The image of the objects we look at falls on the retina region in the inner part of the eye, which we call the retina, through the light. The image of the object we are looking at in the center of our field of vision falls on the macula. The macula is a very small part of the retina located in the center. It is only about 5mm wide, but is responsible for our central vision, most of our color vision, and fine details of our central vision. The macula has a very high concentration of light-sensing cells, which we call photoreceptor cells. Photoreceptor cells process the incoming image and send them to the visual center of the brain as a signal via nerve fibers after photochemical reactions. The image is processed here, and the image from both eyes is combined and interpreted, and converted into a three-dimensional image as we see it. Areas of the retina outside the macula process our peripheral or side vision.
WHY IS IT CALLED YELLOW DOT?
The macula is expressed with the words macula lutea, which comes from the Latin with its use in medicine.
‘Macula’ is a word used for many spots such as sunspots, marks on minerals, and it means spot, speck, spot, paint, stain. It comes from the Latin word “macula” and its origin is unknown. The region corresponding to the ‘corresponse’ retinal points in a way that both eyes see the object in the center alone is called the macula lutea because of the pigments it contains. The word lutea evokes thousands of plants in nature, each more beautiful than the other and each blooming with bright yellow flowers, with the word “Lutea” in its Latin name. The center of the macula, which is the most valuable point of our eye, is the region of the retina responsible for high-resolution vision. It is a pigmented area in the center of the retina of the human eye. In humans, the macula is an oval region with a diameter of about 5.5 mm. Towards the center of the macula region, there are regions called fovea, foveaola and umbo. The most clinically important region of the macula is called the fovea. The fovea is located in the center of the macula; The foveola is located in the center of the fovea. The retina contains two types of photosensitive cells, which we call rod cells and cone cells.
The macula is the region of the retina that contains the most dense cones. The foveola is a small pit-shaped central region of the macula that contains the most concentrated concentration of cone cells.
The umbo is the central point of the foveola, located in the center of the fovea.
The macula contains dense capillaries, except for what we call the foveal avascular zone, which is the avascular zone in the very center. The fovea is the 1.5 mm diameter area in the center of the macula. The foveal avascular zone is approximately 0.5 mm in diameter. The foveola within the foveal avascular zone is 0.35 mm in diameter. The umbo is the point in the center of the macula, which corresponds to the hollow place, occupying a 0.15 mm space and providing the reflex of the macula during the examination. Neural cells such as ganglion cells are not located where the fovea is hollow in the center of the foveal avascular zone, and the light directly reaches the photoreceptors here. This structure is a system that considerably reduces the scattering of light coming into the eye and prevents the formation of glare.
The macula is responsible for the central, high-resolution, color vision possible in good light. If the macula is damaged in macular degeneration or other diseases involving this region, this type of vision is impaired. In the fundus examination, which we call ophthalmoscopy, the macula can be seen when viewed through the pupil.
The term macula lutea comes from the Latin macula, “point” and lutea, “yellow”. That’s why it is popularly known as the yellow spot.
Because the macula is yellow in color, it absorbs excess blue and ultraviolet light entering the eye and acts as a natural sunscreen for this area of the retina. Thanks to the pigments in the macula, the sensitive photoreceptor cells in this shade provide protection like a natural sunglasses. The yellow color expression called macula lutea is due to these pigments in its content. The yellow color comes from the content of dietary yellow xanthophyll carotenoids, lutein and zeaxanthin. Zeaxanthin is more dominant in the macula. There is information that these carotenoids protect this region from some diseases. Although these carotenoids have not been shown to prevent age-related macular degeneration, they have been shown to reduce the risk of disease progression to advanced stages.
The above photo contains a very small area of the base of the eye (retina) that includes only the macula and optic nerve region (posterior pole retina). In this photograph, the yellow round structure where the eye vessels enter and exit shows the head of the optic nerve, while the darker round region than the other areas of the retina shows the central part of the macula, which we call the fovea.
Cellular structures in the macula are specialized for very high acuity vision. The fovea region of the macula contains a high density of cones, which are highly acuity photoreceptor nerve cells.
The normal human eye contains three different types of cone cells with different spectral sensitivity ranges. The brain combines signals from neighboring cones to distinguish different colors. Depending on the lack of these different types of cone cells, different color blindness may occur.
On the other hand, rod photoreceptors, which are more sensitive to visual information than cones, are uniform and are more dominant photoreceptors than cones in dim light. That is, when the light is low, the rod cells are more active.
However, it is impossible to distinguish colors without the information provided by the individual spectral sensitivity of the cones. Cones predominate in the center of the fovea and are found in high density. The macula is therefore responsible for the central, high-resolution color vision possible in good light. For this reason, this type of vision is impaired in diseases where the macula is damaged, such as macular degeneration (yellow spot disease). Therefore, in diseases involving retinal areas outside the macula, the patient may not notice this effect in the eye immediately, whereas in diseases involving the macula, central vision may be blurred. Some patients may not notice early macular involvement in one eye when both eyes are open. For this reason, it is useful to occasionally close one eye and test the vision quality of the eyes one by one.
Beneath the fovea is the RPE, a layer of cells that acts as a protector for the macula. Photoreceptors and RPE cells are interlocked and there is an exchange of neurochemical factors necessary for vision between them. Photoreceptors and RPE cells work together to function. RPE cells are located in a single layer on a very thin layer called Bruch’s membrane, under which there is a layer of vascular network called the choroid. Under normal circumstances, Bruch’s membrane acts as a barrier against blood vessels from the choroid growing into and below the macula. At the same time, oxygen and nutrients pass from the choroid to the macula, and waste products from the macula to the choroid. The macula is very active and uses a lot of energy and oxygen.
There are many congenital and acquired diseases of the retina. Among these diseases, unfortunately, the diseases affecting the macula are more. Visual problems such as age-related macular degeneration, epiretinal membrane, and macular hole directly affect the macula. Diabetic retinopathy also often causes macular edema, resulting in vision loss. prof. Dr. Umit INAN
