Melanopsin-containing retinal ganglion cells (mRGCs) represent a third class of retinal photoreceptors involved in regulating the pupillary light reflex and circadian photoentrainment, among other things. The functional integrity of the circadian system and
melanopsin cells is an essential component of well-being and health, being both impaired in aging and disease. Here we review evidence of
melanopsin-expressing cell alterations in aging and
neurodegenerative diseases and their correlation with the development of
circadian rhythm disorders. In healthy humans, the average density of
melanopsin-positive cells falls after age 70, accompanied by age-dependent
atrophy of dendritic arborization. In addition to aging, inner and outer
retinal diseases also involve progressive deterioration and loss of mRGCs that positively correlates with progressive alterations in circadian rhythms. Among others, mRGC number and plexus complexity are impaired in
Parkinson's disease patients; changes that may explain sleep and
circadian rhythm disorders in this pathology. The key role of mRGCs in circadian photoentrainment and their loss in age and disease endorse the importance of eye care, even if vision is lost, to preserve
melanopsin ganglion cells and their essential functions in the maintenance of an adequate quality of life.