![]() ( C) Polarization of granule cells in the mammalian cerebellum. The axon emerges from the basal process (red) and the dendrite emerges from the apical process (B4). Neuroepithelial progenitors (B1) transform into bipolar cells by first losing their basal attachment which starts branching in the inner plexiform layer (IPL) while the apical process starts branching in the prospective outer plexiform layer (OPL) before (B2) losing its apical attachment (B3). ( B) Polarization of mouse bipolar cells in the mouse retina. The axon (red) therefore emerges from the basal process and the dendrite emerges from the apical process (A6). Upon cell cycle exit, the nucleus undergoes basal translocation (A4) and specifically loses its apical attachment while its basal process starts growing along the basal membrane (A5). Neuroepithelial progenitors characterized by an apical and a basal attachment undergo asymmetrical cell division at the apical surface (A1-A3). ( A) In vivo polarization of retinal ganglion cells in zebrafish ( Danio rerio) and mouse ( Mus musculus). Throughout these figures, the nascent axon is depicted in red and the somatodendritic domain in purple. Therefore, in these two neuronal cell types, there is an important functional relationship between the molecular mechanisms underlying polarized migration and the final axon-dendrite polarity.Ĭell-type specific patterns of neuronal polarization in vivoĮxamples of the sequence of events leading to the polarized emergence of axon and dendrites in four distinct vertebrate neuronal cell types in vivo. ![]() Importantly, both CGN and PN acquire their axon-dendrite polarity from the polarized emergence of the trailing-leading processes during migration. This is the case for cerebellar granule neurons (CGN) as well as cortical and hippocampal pyramidal neurons (PN), three of the best-studied models of neuronal polarization both in vitro and in vivo ( Gao & Hatten 1993, Hatanaka & Murakami 2002, Komuro et al 2001, Noctor et al 2004, Rakic 1971, Rakic 1972, Shoukimas & Hinds 1978). On the other hand, other neurons undergo extensive stereotypical changes of their morphology leading to polarized outgrowth of their axon and dendrite ( Figure 1C-D). This is the case for retinal ganglion cells and bipolar cells in the developing vertebrate retina ( Figure 1A-B) ( Hinds & Hinds 1978, Morgan et al 2006, Zolessi et al 2006). Careful examination of the morphological transition between neural progenitors and post-mitotic neurons reveal that neurons can inherit their axon and dendrite polarity directly from the apico-basal polarity of their progenitors. While migrating, post-mitotic neurons form a leading process and a trailing process which become the axon or the dendrite depending on the cell type ( Figure 1). In vivo, most neurons undergo axon-dendrite polarization during migration. Neuronal polarization can be divided in several specific steps: upon cell cycle exit, mammalian neurons usually migrate over a long distance before reaching their final destination. The current review will provide an updated model synthesizing a recent body of work suggesting that in vivo, neuronal polarity is most probably a result of a complex interaction between extracellular cues directing intrinsic cell polarity pathways. Based on existing data, Craig and Banker provided a conceptual framework for the experiments that, over the past decade, have improved our understanding on how neuronal polarity is established mostly using in vitro approaches. A seminal review published by Craig and Banker fifteen years ago in this journal ( Craig & Banker 1994) observed that “we almost nothing about the cellular mechanisms responsible for the compartmentation in neurons”. How are the axonal and dendritic compartments generated during development? This question has received a lot of attention both at the cellular and molecular levels over the past three decades. Dendrites integrate synaptic inputs triggering the generation of axon potentials at the level of the soma and propagates along the axon making presynaptic contacts onto the dendrite of target neurons. Neurons typically form a single axon and multiple dendrites which underlie the flow of information transfer in the central nervous system. Neurons are among the most polarized cell types in our body and are compartimentalized into two molecularly and functionally distinct domains: the axon and the dendrites. Its disruption is thought to underlie several pathological states including cell transformation and metastasis. Cell polarity lies at the center of many biological processes including epithelial morphogenesis, cell migration and chemotaxis.
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