REVIEWS THE CELL BIOLOGY OF NEUROGENESIS Magdalena Götz* and Wieland B. Huttner‡ Abstract | During the development of the mammalian central nervous system, neural stem cells and their derivative progenitor cells generate neurons by asymmetric and symmetric divisions. The proliferation versus differentiation of these cells and the type of division are closely linked to their epithelial characteristics, notably, their apical–basal polarity and cell-cycle length. Here, we discuss how these features change during development from neuroepithelial to radial glial cells, and how this transition affects cell fate and neurogenesis. DEVELOPMENTAL CELL BIOLOGY MACROGLIAL CELLS During development, neural stem cells give rise to These types of division were first deduced from retroviral Collective term for astrocytes, all the neurons of the mammalian central nervous cell-lineage-tracing experiments19–25 and were subse- oligodendrocytes and Schwann system (CNS). They are also the source of the two quently shown directly in live time-lapse observations cells. types of MACROGLIAL CELL in the CNS — ASTROCYTES and with brain slices26–31 and isolated cells in vitro32–37. OLIGODENDROCYTES1–5 ASTROCYTES . Usually, two criteria are applied to This review mainly discusses the cell-biological basis The main type of glial cell, define a cell as a stem cell — self-renewal, ideally for an of the symmetric versus asymmetric division of neural which has various supporting unlimited number of cell divisions, and multipotency, stem and PROGENITOR CELLS, concentrating on the devel- functions, including that is, the ability to give rise to numerous types of oping CNS of rodents (from which most of the available participating in the formation differentiated cell. However, as it is not clear to what data were derived) and focusing on issues such as CELL of the blood–brain barrier. A subpopulation of astrocytes extent multipotent stem cells exist during the develop- POLARITY, CLEAVAGEPLANE ORIENTATION, apical cell constitu- functions as adult neural stem ment of the CNS1–3,5–16, we use the term stem cells here ents, INTERKINETIC NUCLEAR MIGRATION and cell-cycle length. cells. to describe neural cells that are self-renewing, but not Before addressing these issues, we first briefly describe necessarily for an unlimited number of cell divisions, the main categories of neural stem and progenitor cells, and that might be multipotent or unipotent. as well as some of their basic cell-biological features. The self-renewal of neural stem cells can occur either *Institute for Stem Cell by symmetric cell divisions, which generate two daughter Neural stem and progenitor cells Research, GSF — National cells with the same fate, or by asymmetric cell divisions, Neuroepithelial cells. Before neurogenesis, the NEURAL Research Center for Environment and Health, which generate one daughter cell that is identical to the PLATE and NEURAL TUBE are composed of a single layer of Ingolstädter Landstrasse 1, mother cell and a second, different cell type. Notably, cells, neuroepithelial cells, which form the neuroepi- D-85764 Neuherberg/ during development, neuroepithelial cells, which can be thelium. The neuroepithelium looks layered (‘pseu- Munich, Germany. considered stem cells, first undergo symmetric, PROLIFERA dostratified’), because the nuclei of neuroepithelial ‡Max Planck Institute of TIVE DIVISIONS, each of which generates two daughter stem cells migrate up and down the apical–basal axis dur- Molecular Cell Biology and 17,18 Genetics, cells . These divisions are followed by many asym- ing the cell cycle (interkinetic nuclear migration; see Pfotenhauerstrasse 108, metric, self-renewing divisions, each of which generates below and FIG. 2a). Neuroepithelial cells show typical D-01307 Dresden, a daughter stem cell plus a more differentiated cell such epithelial features and are highly polarized along their Germany. as a NONSTEMCELL PROGENITOR or a neuron (FIG. 1). Neural apical–basal axis, as is obvious from the organization e-mails: 38,39 [email protected]; non-stem-cell progenitors typically undergo symmetric, of their plasma membrane (FIG. 2a). Certain trans- [email protected] differentiating divisions, each of which generates two membrane proteins such as prominin-1 (CD133) are doi:10.1038/nrm1739 neurons — terminally differentiated, postmitotic cells. selectively found in the apical plasma membrane40,41; NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 6 | OCTOBER 2005 | 777 REVIEWS a NE neuroepithelial markers such as the intermediate-fil- Symmetric, proliferative division ament protein nestin52; the maintenance of an apical NE NE Asymmetric, neurogenic division surface and important features of apical–basal polar- ity such as an apical localization of