Research into the cellular and molecular mechanisms of dendritic growth has blossomed in the last 5 years. Although research in the past 20 years has markedly increased our general understanding of the cellular and molecular mechanisms of axonal outgrowth and pathfinding, until recently, relatively little was known about the signals that guide dendritic growth. Such precise interconnections between neurons suggest a high degree of cellular and molecular regulation during development. In many cases, these early synaptic connections are sculpted and remodeled by neuronal activity to achieve the mature pattern of connectivity in the brain ( Goodman and Shatz, 1993 Katz and Shatz, 1996). Finally, highly specific connections between neurons - synapses - are formed. Subsequently, neurons elaborate their axons and dendrites in patterns characteristic for each cell type. First, neurons are born and migrate to their final positions in the nervous system. Nervous system development comprises several stages. Thus, the proper growth and arborization of dendrites are crucial for proper functioning of the nervous system. Consequently, the results of defects in dendritic growth are profound, often accompanying severe neurodevelopmental disorders such as mental retardation ( Purpura, 1975). As dendrites are the site of most synaptic contacts, dendritic development determines the number and pattern of synapses received by each neuron ( Hume and Purves, 1981 Purves and Hume, 1981 Purves et al., 1986). In general, neurons across the nervous system exhibit type-specific patterns of dendritic arbors with highly specified membrane properties that define the computational capability of each cell ( Rall, 1995 Stuart et al., 1999). The nervous system is composed of a vast number of neurons with characteristic afferent and efferent projections, dendritic morphologies and molecular identities. This manuscript reviews our current understanding of the cellular and molecular mechanisms of dendritic growth, the influence of activity in sculpting specific patterns of dendritic arbors, and a potential integral role for dendrites in activity-dependent development of circuits in the nervous system. In the last few years, however, overwhelming evidence has accumulated indicating that dendritic growth is remarkably dynamic and responsive to environmental signals, including guidance molecules and levels and patterns of activity. Until recently, there was relatively little known about the cellular and molecular mechanisms of dendritic growth, perhaps because dendrites were historically considered to be intrinsically determined, passive elements in the formation of connections in the nervous system. Proper growth and branching of dendrites are crucial for nervous system function patterns of dendritic arborization determine the nature and amount of innervation that a neuron receives and specific dendritic membrane properties define its computational capabilities.
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