Asymmetric cell division

An asymmetric cell division produces two daughter cells with different cellular fates. This is in contrast to symmetric cell divisions which give rise to daughter cells of equivalent fates. Notably, stem cells divide asymmetrically to give rise to two distinct daughter cells: one copy of the original stem cell as well as a second daughter programmed to differentiate into a non-stem cell fate. (In times of growth or regeneration, stem cells can also divide symmetrically, to produce two identical copies of the original cell.)

In principle, there are two mechanisms by which distinct properties may be conferred on the daughters of a dividing cell. In one, the daughter cells are initially equivalent but a difference is induced by signaling between the cells, from surrounding cells, or from the precursor cell. This mechanism is known as extrinsic asymmetric cell division. In the second mechanism, the prospective daughter cells are inherently different at the time of division of the mother cell. Because this latter mechanism does not depend on interactions of cells with each other or with their environment, it must rely on intrinsic asymmetry. The term asymmetric cell division usually refers to such intrinsic asymmetric divisions.

In multi-celled organisms this mechanism raises two requirements: first, the mother cell must be polarized; second, the mitotic spindle must be aligned with the axis of polarity. The cell biology of these events has been most traditionally studied in three animal models: the mouse, the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster. Recent work in spiralian development has also discovered insightful mechanisms of asymmetric cell division.

In C. elegans, a series of asymmetric cell divisions in the early embryo are critical in setting up the anterior/posterior, dorsal/ventral, and left/right axes of the body plan. After fertilization, events are already occurring in the one cell stage embryo to allow for the first asymmetric cell division. This first division produces two distinctly different blastomeres, termed AB and P1. When the sperm fertilizes the egg, the sperm nucleus and centrosomes are deposited within the egg, which causes a cytoplasmic flux resulting in the movement of the sperm pronucleus and centrosomes towards one pole. The centrosomes deposited by the sperm seem to be responsible for the establishment of the posterior pole within the one cell embryo. Studies have shown that the pole in which the sperm-derived centrosomes reside always becomes the posterior pole. Furthermore, sperm with mutant or absent centrosomes fail to establish a posterior pole, while enucleated sperm with intact centrosomes successfully fertilize the egg and set up the posterior pole. The establishment of this polarity initiates the polarized distribution of a group of proteins present in the zygote called the PAR proteins (partitioning-defective), which are a conserved group of proteins that function in establishing cell polarity during development. These proteins are initially distributed uniformly throughout the zygote and then become polarized with the creation of the posterior pole. This series of events allows the single celled zygote to obtain polarity through an unequal distribution of multiple factors.

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