Neurogenesis is the process by which nervous system cells, the neurons, are produced by neural stem cells (NSCs). It occurs in all species of animals except the porifera (sponges) and placozoans. Types of NSCs include neuroepithelial cells (NECs), radial glial cells (RGCs), basal progenitors (BPs), intermediate neuronal precursors (INPs), subventricular zone astrocytes, and subgranular zone radial astrocytes, among others.
Neurogenesis is most active during embryonic development and is responsible for producing all the various types of neurons of the organism, but it continues throughout adult life in a variety of organisms. Once born, neurons do not divide (see mitosis), and many will live the lifespan of the animal, except under extraordinary and usually pathogenic circumstances.
During embryonic development, the mammalian central nervous system (CNS; brain and spinal cord) is derived from the neural tube, which contains NSCs that will later generate neurons. However, neurogenesis doesn't begin until a sufficient population of NSCs has been achieved. These early stem cells are called neuroepithelial cells (NEC)s, but soon take on a highly elongated radial morphology and are then known as radial glial cells (RGC)s. RGCs are the primary stem cells of the mammalian CNS, and reside in the embryonic ventricular zone, which lies adjacent to the central fluid-filled cavity (ventricular system) of the neural tube. Following RGC proliferation, neurogenesis involves a final cell division of the parent RGC, which produces one of two possible outcomes. First, this may generate a subclass of neuronal progenitors called intermediate neuronal precursors (INP)s, which will divide one or more times to produce neurons. Alternatively, daughter neurons may be produced directly. Neurons do not immediately form neural circuits through the growth of axons and dendrites. Instead, newborn neurons must first migrate long distances to their final destinations, maturing and finally generating neural circuitry.