Editorial: The neural economy hypothesis: Changes with aging and disease to cones and other central nervous system visual neurons

Photoreceptors and other central nervous system cells are considered post-mitotic and at their maximum numbers before birth, although the human retina undergoes considerable developmental changes in the first few years of life (Hendrickson and Yuodelis, 1984). In the human fovea, the packing of cones becomes denser at the central fovea by migration, not by new cones being formed. Cone outer segments elongate by adding more segments over time, and the inner retinal cells move to more eccentric locations, reducing light scatter to enhance foveal acuity. To provide decades of vision these neurons and their networks must have inherent repair processes and be supported and renewed by glial and other cells. There are limitations to the extent of support available to visual system neurons, especially photoreceptors with their high metabolic rates, the assault by incident light on the retina, and the need for optical clarity that constrains the numbers and locations of blood vessels. Further, these support systems can fail with aging and disease, creating a harsh microenvironment for photoreceptors.