This review highlights important events during the morphological development of retinal ganglion cells (RGCs), focusing on mechanisms that control axon and dendritic arborization as a means to understand synaptic connectivity with special emphasis on the role of neurotrophins during structural and functional development of RGCs. Neurotrophins and their receptors participate in the development of visual connectivity at multiple levels. In the visual system, neurotrophins have been shown to exert various developmental influences, from guiding the morphological differentiation of neurons to controlling the functional plasticity of visual circuits. This review article examines the role of neurotrophins, and in particular of BDNF, during the morphological development of RGCs, and discusses potential interactions between activity and neurotrophins during development of neuronal connectivity. Retinal ganglion cells (RGCs), the sole projection neurons from the retina, integrate, process, and convey all visual information transmitted to the brain (Dowling, 1987). Within the retina, RGCs attain their fates and differentiate early, preceding other types of retinal neurons (Fig. 1). RGCs reside near the lens in the ganglion cell layer (GCL) and exhibit distinctive, characteristic morphologies. Different morphological subtypes of mature RGCs are characterized by soma size and dendritic arborization patterns. To receive synaptic inputs from amacrine and bipolar neurons, each RGC elaborates several branched dendrites into the retina's inner plexiform layer (IPL) and to provide synaptic input to the central targets, each RGC extends a single axon that exits the retina via the optic nerve to synapse on neurons in the brain (the optic tectum in lower vertebrates or superior colliculus, pretectum, and lateral geniculate nucleus in higher vertebrates). When the RGC growth cone has reached and recognized its target region, the growth cone transforms into an actively branching axon terminal, extending and retracting branches it forms synapses with target neurons in the brain. The events of RGC axon extension, growth cone pathfinding, and target recognition all coincide with RGC dendritic arborization in the retina. RGCs begin to extend primary dendrites as their axons course towards their targets, and continue to elaborate complex dendritic arbors as their axon terminals innervate and branch within the target (Holt, 1989).
KEY WORDS: retina, optic tectum, arborization, BDNF, visual systemThis review highlights important events during the morphological development of RGC that influence both the presynaptic (dendritic) and postsynaptic (axon) connectivity of RGC projection neurons. The elucidation of the events that guide development and differentiation of RGCs is due in part to recent advances in in vivo imaging techniques that have permitted visualization of cellular events that underlie the establishment of RGC synaptic connectivity (Cohen-Cory, 2002;Debski and Cline, 2002). The frog and fish retinotectal projections have p...