Neurons are one of the most morphologically diverse cell types, in large part owing to their intricate dendrite branching patterns. Dendrites are structures that are specialized to receive and process inputs in neurons, thus their specific morphologies reflect neural connectivity and influence information flow through circuits. Recent studies in Drosophila on the molecular basis of dendrite diversity, dendritic guidance, the cell biology of dendritic branch patterning and territory formation have identified numerous intrinsic and extrinsic cues that shape diverse features of dendrites. As we discuss in this review, many of the mechanisms that are being elucidated show conservation in diverse systems. IntroductionDendrites -processes of neurons that are primarily specialized for information input -are one of nature's remarkable architectural feats, and the diverse growth patterns shown by dendritic arbors raise important developmental questions. The particular shapes of dendrites are important in neuronal function and circuit assembly. Their targets and complexity influence the range of inputs that a neuron receives. In addition, the morphology of a dendritic arbor can impact the processing and integration of electrical signals (London and Häusser, 2005). Studies of dendrite morphogenesis therefore seek to understand the developmental origin of arbor shape and to shed light on the significance of particular morphologies for nervous system connectivity and function.Dendrite morphogenesis consists of a series of interrelated steps, which include outgrowth and branching, guidance and targeting, cessation of growth and, in some cases, arbor remodeling (see Box 1). Each process is under extensive genetic regulation and has been the subject of intensive study in recent years. In this review, we highlight recent advances in understanding the molecules and mechanisms that function during these key stages of dendrite morphogenesis. We focus primarily on studies carried out in Drosophila (Fig. 1) and refer to known or emerging areas of conservation in vertebrate systems where appropriate. We focus on several key questions, including: what are the cell biological mechanisms that specify the distribution of dendritic branches along an arbor? How do dendrites achieve typespecific branching patterns? How is specific dendritic targeting controlled in different neurons? How are dendrites instructed when to stop branching and growing? How does activity impact dendrite development in Drosophila? We refer readers to other reviews that cover topics that have thus far been studied primarily in vertebrate systems, including dendritic spine morphogenesis and activitydependent dendrite growth (Alvarez and Sabatini, 2007;Chen and Ghosh, 2005;Flavell and Greenberg, 2008;Lippman and Dunaevsky, 2005;Redmond, 2008). Genetic insights into the cell biology of dendrite growth and branchingThe growth and specialized functions of dendritic arbors can require large investments of dendritic plasma membrane and proteins during development, and, inde...

Glia serve many important functions in the mature nervous system. In addition, these diverse cells have emerged as essential participants in nearly all aspects of neural development. Improved techniques to study neurons in the absence of glia, and to visualize and manipulate glia in vivo, have greatly expanded our knowledge of glial biology and neuron–glia interactions during development. Exciting studies in the last decade have begun to identify the cellular and molecular mechanisms by which glia exert control over neuronal circuit formation. Recent findings illustrate the importance of glial cells in shaping the nervous system by controlling the number and connectivity of neurons.

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