centrosomes53 RG N Asymmetric, neurogenic division and prominin-1 REF. 40; the presence, at the apical- most end of the lateral plasma membrane, of adher- N RG Symmetric, neurogenic division ens junctions, proteins that associate with adherens N N junctions in the absence of tight junctions such as ZO1, and proteins that are associated with the apical b NE cell cortex such as PAR3 (partitioning defective pro- Symmetric, proliferative division tein-3)/PAR6/aPKC (atypical protein kinase C)39,42; 54 NE NE and the basal lamina contact TABLE 1. Like neu- Asymmetric, differentiative division roepithelial cells, radial glial cells show interkinetic BP RG nuclear migration, with their nuclei undergoing Symmetric, Asymmetric, differentiative division neurogenic division mitosis at the apical surface of the ventricular zone N N BP RG Symmetric, differentiative division and migrating basally for S phase of the cell cycle Symmetric, (FIG. 2b). However, whereas in neuroepithelial cells neurogenic division N N BP BP Symmetric, neurogenic the nuclei migrate through the entire length of the Symmetric, division cytoplasm (FIG. 2a), this is not the case in radial glial neurogenic division N N N N cells (FIG. 2b; see below). Figure 1 | Lineage trees of neurogenesis. The lineage trees shown provide a simplified view In contrast to neuroepithelial cells, radial of the relationship between neuroepithelial cells (NE), radial glial cells (RG) and neurons (N), glial cells show several astroglial properties. An without (a) and with (b) basal progenitors (BP) as cellular intermediates in the generation of ultrastructural characteristic of astroglial cells, neurons. They also show the types of cell division involved. GLYCOGEN GRANULES55, and various molecules that are characteristic of astrocytes — such as the astrocyte- specific glutamate transporter (GLAST), the Ca2+- TIGHT JUNCTIONS and ADHERENS JUNCTIONS are present at the binding protein S100β, glial fibrillary acidic protein most apical end of the lateral plasma membrane42–44; (GFAP), vimentin and brain-lipid-binding protein and receptors for basal lamina constituents such (BLBP) — start to appear in most ventricular zone OLIGODENDROCYTES α 36,46,47 as integrin 6 are concentrated in the basal plasma cells during, but not before, neurogenesis Glial cells of the central nervous membrane, which contacts the basal lamina39. The (T. Mori and M.G., unpublished observations). In system that form the myelin sheath. apical–basal polarity of neuroepithelial cells seems to mice, this transition occurs throughout most of the require the integrity of adherens junctions. Knocking brain between embryonic day 10 (E10), when no PROLIFERATIVE DIVISION out the adherens-junction-associated protein afadin, astroglial markers can yet be detected, and E12, A division of stem or progenitor also known as AF6, perturbs the polarized organiza- when most CNS regions are dominated by progeni- cells that results in a doubling of tion of these cells43. tor cells that are expressing several of these astro- their number, that is, one stem 52,56 cell divides into two identical glial features (FIG. 3). stem cells or one progenitor cell Radial glial cells. With the generation of neurons, the divides into two identical neuroepithelium transforms into a tissue with numer- Radial glial-cell appearance and cell fate restriction. progenitor cells. ous cell layers, and the layer that lines the ventricle In terms of potential, in contrast to early neuroepi- (the most apical cell layer that contains most of the thelial cells, most radial glial cells are restricted to NONSTEMCELL PROGENITORS progenitor cell bodies) is referred to as the ventricu- the generation of a single cell type, either astrocytes, Cells that are able to generate lar zone (FIG. 2b). With the switch to neurogenesis, oligodendrocytes or — as in most cases in neurogen- differentiated cells such as neuroepithelial cells downregulate certain epithelial esis — neurons22,27–29,35,48–50,56–59. This fate restriction is neurons but that are unable to features, notably tight junctions (but not adherens less often present in neuroepithelial cells and seems to self-renew. junctions, nor ZO1 (zona occludens-1), which in the correlate with the appearance of radial glial-cell prop- PROGENITOR CELLS absence of tight junctions seems to associate with erties. Therefore, transgenic mouse cell lines in which Collective term for stem cells adherens junctions)42 and the apical-versus-basal part of the nestin promoter or the regulatory element and non-stem-cell progenitors. polarity of delivery of certain plasma-membrane pro- of the Blbp gene was used to drive the expression of the teins45. Concomitantly, ASTROGLIAL hallmarks appear. In Cre-recombinase gene at early embryonic stages,