Agerelated Changes In The Diameter Of The Acromeres And The Coverage Of The Photoreceptors

The area of selective rod loss is located 3 mm from the fovea. The spaces left empty by the cell loss are occupied by larger rods. The diameter of the rod acromeres ranges from 2.2 to 2.8 mm in young retinas, and those of surviving rods in older retinas are 13.5% larger in the same eccentricity range. One of the most significant results of Curcio et al. (1993) is that the spatial density of rods reduces by around 30% in the central 28.5° during the period from 34 to 90 years in apparently healthy eyes. This loss is less in the fovea between 45 and 61 years and from the ninth decade is larger in a parafovial ring from 0.5 to 3 mm eccentricity. Notwithstanding the diffuse loss of rods in the central retina, the appearance of the photoreceptor mosaic is qualitatively normal, as only around 2 rods/mm2 are lost per day. Furthermore, the retina does not show either gaps or the insertion of other types of cells (glia), such that the extension of the acromeres of the remaining rods is equal to that of younger retinas. There is no evidence of an age-related decrease in the quantity of rhodopsin as might be expected with the loss of photoreceptors. The remaining photoreceptors are sufficient to perform their function.

Using densitometry, it has been shown that there is a small but significant increase in the density of the pigment of the rods between 12 and 78 years up to the temporal 16°. As the number of rods in this area diminishes with age, the acromeres of the remaining rods should contain more pigment in older retinas than in younger ones. This increase may be at least partly due to the twisting that effectively increases the total length off the acromeres by around 40% without varying the distance between the outer limiting membrane and the RPE. It is calculated that this process affects from 10 to 20% of the total rod population from the seventh decade, whereas in the central retina it is from 2% in both young and old donors, suggesting that the increase in rhodopsin can be due to the fact that the acromeres of elderly eyes are larger than those of younger eyes but morphologically similar. For example, if the diameter of the acromeres increases in proportion with the inner segments, the membrane of the disk would contain 29% more pigment in older eyes than in younger. The compensatory hypertrophy of the acromeres of the rods is in agreement with the proof of the plasticity of the rod system in adults after its partial loss.

At this point we ask ourselves if the age-related loss of rods contributes to the reduced scotopic function. Since the extension of the rods is the same in young and old retinas, it is thought that other factors such as changes in the sensitivity of the photo-pigments, regeneration, response, neuro-transmitters, and the membrane charge weaken the sensitivity of the rods.

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Blood Pressure Health

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