Output of the Subventricular Zone

The SVZ produces both neurons and glia, but the types of cells produced differ both regionally and temporally. For example,

Subventricular Zone Mouse

FIGURE 15. Expression domains of Dlx1, Dlx2, Dlx5, and Dlx6 during mouse brain development. (Top) Schema of a transverse section through the E12.5 mouse telencephalon showing the combined expression of Dlx transcripts. Most cells in the subpallial telencephalon express Dlx1, Dlx2, Dlx5, or Dlx6 at some stage of their differentiation. The arrows indicate the migration from the subpallium to the pallium (cortex) (Marin and Rubenstein, 2001). The boxed region on the left is used in the middle section to show the expression of Dlx2, Dlx1, Dlx5, and Dlx6. Dlx2 is primarily expressed in undifferentiated cells; it is expressed in scattered cells in the ventricular zone, in most cells in the subventricular zone and in scattered cells in the mantle zone. Dlx6 is primarily expressed in differentiated cells in the mantle zone. Dlx1 and Dlx5 are expressed in intermediate patterns. (Bottom) A hypothesized genetic and biochemical pathway that proposes the sequential role of Dlx2, Dlx1, Dlx5, and Dlx6 at different stages of differentiation. Telencephalic regions are as follows. Pallium: neocortex (NCX) and pallio-cortex (PCX). Subpallium: lateral ganglionic eminence (LGE). Medial ganglionic eminence (MGE). Stages of differentiation: ventricular zone (VZ); subventricular zone (SVZ); mantle zone (MZ). LV, lateral ventricle (ventricle of telencephalon); III, third ventricle (ventricle of the diencephalon). Figure modified from Panganiban and Rubenstein 2002.

although the neocortical SVZ coexists with the VZ for much of the time that neurons are produced for the neocortex (Takahashi et al., 1995b), and, thus, it is possible that the SVZ may produce a small number of neurons destined for the neocor-tex (Reynolds and Weiss, 1992; Levinson and Goldman, 1993), during this time virtually all of the daughter cells of the SVZ re-enter the cell cycle, and, thus, the proportion of neurons produced is estimated to comprise only at most 5-10% of the total (Takahashi et al., 1995b). The anterior part of the SVZ is, however, a major producer of the neurons of the olfactory bulb both during the prenatal and postnatal periods and also into adulthood (Hinds, 1968a, b; Luskin, 1993; Lois and Alvarez-Buylla, 1994; Luskin and McDermott, 1994). Neurons are also produced by the lateral and medial ganglionic eminence. Many of these neurons take up residence locally and comprise the future striatum

Non-Myeliriatirig OL Myelinating OL Myelinating OL Satellite OL

Sub Ventricular Zone

FIGURE 16. The descendants of the perinatal SVZdl. Depicted are the types of cells that are generated from postnatal day 2 SVZdl cells. Progenitors that leave the SVZdl and differentiate within the subcortical white matter become either myelinating or nonmyelinating oligodendrocytes. Few become astrocytes. Those progenitors that differentiate within the neocortex become myelinating oligodendrocytes as well as satellite oligodendrocytes and cells that label with the NG2 proteoglycan. Additionally, those progenitors that make contact with naked cerebral endothelial cells become protoplasmic astrocytes. Figure modified from Brazel et al., 2003.

FIGURE 16. The descendants of the perinatal SVZdl. Depicted are the types of cells that are generated from postnatal day 2 SVZdl cells. Progenitors that leave the SVZdl and differentiate within the subcortical white matter become either myelinating or nonmyelinating oligodendrocytes. Few become astrocytes. Those progenitors that differentiate within the neocortex become myelinating oligodendrocytes as well as satellite oligodendrocytes and cells that label with the NG2 proteoglycan. Additionally, those progenitors that make contact with naked cerebral endothelial cells become protoplasmic astrocytes. Figure modified from Brazel et al., 2003.

(Bhide, 1996). Another group of them have an interesting fate in that they migrate laterally and populate the neocortex (Marin and Rubenstein, 2001; Letinic et al., 2002). This laterally migrating group produces many of the inhibitory interneurons (i.e., GABAergic) of the neocortex. An additional population of inhibitory interneurons (GABAergic) is produced in the gan-glionic eminence of humans (but not in other primates or mice) that are destined for the dorsal thalamic nuclei in the thalamus (Rakic and Sidman, 1969; Letinic and Rakic, 2001). The migration of these telencephalic neurons into the dorsal thalamus forms a structure that is large enough to warrant a name, "corpus gan-gliothalamicum" (Rakic and Sidman, 1969). This special output population suggests that the SVZ proliferative population is available for recent evolutionary modification.

During perinatal life, the SVZ is a principal spawning ground for neuroglial cells (Smart, 1961; Smart and Leblond, 1961; Privat, 1975; Mares and Bruckner, 1978; Smart and McSherry, 1982; LeVine and Goldman, 1988b; Levinson and Goldman, 1993). The best studied of the gliogenic portions of the SVZ is the dorsal-lateral portion of the SVZ, that is, the SVZdl (Fig. 14). Mapping studies using retroviral markers (reviewed by Brazel and Levison, 2003) show that this zone produces a variety of glial cell types (Fig. 16). During postnatal life and into adulthood, parts of the SVZ persist as a population of stem/progenitor cells that seem to proliferate for the lifetime of the animal. These stem/progenitor cells produce both neurons and glia; the largest portion of these seems to be destined for the olfactory bulb, which they reach through the rostral migratory stream (Alvarez-Buylla and Garcia-Verdugo, 2002).

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