Retina

The retina is a layer of nerve tissue that covers the interior of the eyeball's back two-thirds, where light stimulation occurs, causing the illusion of vision. Actually, the Retina is an extension of the brain, which is formed embryonically from the neural tissue and is connected to the brain properly by the optic nerve. Retinal detachment is an emergency situation that the eye’s part (retina) pulls away from supportive tissue.

About the Retina Tissue

The Retina is given as a complex transparent tissue that consists of many layers, only one of which has light-sensitive photoreceptor cells. Light can pass through overlying layers to enter photoreceptor cells, which are divided into two groups: rods which cones, and are functionally and structurally distinguished by their response to different types of light. Rods predominate in the nocturnal animals, and they are most sensitive to reduced light intensities. At the same time, humans provide night vision and aid in visual orientation of the retinal display.

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Cones are very prominent in humans and in some of the animals that are active during the day and provide a detailed vision (it means, as for reading) and perception of color. The more cones per unit area of the Retina, the better the detail that can be distinguished by that area. These rods are fairly well-distributed over the total retina part, but cones tend to concentrate at two sites, as given below:

• Surrounding macula lutea, which is the circular patch of yellow-pigmented tissue with the size of up to 5 to 6 mm (0.2 to 0.24 inch) in diameter.

• Fovea centralis, which is a pit at the rear of the Retina that contains no rods and holds the densest concentration of cones in the eye.

When light enters the eye, it passes through the lens and cornea and is refracted by focusing an image onto the Retina. Light-sensitive molecules present in the rods and cones react to a particular wavelength of light and trigger nerve impulses. Complex interconnections (which are called synapses) within and between the retinal cell layers assemble these impulses into the coherent pattern that, in turn, is carried out through the optic nerve to the brain's visual centers, at which they are further organized and interpreted.

Structure

Inverted Versus Non-Inverted Retina

The inverted Retina of vertebrates is characterized as having light-sensing cells in the back of the retina, requiring light to pass through layers of capillaries and neurons before reaching the cones and rods. The ganglion cells, whose axons form optic nerves, are at the Retina's front; thus, the optic nerve should cross via Retina en route to the brain. There are no photoreceptors in this region that give rise to the blind spot. Also, in contrast, in the cephalopod retina, the photoreceptors are in the front part, with processing capillaries and neurons behind them. Due to this, cephalopods do not have a blind spot.

Although the overlying neural tissue is partially transparent, and the accompanying glial cells have been represented to act as fiber-optic channels to transport the photons directly to the photoreceptors, light scattering does take place. A few vertebrates, including humans, hold an area of the central Retina adapted for a high-acuity vision. This region, known as the fovea centralis, is avascular (meaning it lacks blood vessels) and has very little neural tissue in front of the photoreceptors, reducing light scattering.

Development

Retinal development starts with the establishment of the eye fields, which are mediated by the SIX3 and SHH proteins, with subsequent development of the optic vesicles, which are regulated by the LHX2 and PAX6 proteins. The Pax6 role in eye development was elegantly demonstrated by Walter Gehring with his colleagues, who represented that ectopic expression of Pax6 can lead to the eye formation in Drosophila antennae, legs, and wings. The optic vesicle will give rise to 3 structures as given below:

• The retinal pigmented epithelium, 

• The neural Retina, and

• The optic stalk.

The neural Retina has the retinal progenitor cells (RPCs), which give rise to seven Retina's cell types. The differentiation starts with the retinal ganglion cells and concludes with the production of Muller glia. Although every cell type varies from the RPCs in sequential order, there is a considerable overlap in timing, where the individual cell types vary. The cues, which determine an RPC daughter cell fate are coded by the multiple transcription factor families with the homeodomain and bHLH factors.

Blood Supply

The Retina can be divided into layers, each with its own set of cellular compartments or cell types, each with its own metabolism and nutritional requirements. To satisfy such requirements, the ophthalmic artery will bifurcate and supply the Retina through two distinct vascular networks, which are given below:

• The choroidal network that supplies the outer Retina and choroid,

• The retinal network supplies the inner layer of the Retina.

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Circulatory Mechanisms

At first glance, one can think that the vertebrate Retina is "badly designed" or "wired wrongly," but it's a fact that the Retina could not function if it were not inverted. The photoreceptor layer should be embedded in Retinal Pigment Epithelium (RPE) that performs at least 7 vital functions, one of the most obvious being to supply the oxygen and the other required nutrients, that are needed for the photoreceptors to function.