally oriented (parallel to the retinal surface) interneurons, which aid in signal processing. Bipolar cells are vertically oriented (perpendicular to the retinal surface).

Cones synapse with eight different types of bipolar cells. Five of these are called diffuse bipolar cells and make synaptic contact with up to 20 cones. The other three types contact only single cones and are called midget bipolar cells. Because there are 150 million photoreceptors and only 1 million optic nerve fibers, there must be convergence of signals and individual cone signals mix with others. In addition, the horizontal action of the horizontal and amacrine cells can allow one area of the retina to influence another.

Transmission to ganglion cells

The bipolar cells in the inner nuclear layer process input from photoreceptors and horizontal cells. They transmit the signal to their axons. In the inner plexiform layer, bipolar axons contact ganglion cell dendrites and amacrine cells, another class of interneurons, through synapses. Ganglion cells are vertically oriented while amacrine cells are horizontally oriented.

Transmission along the optic nerve

The ganglion cells of the ganglion cell layer send their axons through the nerve fiber layer and converge at a point nasal to the center of the retina, forming the optic nerve. The ganglion cell axons all leave the eye at the optic disk. Theses axons travel all the travel all the way to the lateral geniculate nucleus in the brain stem. At the optic disc, no retinal photoreceptors, bipolar cells, ganglion cells, or accessory cells are present. Each human optic nerve has 1,000,000 axons.