For the WT strain, however, the dividing cells displayed the normal tight coordination of septum formation and constriction, showing characteristics of a symmetrical division.
Therefore, seam cells must have some mechanism of overriding WRM-1/POP-1 asymmetry when polarity needs to be reversed, or when a symmetrical division is required.
In ceh-20 and unc-62 mutants this balance is lost and both daughters adopt the proliferative fate, resulting in a symmetrical division pattern.
In addition, there is a single symmetrical division at L2 in the V lineage, where both daughter cells retain proliferative ability and consequently expand the pool of seam cells so that adult hermaphrodites have 16 seam nuclei per side.
Upon division, a neural progenitor can either give rise to two new progenitors (P-P) or two neurons (N-N), in a so-called symmetrical division, or to another progenitor and a neuron (P-N, asymmetrical division).
NSCs are maintained in specialized microenvironments in these brain regions (Alvarez-Buylla and Lim 2004), in which they may undergo symmetrical division at a very low rate maintaining their pluripotency, or alternatively undergo asymmetric division to differentiate into neuronal precursors (NPs) (Ohnuma and Harris 2003; Sommer and Rao 2002).
While symmetrical division of a neuroprogenitor cell results in the generation of two identical neuroprogenitor cells (thereby increasing the progenitor pool), asymmetrical division leads to the production of one progenitor cell (thereby maintaining the progenitor pool) and a committed precursor, which eventually undergoes migration and differentiates into neurons [ 20, 21].
Moreover, the cells in neural rosettes are capable of multiplying by symmetrical division over a period of time and are able to differentiate cell types of both anterior-posterior and central-peripheral types of the nervous system and could be maintained in a long-term culture by stimulating Sonic hedgehog (Shh) and Notch pathways [ 35].
Neurons may also be generated by the symmetrical division of an apically-bound cell type called short neural precursors (SNP) [21], [22].
Interestingly, similar division angles produced ICM cells if the mother was the slightly 'more inside' daughter of a symmetrical (≥60°) division at the eight- to 16-cell stage (the bottom-left quadrant) (Fig. 5B, compare regions a,a′ and b,b′).
What they do know is that when they grow these cells in culture, they can push them toward either asymmetrical or symmetrical division by controlling the density of cells and the amount of oxygen available.
