Retina

The retina of vertebrates is a light-sensitive tissue located on the inner surface of the eye. It is similar to a cloth where images are projected. The light that falls on the retina triggers a series of chemical and electrical phenomena that are finally translated into nerve impulses that are sent to the brain through the optic nerve.

The retina has a complex structure. It is basically made up of several layers of neurons interconnected by synapses. The only cells that are directly sensitive to light are the rods and cones. The human retina contains 6.5 million cones and 120 million rods. The rods function mainly in low light conditions and provide black and white vision, the cones, however, are adapted to bright light situations and provide color vision.

Etymology

The name retina is a diminutive of the Latin word rete which means 'net', since its discoverer, Herophilus of Chalcedon, described it as a small network.

Embryological origin

The retina comes from a bilateral outpouring of the forebrain called the primary optic vesicle, which after local invagination becomes the cup-shaped secondary optic vesicle. Each optic cup remains connected to the brain by a stalk, the future optic nerve. In the adult, the retina is made up of an outer pigmented epithelial layer, the pigment epithelium, and an inner layer, the neural retina or retina proper, which contains elements similar to those of the brain, for which reason it can be considered as a particularly important part. differentiated from the central nervous system.

Macroscopic structure of the retina

In this image you can see the human retina, the most colorful area located in the center is the fóvea and the whitening area on the left is the optical disk or papilla.

The retina is a thin and partially transparent layer, it is in contact with the inner face of the choroid and vitreous humor. Various structures can be observed on its surface:

• Papila or optical disk: Papilla is the point where the optical nerve enters the eyeball, crossing the sclerotic membrane, the choroid and finally the retina. It is a pink disc that is located at the back of the eyeball and is located about 3 millimeters medially to the posterior pole of the eye. It has a dimensions of 2 x 1.5 mm. In the papilla there are no photoreceptors, so it is called a blind spot.
• Fóvea: It is located about 2.5 mm from the temporal edge of the optic papilla, where the retina surface is depressed and is shallow. It has a greater number of ganglion cells, with a more regular and accurate distribution of structural elements; it has only cones. Blood vessels surround the fovea above and below, while there are only small capillaries within it. In the same center of the fóvea, in an area of about 0.5 mm in diameter, there are no capillaries to maximize the transparency of the retina.
• Ora serrata: It is the previous limit of the retina. There is a nasal or medial serrata or a lateral or temporary serrata.

It can also be divided macroscopically into two zones:

• Central area of the retina: It is the portion of the retina that surrounds the fóvea and where the greatest photoreception occurs. The fóvea and the small area around it contain a yellow pigment and that is why it is called Mácula lutea.
• Peripheral area of retina: This area has less capacity for photoreception, for having fewer cones and canes.

Structure of the retina

Layers of the retina

Simplified retina organization. The light enters above and must cross all the cellular layers to the cones and canes.

The retina contains ten parallel layers that are, starting with the most superficial zone, until the most internal:

1. Pigmentary epithelium: It is the outermost layer of the retina. It is formed by cubic cells that are not neurons and have melanin granules that give it a characteristic pigmentation.
2. Capa de células fotorreceptoras: It is made up of the outermost segments of the cones and the canes.
3. External limiting capacity: It is not a membrane, but intercellular unions of the adherent zonula type between the photoreceptor cells and the Müller cells.
4. Nuclear or external granular layer: It is formed by cell nuclei of the photoreceptor cells.
5. External plexiform layer o external synaptic: It is the region of synaptic connection between photoreceptor cells and bipolar cells.
6. Nuclear or internal granular shell: It is formed by cell nuclei of bipolar cells, horizontal cells and amacrine cells.
7. Internal plexiform layer or internal synaptic: It is the region of synaptic connection between bipolar cells, amacrinas and ganglionars.
8. Node cell duct: It is formed by the nuclei of the lymph node cells.
9. Fiber layer of the optic nerve: It is formed by the axons of lymph node cells that form the optical nerve.
10. Internal limiting capacity: Separate the retina of the vitreous humor.

Cells of the retina

The retina has three types of cells:

• Pigmented: They take care of the metabolism of the photoreceptors.

• Neurons:

1. Photoreceptive cells: These are cones and canes. They transform the luminous impulses into electrical signals.
2. Retina bipolar cells. They connect the photoreceptor cells with the lymph nodes.
3. Amacrine cells. They're modulating interns.
4. Horizontal cells. They perform a function similar to amacrine cells, they are modulator interneurons.
5. Retina lymph node cells. Of these neurons part the optical nerve that connects the retina with the brain.

• Brass cells:

1. Astrocytes.
2. Cells of Müller. Its function is support, synthesize glucogen and glucose to other nerve cells.

Some milestones in the study of the retina

At the end of the 19th century and beginning of the XX the Spanish Santiago Ramón y Cajal made histological sections of the retina and observing them on the stage of an optical microscope described the main types of cells that compose it: photoreceptors, bipolar cells, horizontal cells, amacrine cells and ganglion cells. In 1952, the German Stephen Kuffler, studying the night vision of cats, recorded the presence and activity of ganglion cells that react individually to light stimuli and even to the lack of light. In 2007 King-Wai Yau detected the presence of flat ganglion cells in the retinas of fish. It is noteworthy that many nocturnal animals (cats, canids, predatory fish such as sharks, deep-sea fish, etc.) have a structure called Tapetum lucidum behind the retina. The tapetum lucidum works like a mirror that reflects light that has passed through the retina back to the cornea, thus improving vision in the dark.

Diseases of the retina

The retina can be affected by different diseases that sometimes considerably reduce visual capacity. Some of the most common are:

• Retina surrender. Retinal detachment is an eye disease caused by the separation of the neurosensorial retinal from the pigmentary epithelium to which in normal conditions it is adhered. As a result, the neurosensorial retina is without blood irrigation and vision loss occurs. It can occur spontaneously or after eye trauma. People with advanced myopia are more predisposed.
• Retinosis pigmentaria.
• Macular degeneration associated with age.
• Diabetic retinopathy.
• Hypertensive retinopathy.
• Macular hole.
• Retinitis.