Retina

The retina (Latin: retina) is the inner layer of the eyeball extending between the site where the optic nerve enters the eyeball and the posterior margin of the ciliary body. The retina is where the light signals are transformed into neural impulses followed by their transmission to the brain.

Structure of the retina

The retina is a thin, transparent membrane. The retina is housed between Bruch’s membrane of the choroid externally (the retina’s outer surface) and the aqueous humor internally (the retina’s inner surface). The retina has two parts or main layers: the inner neurosensory retina and the outer retinal pigmented epithelium. Both layers are derived from the optic cup. The space between both of the layers is called the subretinal space

Under normal circumstances, this space should be empty as both of the layers adhere to each other. The place where the anterior end of the retina meets the ciliary body is known as the ora serrata. At the ora serrate, both retina layers tightly attach to each other. The anterior part of the retina is continuous with the columnar cell layers of the ciliary body. The retina is the thinnest at the center of the fovea.

Layers of the retina

The retina has numerous types of cells and layers. The retina is made up of ten layers. When reading about the layers, one should be careful how the layers are numbered and from where to where it is said to be going. Here, we classify the layers from the outside inwards or superficial to deep:

  • The retinal pigment epithelium
  • The rods and cones
  • The external limiting membrane
  • The outer nuclear layer
  • The outer plexiform layer
  • The inner nuclear layer
  • The inner plexiform layer
  • The ganglion cell layer
  • The nerve fiber layer
  • The inner limiting membrane

The epithelium is part of the outer retinal pigmented epithelium, while the other nine layers are a part of the inner neurosensory retina (neural retina). The layers are not present in the optic nerve head, the fovea, and foveola, as well as the ora serrata. The reason for the absence of the layers in the optic nerve head is that the axons of the retinal ganglion cells leave the retina to form the optic nerve, and all other cells are missing. 

Within the fovea and foveola, the inner five layers are, in a way, pushed to the sides. At the ora serrata, the retina is the thinnest because the retinal pigment epithelium connects with the outer pigmented epithelium of the ciliary body, but the neural retina meets the inner unpigmented ciliary epithelium.

Retinal pigmented epithelium

The pigmented layer of the retina or the retinal pigmented epithelium has a single cell layer extending between the optic nerve and the ora serrata. The retinal pigmented epithelium consists of cuboidal cells. These contain a large amount of dark pigment. The epithelial cells absorb the light passing through the retina and prevent the light from reflecting back to the neurosensory layer. At the ora serrata, the retinal pigmented epithelium continues as the pigmented ciliary epithelium. 

The cells of the retinal pigmented epithelium are tall and narrow with becoming more flat, traveling towards the ora serrata. The basal part of these cells lies on the basement membrane that is a part of Bruch’s membrane of the choroid. The apical ends have microvilli projecting between the outer segments of the rods and cones. Glycosaminoglycans, in which the microvilli are swimming, possibly work as a glue between the pigment and neural zones. 

The cells of the pigmented layer take part in the absorption of light, turnover of the outer segments of the photoreceptors, and the formation of rhodopsin and iodopsin. The retinal pigmented epithelium forms the blood-retina barrier. The pigmented cells join each other via tight junctions. The tight junctions completely surround the cells forming a barrier that limits the flow of ions and forbids diffusion of large toxic molecules from the choroid capillaries.

Neural retina

The neural retina comprises different cells: the photoreceptors, the bipolar cells, the ganglion cells, the horizontal cells, the amacrine cells, and the supporting cells. The outer layer of the neural retina has photoreceptors that convert the optical image into neural activity. Neural activity from the photoreceptors continues radially to bipolar and ganglion cells and laterally through horizontal cells and amacrine cells. Photoreceptors create synapses with each other, bipolar and horizontal cells in the outer plexiform layer. 

The bipolar, amacrine, and ganglion cells synapse in the inner plexiform layer. Ganglion cell axons go in the direction of the optic disc in the nerve fiber layer. Here, the axons leave the retina as the optic nerve. Even though most of the information travels from photoreceptors to the brain, some information travels other way.

Photoreceptors

The photoreceptors are similar to sensory receptors anywhere else in the body. Two types of photoreceptor cells are present: the rods and the cones. The rods are responsible for vision in poor light and creating images in black and white, but the cones work in bright light and allow colour vision. The number of rods is around 110-125 million, while cones are about 6.3-6.8 million. 

The amount and density of photoreceptors are different throughout the retina. In the fovea centralis, no rods are present, while their numbers increase, getting closer to the periphery and starting to decrease at the extreme edge of the retina. Meanwhile, the cones are present in large amounts in the fovea, and their numbers fall towards the periphery. The number of photoreceptors is higher than ganglion cells. 

