Visual System Development in Vertebrates

Abstract

The development of a functional visual system involves a complex series of inductive and signalling interactions essential for the formation of the eye and its central connections with the brain. Outpouchings from the forebrain, together with the overlying surface ectoderm and neural crest cells give rise to the major structures of the eye (neural retina, pigmented epithelium, lens and cornea). Central connections between the eye and brain regions that receive direct connections from the eye (visual targets) are formed by the axons of retinal ganglion cells. Attractive and repulsive cues in the extracellular environment guide the retinal axon along specific pathways in the brain. Gradients of signalling molecules, together with spontaneous neural activity, drive a point‐to‐point mapping of retinal axons in visual targets, ensuring accurate reconstruction of the visual image. The cellular, molecular and inductive mechanisms that sculpt each of these key developmental processes essential for normal visual system development are beginning to be understood. Key Concepts: The eye primordia (optic cups) develop as outgrowths from the forebrain. Contact of the optic cup with the overlying surface ectoderm induces formation of the lens. Signals from the lens induce the formation of the cornea. Retinal cells differentiate into overlapping central–peripheral waves to form three layers of cell bodies separated by two plexiform (synaptic) layers. Inhibitory signalling and local adhesive interactions control synaptic specificity in the retina. Attractive and repulsive signals in the extracellular environment direct growth of retinal axons to target regions in the brain. Retinal axons map topographically in target regions of the brain. Map formation is controlled through the graded action of repulsive signalling molecules and spontaneous retinal activity. Activity‐dependent mechanisms drive the formation of eye‐specific layering of synaptic inputs in visual targets.