Neural Stem Cells
The mature mammalian central nervous system (CNS) is composed of three major differentiated cell types: neurons, astrocytes and oligodendrocytes. Neurons transmit information through action potentials and neurotransmitters to other neurons, muscle cells or gland cells. Astrocytes and oligodendrocytes, collectively called glial cells, play important roles of their own, in addition to providing a critical support role for optimal neuronal functioning and survival. During mammalian embryogenesis, CNS development begins with the induction of the neuroectoderm, which forms the neural plate and then folds to give rise to the neural tube. Within these neural structures there exists a complex and heterogeneous population of neuroepithelial progenitor cells (NEPs), the earliest neural stem cell type to form.1,2 As CNS development proceeds, NEPs give rise to temporally and spatially distinct neural stem/progenitor populations. During the early stage of neural development, NEPs undergo symmetric divisions to expand neural stem cell (NSC) pools. In the later stage of neural development, NSCs switch to asymmetric division cycles and give rise to lineage-restricted progenitors. Intermediate neuronal progenitor cells are formed first, and these subsequently differentiate to generate to neurons. Following this neurogenic phase, NSCs undergo asymmetric divisions to produce glial-restricted progenitors, which generate astrocytes and oligodendrocytes. The later stage of CNS development involves a period of axonal pruning and neuronal apoptosis, which fine tunes the circuitry of the CNS. A previously long-held dogma maintained that neurogenesis in the adult mammalian CNS was complete, rendering it incapable of mitotic divisions to generate new neurons, and therefore lacking in the ability to repair damaged tissue caused by diseases (e.g. Parkinson’s disease, multiple sclerosis) or injuries (e.g. spinal cord and brain ischemic injuries). However, there is now strong evidence that multipotent NSCs do exist, albeit only in specialized microenvironments, in the mature mammalian CNS. This discovery has fuelled a new era of research into understanding the tremendous potential that these cells hold for treatment of CNS diseases and injuries.
