The human retina shows considerable age-related structural changes, particularly in its peripheral zones, although the macula and its fovea centralis are not spared. The aged retinal periphery is thinner (10-30 mm), containing a lesser number of rods and other nerve cell types. The aging-related loss of rods appears to be a slow process, beginning in the third and fourth decade, and may be related to accumulated damage due to physiological exposure to light (8-10). With aging rods, outer segments shorten and disengage from the microvilli of pigment epithelium, resulting in lesser amounts of membranous discs and their major constituent rhodopsin, the rods' photoreceptor molecule. These events may be related to changes in the turnover of rod discs with aging (8). Normally, the entire population of rod discs turns over every two weeks. This process and packing orderliness of discs slowly declines with aging, perhaps due to changes in the function of pigment epithelial cells that regulate the turnover of photoreceptor cells. The result is reduced efficiency of phototransduction (see below).
Aging of Retinal Neurons Early Studies on Retinal Aging
Retinas of humans and monkeys lose cones at a rate of 3% per decade (8). The turnover rate of cones was believed to be about a year, making them susceptible to accumulated effects of light damage and posttranslational modification of their photoreceptor proteins. Cone pigment density decreases with aging, presumably as a result of cone cell loss (9). The remaining cones increase in average diameter. Because size and geometry of foveal cones are important in visual acuity, these changes may contribute to the observed losses in visual acuity with age (see below). Nerve cell loss during aging in other neuronal types of retina, i.e., bipolar, amacrine, and horizontal cells, is around 30%. The loss for the ganglion cells is believed to be higher, about 50%. Mueller cells (a type of glia cell in the retina) take over the space left by the lost neurons and form cysts, which are common in the aged retina (7,8,11). The degeneration of macula in advanced age is discussed in section titled Aging and Eye Diseases.
Photoreceptor density decreases with age at a rate of 0.2% to 0.45% per year, with rods showing a more marked loss than cones (42). Cone mosaics are more organized in temporal regions compared to peripheral regions and show no change with aging, but aging distorts the regularity of the nasal peripheral cone mosaic (43). While foveal cone numbers remain stable with age, the parafoveal rods show a decrease of 30% by old age. This condition leads to greater loss of scotopic sensitivity compared to phototropic sensitivity (44). Short-, middle-, and long-wave-sensitive cones were analyzed for their sensitivity. A decrease in sensitivity, that is, an increase in threshold, was observed for all three types of cones (45).
In another recent study, Cavallotti et al. (46) studied numerous structural and biochemical changes in the human retinal tissues with aging. The study compared retinas from young adults and donors aged 60 and older, using scanning electron microscopy and biochemical methods. Highly significant changes in retinal thickness, numbers of ganglion cells, numbers of capillaries, synaptic bodies, and cellular processes as well as in intercellular connections were observed with aging. Also, tissue protein and cytoplasmic protein concentration were decreased significantly with age, but not structural protein concentrations.
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