The subependymal zone neurogenic niche: a beating heart in the centre of the brain: how plastic is adult neurogenesis? Opportunities for therapy and questions to be addressed.

The mammalian brain is a remarkably complex organ comprising millions of neurons, glia and various other cell types. Its impressive cytoarchitecture led to the long standing belief that it is a structurally static organ and thus very sensitive to injury. However, an area of striking structural flexibility has been recently described at the centre of the brain. It is the subependymal zone of the lateral wall of the lateral ventricles. The subependymal zone--like a beating heart--continuously sends new cells to different areas of the brain: neurons to the olfactory bulbs and glial cells to the cortex and the corpus callosum. Interestingly, the generation and flow of cells changes in response to signals from anatomically remote areas of the brain or even from the external environment of the organism, therefore indicating that subependymal neurogenesis--as a system--is integrated in the overall homeostatic function of the brain. In this review, it will be attempted to describe the fundamental structural and functional characteristics of the subependymal neurogenic niche and to summarize the available evidence regarding its plasticity. Special focus is given on issues such as whether adult neural stem cells are activated after neurodegeneration, whether defects in neurogenesis contribute to neuropathological conditions and whether monitoring changes in neurogenic activity can have a diagnostic value.