Abstract
The cerebral cortex is composed of billions of neurons that can grossly be subdivided into two broad classes: inhibitory GABAergic interneurons and excitatory glutamatergic neurons. The majority of cortical neurons in mammals are the excitatory type and they are the main focus of this review article. Like many of the cells in multicellular organisms, fully differentiated neurons are both morphologically and functionally polarized. However, they go through several changes in polarity before reaching this final mature differentiated state. Neurons are derived from polarized neuronal progenitor/stem cells and their commitment to neuronal fate is decided by cellular and molecular asymmetry during their last division in the neurogenic zone. They migrate from their birthplace using so-called multipolar migration, during which they switch direction of movement several times, and repolarize for bipolar migration when the axon is specified. Therefore, neurons have to break their previous symmetry, change their morphology and adequately respond to polarizing signals during migration in order to reach the correct position in the cortex and start making connections. Finally, the dendritic tree is elaborated and the axon/dendrite morphological polarity is set. Here we will describe the function, establishment and maintenance of polarity during the different developmental steps starting from neural stem cell (NSC) division, neuronal migration and axon specification at embryonic developmental stages.
Highlights
DEVELOPMENT OF THE CEREBRAL CORTEXThe process of brain development is quite amazing
With the accumulation of neurons above the ventricular zone (VZ), the thickness of the cerebral wall increases and the somal translocation mode of migration becomes less frequent. It is replaced by a multi-phase mode of migration: a very short bipolar migration when neurons exit the VZ followed by an extended multipolar migration occurring in the multipolar morphology zone (MMZ), comprising the sVZ and the lower part of the intermediate zone (IZ)
We have seen earlier that molecular pathways are involved in polarizing the movement of multipolar neurons towards the cortical plate (CP). Once they reach the top of the MMZ, they initiate their attachment to the radial glia fibers which might give them the signal to transform into bipolar cells but there is no hard proof for this and cells might become bipolar before they attach to the fibers
Summary
Like many of the cells in multicellular organisms, fully differentiated neurons are both morphologically and functionally polarized. They go through several changes in polarity before reaching this final mature differentiated state. Neurons are derived from polarized neuronal progenitor/stem cells and their commitment to neuronal fate is decided by cellular and molecular asymmetry during their last division in the neurogenic zone. They migrate from their birthplace using so-called multipolar migration, during which they switch direction of movement several times, and repolarize for bipolar migration when the axon is specified.
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