Neurogenesis is the process by which new neurons are formed in the brain. Neurogenesis is crucial when an embryo is developing, but also continues in certain brain regions after birth and throughout our lifespan.   

The mature brain has many specialised areas of function, and neurons that differ in structure and connections. The hippocampus, for example, which is a brain region that plays an important role in memory and spatial navigation, alone has at least 27 different types of neurons.

The incredible diversity of neurons in the brain results from regulated neurogenesis during embryonic development. During the process, neural stem cells differentiate—that is, they become any one of a number of specialised cell types—at specific times and regions in the brain.

Neural stem cells can produce new neural cells of any type. When stem cells from the brain are isolated and grown in a dish, they continuously divide and create large spherical masses of cells, similar to the two shown here. Each spherical mass, called a neurosphere, is produced by a single stem cell. When exposed to different chemicals, the cells turn into either neurons (red) or glia (cyan). Cell nuclei are shown in dark blue. (Image: Chanel Taylor / QBI)

Stem cells can divide indefinitely to produce more stem cells, or differentiate to give rise to more specialised cells, such as neural progenitor cells. These progenitor cells themselves differentiate into specific types of neurons. As seen in the diagram below, neural stem cells can also differentiate into glial progenitor cells, which give rise to glial cells such as astrocytesoligodendrocytes and microglia.

Neural stem cells have the potential to generate all neural cell types. They differentiate into neuronal progenitor cells, which give rise to neuron, or glial progenitors, which give rise to glial cells. (Image Credit: NCD Project / CC BY-SA 3.0 via Commons)

Until recently, neuroscientists believed that the central nervous system, including the brain, was incapable of neurogenesis and unable to regenerate. However, stem cells were discovered in parts of the adult brain in the 1990s, and adult neurogenesis is now accepted to be a normal process that occurs in the healthy brain.