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Retina

9. Februar 2017 - Eye, Ophthalmology

The vertebrate retina is a light-sensitive tissue located on the inner surface of the eye . It is similar to a canvas where the images are projected. The light that impinges on the retina triggers a series of chemical and electrical phenomena that finally translate into nerve impulses that are sent to the brain by the optic nerve .

The retina has a complex structure. It consists basically of several layers of neurons interconnected by synapses . The only cells directly sensitive to light are cones and rods . The human retina contains 6.5 million cones and 120 million canes. The rods work mainly in low light conditions and provide black and white vision, however the cones are adapted to situations of high luminosity and provide color vision. 1

Etymology

The name retina is the diminutive of the Latin word rete which means ‚red‘ because who discovered it, Herophilus of Chalcedon , described it as a small network.

Embryological origin

The retina comes from a bilateral evagination of the forebrain called the primary optic vesicle , which, after a local invagination, is transformed into the secondary , glass-shaped vesicle . Each optic cup remains connected to the brain by a stem, the future optic nerve . In the adult, the retina is formed by an external pigmented epithelial layer, the pigment epithelium, and an internal lamina, neural retina or retina proper, which contains elements similar to those of the brain, so that it can be considered as a particularly Differentiated from the central nervous system .

Macroscopic structure of the retina

In this image can be observed the human retina, the most colored area located in the center is the fovea and the whitish area on the left is the optic disc or papilla.

The retina is a thin, partially transparent layer that is in contact with the inner face of the choroid and with the vitreous humor . On its surface can be observed various structures: 2 1

  • Papilla, or optic disc : The papilla is the point where the optic nerve enters the eyeball, traversing the sclera membrane, the choroid and finally the retina. It is a pink disk 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 dimensions of 2 x 1.5 mm. In the papilla there are no photoreceptors, so it is called the blind spot.
  • Fovea :is located about 2.5 mm or 17 degrees of temporal edge of the optic disc, where the surface of the retina is depressed and is shallow. It presents a greater number of ganglion cells, with a more regular and accurate distribution of the structural elements, has only cones . The blood vessels surround the fovea from above and below, while within it only small capillaries exist. In the center of the fovea, in an area of ​​about 0.5 mm in diameter, there are no capillaries to maximize the transparency of the retina.
  • Ora serrata : The previous limit of the retina. There is one nasal or medial serrata and one lateral or temporal serrata.

It can also be divided macroscopically into two zones:

  • Central area of the retina : The portion of the retina surrounding the fovea and where most photoreception occurs. The fovea and the small area around it contains a yellow pigment and is therefore called macula .
  • Peripheral area of the retina : This area is less able to photoreception, by having fewer rods and cones.

Microscopic structure of the retina

Layers of the retina

Simplified organization of the retina (modification of a drawing of Santiago Ramon y Cajal ). The light enters from the left and it must cross all the cellular layers until arriving at the cones and poles that are to the right of the scheme.

The retina contains ten parallel layers which are, beginning with the outermost zone to the innermost: 3

  1. Retinal pigment epithelium : is the outermost layer of the retina. It consists of cubic cells are not neurons and have granules of melanin that give a characteristic pigmentation.
  2. Layer of photoreceptor cells : It is formed by the outer segments of the rods and cones.
  3. External limiting layer : It is not a membrane but the adherent junctions between zonula type photoreceptor cells and Müller cells.
  4. Nuclear or external granular layer : It consists of the cell nuclei of the photoreceptor cells .
  5. Layer outer plexiform : The region of synaptic connection between photoreceptor cells and bipolar cells.
  6. Nuclear or internal granular layer : It consists of the cell nuclei of bipolar cells, horizontal cells and amacrine cells.
  7. Layer inner plexiform : The region of the synaptic connection between bipolar cells, amacrine and ganglion.
  8. Layer of ganglion cells : It is formed by the nuclei of ganglion cells.
  9. Layer optic nerve fibers : It is formed by the axons of ganglion cells form the optic nerve .
  10. Internal limiting layer : Separate the retina of the vitreous humor . 4

Retinal cells

The retina has three types of cells: 1

  • Pigmented : is responsible of the metabolism of photoreceptors.
  • neurons :
  1. Photoreceptor cells: They are the cones and rods . They transform light pulses into electrical signals.
  2. Retina bipolar cell . They connect photoreceptor cells to ganglion cells.
  3. Amacrine cells. They are modulating interneurons.
  4. Horizontal cells. They fulfill a function similar to amacrine cells, they are modulating interneurons.
  5. Retinal ganglion cell . Of these neurons part the optic nerve that connects the retina with the brain.
  • Supporting cells :
  1. Astrocytes.
  2. Müller cells. Its function is to support, synthesize glycogen and yield glucose to other nerve cells.

Some milestones in the study of the retina

In the late nineteenth century and early twentieth century the Spanish Santiago Ramon y Cajal made histological sections of the retina and watching on the stage of an optical microscope described the main types of cells that compose it : photoreceptor, bipolar cells, horizontal cells, Amacrins and ganglion cells. In 1952 the German Stephen Kuffler are studying the night vision of cats registered the presence and activity of ganglion cells individually react to light stimuli and even 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, dogs, predatory fish such as sharks, deep – sea fish, etc.) have a structure called tapetum lucidum behind the retina. The tapetum lucidum acts as a mirror that reflects the light that has passed through the retina, thus improving vision in the dark.

Retinal diseases

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

  • Retinal detachment . Retinal detachment is an eye disease caused by the separation of the neurosensory retina from the pigment epithelium to which it is normally attached. As a result, the neurosensory retina is without blood supply and loss of vision occurs. It can occur spontaneously or after trauma to the eye. People are more likely to have nearsightedness advanced. 5
  • Retinitis pigmentosa .
  • Macular degeneration .
  • Diabetic retinopathy .
  • Hypertensive Retinopathy .
  • Macular hole .
  • Retinitis .

References

  1. ↑ Jump to:a b c Felix Jesus Alañón Fernández, Manuela Lara Cardenas, Miguel Angel Fernandez and Ana Martos Alañón Aguilera: Anatomy and physiology of the ocular apparatus. Retrieved on September 1, 2011
  2. Back to top↑ Encarna García Garrido: ocular anatomy. Retrieved on September 1, 2011
  3. Back to top↑ Kierszenbaum, AL (2007). «Ch.9 Sensory organs». Histology and Cell Biology: an introduction to pathology (2nd edition). Mosby Inc. ISBN  0-3230-4527-8 .
  4. Back to top↑ A. Cordova : Dynamic Physiology. Masson SA, 2003, ISBN 84-458-1270-X . Accessed September 11, 2011
  5. Back to top↑ Shukla Manoj, OP Ahuja, Jamal Nasir. Epidemiological study of nontraumatic phakic rhegmatogenous retinal detachment. Indian J Ophthalmol 1986; 34: 29-32 .