This chapter deals with adult neurogenesis and examines what is known about the behavior and function of precursor cells in the adult brain. It outlines few examples of normally occurring neurogenesis in the mammalian central nervous system (CNS), and describes adult neural precursors. Functional adult neurogenesis occurs in many non-mammalian vertebrates. The chapter reviews a few lines of recent research demonstrating that endogenous neural precursors can be induced to differentiate into neurons in regions of the adult brain that do not normally undergo neurogenesis. In the adult mammalian brain, neurogenesis normally occurs only in the olfactory bulb and the dentate gyrus (DG) of the hippocampus. Transplantation studies support the concept of neurogenic and non-neurogenic regions, and provide evidence about the role of the microenvironment in realizing the potential of neuronal stem or progenitor cells. Neuronal replacement therapies based on manipulation of endogenous precursors may be possible in the future.

This chapter provides an overview of molecular mechanisms that guide axon extension during neural development. It introduces the growth cone, a specialized motile structure at the tip of the axon responsible for sensing and responding to guidance cues. Growth cone morphology is a direct consequence of the organization of the two main components of its cytoskeleton, microtubules, and filamentous actin. The chapter describes the trajectory of embryonic spinal commissural axons, and reviews axonal guidance cues. It presents a description of the growing understanding of the cellular and molecular mechanisms that transduce extracellular guidance cues into directed axon growth. As an axon extends along its trajectory, its growth cone has the capacity to change its response to local guidance cues. The chapter concludes with a discussion on the possibility that cues now known to regulate axon guidance during development may subsequently influence axon regeneration in the adult central nervous system (CNS).