For example, electron microscopic observations of serial thin sections indicated that axon extensions from RGCs occur from basal processes that contact the inner limiting membrane of the neuroepithelium (Hinds & Hinds 1974). Of these, the emergence of axons from retinal ganglion cells (RGCs) has been extensively studied. 2007), and optic tectum of chicks (Martínez et al. 2006 Zolessi et al., 2010), spinal cord of mice and chicks (Wentworth & Hinds 1978 Wilcock et al. These include axons in the retina of mice and zebrafish (Hinds & Hinds 1974 Morgan et al. Emergence of axons from polarized cellsĪ number of studies on developing neurons in situ have suggested axons emerge from polarized immature neurons. In this review, we compare cellular events during axon formation studied in various systems both in vitro and vivo and discuss possible common mechanisms underlying axon-dendrite polarization. Recent progress in live imaging has enabled such observations in vivo or in in vivo-like preparations, shedding light on the fundamental but long unanswered question of how neurons establish axon-dendrite polarity. Whether this is true or not requires direct observation of axon initiation processes in vivo. Moreover, since neuroepithelial cells that give rise to neurons are polarized apico-basally, neurons that emerge from such cells may maintain this polarity in vivo at the time of axon initiation. However, these mechanisms have been questioned, because the dissociation procedure removes all existing processes and the neurons resume the elongation of processes from an artificially created non-polarized condition. Based on these observations, a number of studies have addressed the molecular mechanisms underlying this process, identifying the roles of several molecules (reviewed in Arimura & Kaibuchi 2007 Barnes & Polleux 2009 Bradke & Dotti 2000), while a quantitative mathematical model has also been proposed (Inagaki et al.
These findings imply that the axon–dendrite polarity is intrinsically regulated such that the “winner” among equivalent early processes becomes an axon (reviewed in Bradke & Dotti 2000). Upon this moment, none of the remaining processes will ever become an axon. After a period of neurites alternately elongating and retracting, one of these neurites acquires an axonal nature. These cells, shortly after plating, extend lamellipodia, which then develop into several short immature neurites (Dotti et al. Historically, how neurons acquire their axon-dendrite polarity has been extensively studied using dissociated hippocampal cells in culture. Neurons are polarized cells that extend a single axon and several dendrites.
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