Subventricular zone

The subventricular zone (SVZ) is present in both embryonic and adult neural tissues in the vertebrate central nervous system (CNS). In embryonic life, the SVZ refers to a secondary proliferative zone containing neural progenitor cells, which divide to produce neurons in the process of neurogenesis. The primary neural stem cells of the brain and spinal cord, termed radial glial cells, reside in the ventricular zone (VZ) (so-called because the VZ lines the developing ventricles). In the developing cerebral cortex, which resides in the dorsal telencephalon, the SVZ and VZ are transient tissues that do not exist in the adult. However, the SVZ of the ventral telencephalon persists throughout life. The subventricular zone (SVZ) is present in both embryonic and adult neural tissues in the vertebrate central nervous system (CNS). In embryonic life, the SVZ refers to a secondary proliferative zone containing neural progenitor cells, which divide to produce neurons in the process of neurogenesis. The primary neural stem cells of the brain and spinal cord, termed radial glial cells, reside in the ventricular zone (VZ) (so-called because the VZ lines the developing ventricles). In the developing cerebral cortex, which resides in the dorsal telencephalon, the SVZ and VZ are transient tissues that do not exist in the adult. However, the SVZ of the ventral telencephalon persists throughout life. The adult SVZ is a paired brain structure situated throughout the lateral walls of the lateral ventricles. It is composed of four distinct layers of variable thickness and cell density, as well as cellular composition. Along with the dentate gyrus of the hippocampus, the SVZ is one of two places where neurogenesis has been found to occur in the adult mammalian brain. The innermost layer (Layer I) contains a single layer (monolayer) of ependymal cells lining the ventricular cavity; these cells possess apical cilia and several basal expansions that may stand in either parallel or perpendicular to the ventricular surface. These expansions may interact intimately with the astrocytic processes that are interconnected with the hypocellular layer (Layer II). The secondary layer (Layer II) provides for a hypocellular gap abutting the former and has been shown to contain a network of functionally correlated Glial Fibrillary Acid Protein (GFAP)-positive astrocytic processes that are linked to junctional complexes, yet lack cell bodies except for the rare neuronal somata. While the function of this layer is yet unknown in humans, it has been hypothesized that the astrocytic and ependymal interconnections of Layer I and II may act to regulate neuronal functions, establish metabolic homeostasis, and/or control neuronal stem cell proliferation and differentiation during development. Potentially, such characteristics of the layer may act as a remainder of early developmental life or pathway for cellular migration given similarity to a homologous layer in bovine SVZ shown to have migratory cells common only to higher order mammals. The third layer (Layer III) forms a ribbon of astrocyte cell bodies that are believed to maintain a subpopulation of astrocytes able to proliferate in vivo and form multipotent neurospheres with self-renewal abilities in vitro. While some oligodendrocytes and ependymal cells have been found within the ribbon, they not only serve an unknown function, they are uncommon by comparison to the population of astrocytes that reside in the layer. The astrocytes present in Layer III can be divided into three populations through electron microscopy, with no unique functions yet recognizable; the first type is a small astrocyte of long, horizontal, tangential projections mostly found in Layer II; the second type is found between Layers II and III as well as within the astrocyte ribbon, characterized by its large size and many organelles; the third type is typically found in the lateral ventricles just above the hippocampus and is similar in size to the second type but contains few organelles. The fourth and final layer (Layer IV) serves as a transition zone between Layer III with its ribbon of astrocytes and the brain parenchyma. It is identified by a high presence of myelin in the region. Four cell types are described in the SVZ: 1. Ciliated Ependymal Cells (Type E): are positioned facing the lumen of the ventricle, and function to circulate the cerebrospinal fluid. 2. Proliferating Neuroblasts (Type A): express PSA-NCAM (NCAM1), Tuj1 (TUBB3), and Hu, and migrate in line order to the Olfactory Bulb