The photoreceptor cells are long and narrow. The outer ends of the cells connect with the pigment epithelium – the outer segment. The inner segment is joined to the outer segment via connecting stalks. The rods and cones have a similar organization, but some details differ. The nuclei of the rods and cones form the outer nuclear layer.

Rod cells

The rod cells are long cells. The actual receptor is the outer segment of the rod that contains rhodopsin. The inner segment of the rod cells has two zones: the ellipsoid (next to the connecting stalk) and the myoid (towards the vitreous). The ellipsoid has mitochondria, but the myoid has ribosomes, Golgi apparatus, and endoplasmic reticulum. 

The rest of the rod is made of the outer fiber, the cell body, the inner fiber, and the spherule. The spherule contains presynaptic vesicles and creates synapses with the dendrites of the bipolar cells. Rod outer segments are cylindrical and consist of membranous discs.

Cone cells

The cone cells are also long. Their structure is similar to that of the rods. The outer segment of the cone is conical, wide, and tapering down to a rounded tip. The cones contain other photochemical – iodopsins. The outer and inner segments are joined via modified cilium. The cone’s inner segment resembles the rod’s inner segment. The cone’s body is connected to the expanded end – the cone pedicle – through the inner fiber. The cone pedicles synapse with the dendrites of the bipolar cells.

Bipolar cells

The bipolar cells have an axon on one end and a dendrite on the other. The bipolar cells are orientated radially. The cell has multiple dendrites. The dendrites synapse with the rods and cones, while the axons extend to the deeper layer of the retina, synapsing with the ganglion and amacrine cells. The bipolar cells are the first-order neurons in the visual pathway. 

The bipolar cells collect the information from the photoreceptors and send it further to the ganglion cells. The bipolar cells can be divided based on their type: the rod bipolar cells (connect several rod cells to one to four ganglion cells), flat or diffuse bipolar cells (connect many cone cells to many ganglion cells), midget bipolar cells (connect a single cell to a single midget ganglion cell). The connection between midget cells forms a direct pathway from the cone to a single optic nerve fiber.

Ganglion cells

The ganglion cells are the second-order neurons in the visual pathway. The ganglion cells are multipolar cells synapsing with the bipolar and amacrine cells through the dendrites. The cells have nonmyelinated axons, which start from their basal ends. The axons have a sharp horizontal turn and turn towards the disc of the optic nerve. 

The axons go through the lamina cribrosa of the sclera, and then they become myelinated. The axons of the ganglion cells form the optic nerve. The ganglion cell dendrites create synapses with the bipolar and amacrine cells in the inner plexiform layer. The axons of the ganglion cells form the nerve fiber layer on the inner surface of the retina. 

The axons go parallel to the surface of the retina, confluence on the optic nerve head where they leave the eye as the optic nerve. Most of the ganglion cells are small, but some are small. In most of the retina, the ganglion cells form a single layer. From the periphery to the macula, the layers of the ganglion cell increase even up to ten layers. Within the fovea, the ganglion cells are not present.

Horizontal cells

The horizontal cells are located around the apices of the rods and cones and create synapses with them. The horizontal cells form synapses also with the distant ganglion cells. The cells release the inhibitory neurotransmitter. The release of the neurotransmitter allows the optic nerve to transmit the signals from the photoreceptors. The horizontal cells are multipolar with one long and several short processes. The short processes contact with the cones, but the long processes – with the rods.

Amacrine cells

The amacrine cells lie near the ganglion cells and synapse with the ganglion cells and the bipolar cells. The amacrine cells are stimulated by the bipolar cells, followed by the stimulation of the ganglion cells via the amacrine cells. The amacrine cells are the bridge between the bipolar and ganglion cells. They are responsible for ensuring that all of the relevant ganglion cells are stimulated. The amacrine cells have large bodies with multiple processes.

Supporting cells

The most prosperous supportive cells are the Muller cells that are located all over the neural retina. The Muller cells are long and narrow cells. The Muller cells have numerous processes connecting them to the photoreceptor cells. These connections form the outer limiting membrane. The termination of the Muller cells forms the inner limiting membrane. 

The Muller cells contact blood vessels. The basal laminae of the Muller cells fuse with perivascular cell or vascular endothelia taking part in forming the blood-retina barrier. The cells maintain the stability of the retinal extracellular environment. Apart from the Muller cells, the retina also has astrocytes, perivascular glial cells, and microglial cells.

