Spatial density of retinal photoreceptors
This page is dedicated to the study of simplified distribution of the retinal photoreceptors, which are of two types in the human retina. the rods and cones. These cells contain the Visual pigment in their external articles (the deeper layer of the retina). The cones are the cells that allow the perception of the details, and who are sensitive to colors (there are 3 types of cones, which are more or less sensitive to various wavelength with a peak of sensitivity near 555 nm): they require a sufficient amount of light to "work." The cones are the cells that determine the level of the' eyesight and of the resolving power of the eye.
The sticks are very sensitive to light, but their density and their connections with other retinal cells (several sticks for a bipolar cell) do not allow them to discern fine details of an image.
Curcio et al. (The journal of comparative neurology - 1990) measured the density space of photoreceptors (rods and cones) on retinas of 7 human eyes taken post-mortem, and analyzed in histological way.
The human retina contains 4.6 million cones on average, (4.08 - 5.29 million). The peak density of cones in the fovea is 199000 cones by mm2: this figure is an average that masks a significant inter individual variability. In this study, the extreme densities were 100000 cones to 324000 cones per mm2. This maximum density can be measured on a surface which the angular extension can reach 0.032 deg2 (about 0.2 degree of Visual field, or 12 minutes of arc 0.0025 mm2 surface foveolaire). However, at 1 mm of this peak, the density is cones is ten times less. The size of the outer segment of the cones varies between the central region (1.6 to 2.2 microns) and the periphery macular (4 microns and more).
The overall density of the cones is more important in the nasal retina than the temporal retina. However, the number of cones seems relatively constant from one eye to the other: their spatial distribution is however quite variable between subjects. The fovea is devoid of sticks on a diameter of 600 microns. Beyond sticks appear progressively and their density becomes equal to that of the cones at a distance of 400 to 500 microns to the center of the fovea. The cones have a diameter of about 4 microns and are one of the sticks is close to 2 microns.
At the center of the fovea, the cones have a more often hexagonal perimeter: they form a joined structure in honeycomb. It has been mathematically demonstrated in 1999 that this kind of arrangement is optimal: it promotes a "paving" of maximum density, by minimizing the perimeter of the contiguous 'cells'. Bees use this type of motive for the realization of the inserts of the booty, the hexagonal arrangement allows them to minimize the wax used for the creation of the alveoli.
The retina contains on average 92 million sticks (77.9 to 107.3 million). The fovea is devoid of sticks on a horizontal diameter of approximately 0.350 mm (either 1.25 degree of Visual field). The density of the sticks is maximum along an elliptic perimeter area at the same distance as the optic nerve of the fovea. The density of sticks seems maximum within the superior retina.