Macula lutea ("the yellow spot") – the posterior part of the retina which contains pigment due to xanthophyll. Histologically, the macula is defined as an area with a diameter of 5-6 mm with 2 or more layers of ganglion cells, located between the temporal arcades (the border of the macula is where the ganglion layer becomes a single layer). Oxidized carotenoids, in particular lutein and zeaxanthin, accumulate in the center of the macula and give it its yellow color. These carotenoids have an oxidizing capacity that filters wavelengths of the blue color and thus prevents phototoxic damage. The yellow pigment in the macula contributes to the hypofluorescence seen in this area on fluorescein angiography.
Fovea – the central 1.5 mm which is responsible for visual acuity and color vision – the entire photoreceptor layer here is cones (without rods).
Foveal avascular zone – an avascular area within the fovea whose center is considered the center of the macula.
Foveola – a central point in the fovea with a diameter of 0.35 mm (almost equivalent to the foveal avascular zone); in the foveola the cones are thin and dense. Here the following layers are absent: the ganglion cell layer and the inner nuclear cell layer.
Umbo – a small depression within the foveola.
The parafoveal ring – a ring around the fovea with a thickness of 0.5 mm where the following layers are thickest: the ganglion layer, the inner nuclear layer, and the outer plexiform layer. This is in fact the area where the retina is thickest.
The perifoveal ring – a ring with a thickness of 1.5 mm which surrounds the parafoveal area. This is in fact the area between the parafovea and the border of the macula.
The retina outside the macula is divided into several regions: equatorial, peripheral, ora serrata.
Equatorial retina – the retina around the equator.
Anterior/peripheral retina – everything anterior to the equatorial retina.
Ora serrata – the border between the retina and the pars plana is also called the ora serrata.
Complexes at the border between the ciliary body and the retina:
Dentate processes – projections of retina into the pars plana which are more common on the nasal side.
Ora bays – projections of the pars plana into the retina.
Sometimes the dentate processes surround the ora bays and give a mistaken impression of a peripheral retinal hole.
Meridional fold – a thickening of the retina which continues and penetrates into the pars plana and looks like large, exaggerated dentate processes. When they are continuous with a ciliary process, the two together are called a meridional complex.
The layers of the retina from the inside (adjacent to the vitreous) to the outside (adjacent to the retinal pigment epithelium):
a. Internal limiting membrane – also known by the abbreviation ILM.
b. Nerve fiber layer – the axons of the ganglion layer – also known by the abbreviation RNFL.
c. Ganglion cell layer
d. Inner plexiform layer
e. Inner nuclear layer
f. Outer plexiform layer
g. Outer nuclear layer (the nuclei of the photoreceptors)
h. External limiting membrane – also known by the abbreviation ELM.
i. Inner and outer segments of the rods and cones (photoreceptors)
In order for light to reach the retina, it must first pass through all the layers of the retina in order to reach the photoreceptors (rods and cones). The density and distribution of the photoreceptors differ in different areas of the retina:
In the fovea area there are only cone cells and Müller cells (without rod cells), with the cones at a density of 140 thousand cones per square mm of retina. They are mainly sensitive to the colors red and green. As one moves away from the fovea, the number of cones decreases, so that in the periphery of the retina there are almost no cones.
The maximum density of the rods is about 20 degrees from fixation, where it reaches 160,000 per square mm. Despite the high density of rods in this area, visual acuity in this area is still not high, because it is a product of the responses of all the rods in a given area. The density of the rods also decreases in the periphery of the retina.
The molecules that respond to light: in rods and also in cones they derive from vitamin A and are linked to a protein called opsin. In rods the molecule in question is called rhodopsin, and in cones there are 3 different opsins for the colors red, green and blue. All these molecules are located in the outer segment of the photoreceptors. The rods can contain up to 1000 discs which are stacked one on top of another like coins and are shed from the outer retina and eaten (phagocytosis) by the retinal pigment epithelium for processing and recycling. One protein called ABCR (ATP-binding cassette transporter of the retina), which is encoded by the ABCA4 gene, is involved in the process of transporting the retinoids so that they can be converted into vitamin A and later transferred to the retinal pigment epithelium for recycling (as explained earlier). Therefore, defects in ABCR impair the metabolism of retinoids and lead to their accumulation in the retina and RPE, as seen in Stargardt disease.
The photoreceptors have a graded response according to the amount of light. Horizontal cells which are in synapse with neighboring photoreceptor cells modulate the response. The photoreceptors are also connected to bipolar cells (in a ratio of 1:1 in cones and a ratio of 100:1 in rods). The bipolar cells also respond in a graded manner to the cells connected to them. The bipolar cells connect to ganglion cells which respond to the signal from them and also from amacrine cells (which also modulate them according to the speed of the change in light). The NFL layer is in fact continuous with the ganglion cells and passes through the inner part of the retina and gathers to form the optic nerve.
Pathway: photoreceptors => bipolar cells => ganglion cells => dorsolateral geniculate nucleus.
Membranes: there is one true membrane called the ILM which is continuous with the vitreous and formed from the feet of the Müller cells. There are two false membranes called the ELM which are formed from the connections between the Müller cells and the photoreceptors. Hence the Müller cells pass through almost all the layers of the retina. The third membrane, called the middle limiting membrane, is formed from the connections between the photoreceptors and the bipolar cells.
Blood supply: the central retinal artery, which is in fact the first branch of the ophthalmic artery, enters the eye and divides into four branches, each of which supplies a quadrant of retina (with the help of further divisions). In about 15%-30 of eyes in the population, an artery called the cilioretinal artery, which is a branch of the ciliary circulation, supplies blood to the retina in the area between the optic nerve and the center of the macula. The blood supply from the retinal artery is divided into sub-layers of capillaries: the first, superficial, which sits in the RNFL layer and the ganglion layer, and the second, deeper, in the inner nuclear layer. The metabolic needs of all the other layers are supplied by the choriocapillaris – a layer of capillaries from the ciliary arteries.
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