All of the retina cells are dispersed throughout the neural retina’s layers:

  • The inner limiting membrane – terminal ends of the Muller cells’ processes;
  • The nerve fiber layer – the axons of the ganglion cells blending towards the optic disc;
  • The ganglion cell layer – nuclei of the ganglion cells;
  • The inner plexiform layer – synapses between bipolar, amacrine, and ganglion cells;
  • The inner nuclear layer – nuclei of the bipolar, horizontal, amacrine, and Muller cells;
  • The outer plexiform layer – synapses between the terminal processes of the rods and cones, bipolar and horizontal cells;
  • The outer nuclear layer – nuclei of the cones and rods;
  • The outer limiting membrane – Muller cells’ synapses with the photoreceptors;
  • The layer of rods and cones – photoreceptor cells.

Landmarks of the retina

Within the retina, it has characteristics that work as topographic landmarks for orientating around the retina. The two notable landmarks are the macula lutea and optic disc.

Macula lutea

The macula lutea is an oval-shaped, highly pigmented area in the center of the inner retinal layer. The macula is approximately 5.5 mm in diameter. A depression in the middle of the macula is called the fovea centralis. Fovea centralis has the most significant number of cone cells. The sides of the fovea centralis are called the clivus. Retinal blood vessels use the fovea centralis as a passage to enter the eye. 

The fovea centralis is formed by the nerve cells and fibers of the inner layers of the retina. The macula consists of at least two ganglion cell layers. The macula lutea has regions within it: the umbo (the center of the foveola), foveola (in the center of the fovea), foveal avascular zone, fovea, parafovea, and perifovea areas.

The yellowish colour of the macula is caused by a pigment – xanthophyll. The colour allows absorbing excess blue and ultraviolet colour, working as a natural sunblock. The macula lutea has the most significant amount of photoreceptor cells, making it a site where the clearest vision is. The macula lutea responsibilities are the central, high-resolution, colour vision, possibly only in good, bright light.

A vertical line going through the fovea centralis divides the retina into the nasal and temporal halves. In contrast, a horizontal line divides these halves into four quadrants: the upper and lower temporal and the upper and lower nasal.

Optic disc

A few millimeters medially to the macula lutea is the optic disc also known as the optic nerve head. The optic disc is pale pink or almost white. The optic disc is where the optic nerve leaves the eye and can be considered the beginning point for the optic nerve. The optic disc is known as the blind spot due to the lack of photoreceptors. The optic disc center is slightly depressed because of the central retinal artery and vein passing through. The optic disc is also an entry point for the blood vessels that supply the retina.

Visual pathway

The visual pathway starts in the retina. The photoreceptor cells generate action potentials. The ganglion cell layer and nerve fiber layer are the origin place for the optic nerve. The ganglion cell layer has the cells' bodies, while the nerve fiber layer has axons. The axons consist of temporal and nasal fibers and join together at the optic disc. After the optic disc, the fibers go posteriorly out of the eye. 

These axons leave the orbit by going through the orbital foramen. After exiting the orbital foramen, the optic nerve enters the optic canal and exits in the middle cranial fossa. After traveling a bit, the two optic nerves join, creating the optic chiasm. Within the optic chiasm, half of the fibers from one eye join the fibers of the other eye and vice versa. This connection allows binocular vision

After the connection, the pathway continues as two optic tracts. The fibers travel to the lateral geniculate body. After the lateral geniculate, it continues to the visual cortex in the occipital lobe. Some fibers leave the track before entering the lateral geniculate and travel to the brain stem sending information to affect the pupil size.

Vasculature and innervation of the retina

Blood supply and venous drainage

The retina has two types of blood supply. The outer five layers of the retina are avascular and have an indirect supply from the choroidal capillaries. The inner layers are supplied directly from the capillaries connected to branches of the central retinal artery and vein.

The central retinal artery enters the optic nerve as a branch of the ophthalmic artery and travels within the optic nerve to its head. Then the artery passes through the lamina cribrosa and divides into superior and inferior branches, with each giving off nasal and temporal branches supplying the quadrants of the retina. These branches do not overlap. The nasal branches are straight going towards the ora serrata, while the temporal branches arch below and above the fovea centralis and then pass to the ora serrata. The arterial branches go in the nerve fiber.

Venous drainage happens similarly to the blood supply, with all tributaries ending up in the central retinal vein.

The retina has no lymphatic drainage.

Innervation

The retina receives its efferent innervation through axons within the optic nerve. A small number of neurons branch widely within the retina. These branches origin at the optic disc, travel through the nerve fiber layer, and branches again in the inner plexiform layer.