Morphological and Neurochemical Development of the Human Neural Retina

The human retina, a derivative of the diencephalon, differentiates into its components in sequential patterns following a spatiotemporal order. The neuronal differentiation in the fovea and central retina begins earlier than that occurring elsewhere, completes by midgestation (19–21 weeks) and continues in the periphery until 30 weeks of gestation. By birth, all retinal layers and neurons are mature over its entire extent, except at the fovea, where photoreceptor differentiation (outer segments) continues up to 5 months postnatally. During differentiation, the neurons express an array of marker molecules, such as amino acid neurotransmitters, neuropeptides, calcium-binding proteins, nitric oxide and proteins of phototransduction pathways. Although their precise involvement in retinal development is unclear, their patterns of expression indicate that many of them may be necessary for neuronal differentiation. In order to gain knowledge on this issue, future studies utilizing embryonic human retinal culture and transplantation of the tissue into suitable hosts and characterization of the role of neurochemicals involved in retinal development appears to be worthwhile. This approach may highlight novel information about the specific neurochemical requirements for the adult retina in normal and pathological states.