EPENDYMA AND SUBEPENDYMA OF ADULT MAMMALIAN BRAIN AS A SOURCE OF NEURAL STEM CELLS

The brain of adult animals, including humans, sustained the ability to create new glial cells and neurons. This process is called postnatal neurogenesis. It is possible owing to the presence of neuronal stem cells in the neurogenic zones. The subependymal layer, a part of periventricular zone, demonstrates the highest intensity of neurogenesis. Ependymal cells, forming the external lining of the lateral ventricles, can also effectively differentiate into cells of the nervous system. Ependymal cells (ependymocytes) originate from radial glia and are created during embrional and early postnatal development. Ependymocytes of adult mammalian brain form migrating cells differentiating into astrocytes and neurones. Ependymal cells express neuroprogenitor markers like Sox2 or nestin and adult stem cells marker - CD133. Subependyma is layered below the uniform ependyma lining. It is formed of a few diverse types of cells. There are numerous data indicating that subependymal, GFAP-positive astrocytes proliferate intensively and they have the characteristics of neural stem cells. The subependymal zone is a niche for neural stem cells, neuronal and glial precursor (progenitor) cells, neuroblasts and glioblasts that are immature neurons and glia, respectively. Ependymal and subependymal zone emerges to be a rich source of neural stem cells. Neural stem cells (NSCs) are primary cells with the ability of self-renewal or differentiation into one of three types of nervous system cells: neurons, oligodendrocytes or astrocytes. NSCs isolated from neurogenic zones of adult mammalian brain can be cultured in vitro in two systems: as monolayer system or as culture of neurospheres. Neurospheres cultured in vitro are spherical or elipsoidal structures formed of hundreds of cells surrounded by an envelope rich in extracellular matrix components. The cells forming neurosphere differ in their size, presence of cytosolic granules, number of mitochondria, the phase of cell cycle. Neurospheres appear as complex biological structures in which events such as phagocytosis, mitosis, apoptosis and even necrosis occur at the same time. The amount of NSCs in neurospheres is estimated as about 0.16%. In physiological conditions proliferation and differentiation of neural stem cells are precisely regulated through different interactions and signalization systems within the niche. The attempt to recognize these complex interactions and to fully characterize all the factors and signaling systems is still a challenge for the researchers. In this article we describe only these molecules and pathways of signalization that are quite well characterized. Our understanding of the mechanisms regulating proliferation and differentiation of NSCs will allow to precisely control these processes in in vitro cultures. These findings will help to applicate NSCs in the therapy of neurodegenerative diseases like Alzheimer or Parkinson disease, brain injury and stroke.