Division Types (Symmetrical and Asymmetrical)

Asymmetric

Animals are made up of a vast number of distinct cell types. During development, these cell types are generated from a single cell, the zygote. Asymmetric divisions contribute to this expansion in cell type diversity by making two types of cells from one. For example, it is thought that many of the cells in the central nervous system derive from asymmetric divisions. Cells may divide asymmetrically to produce two novel cells at the expense of the mother cell. For example, in plants, an asymmetric division of an unspecialized epidermal cell can produce a guard cell mother cell that divides again to produce two guard cells—the cells that control the closing and opening of stomata.

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. In another, the prospective daughter cells are made different at the time of division of the mother cell. Because this latter mechanism does not depend on the interactions of the cells with their environment, it must rely on intrinsic asymmetry. The term asymmetric cell division usually refers to such intrinsic asymmetric divisions. Intrinsic asymmetric divisions rely on the following mechanism: At mitosis, certain proteins are localized asymmetrically to one half of the cell. Next, the cell is cleaved to separate the two halves. Thus, the asymmetrically localized proteins are inherited to only one of the daughter cells, causing that cell to be different from its sibling. Because these proteins determine what becomes of a cell, they are called cell fate determinants. This mechanism has two requirements: first, the mother cell must be polarized, and second, the mitotic spindle must be aligned with the axis of polarity. The cell biology of these events has been most successfully studied in three animal models: the mouse, the nematode Caenorbabditis elegans, and the fruit fly Drosopbila melanogaster.

Most mechanistic insights into asymmetric cell division come from invertebrate experiments. However, discoveries in work on mammalian stem cells have revealed enormous flexibility among the progeny of individual cells. Many different cell fates can be induced by changing growth factors in the culture medium, suggesting that lineage restrictions and intrinsic asymmetries have only minor functions. However, time—lapse video microscopy shows that cortical progenitor cells divide in stereotyped lineages—even in culture, where directional extrinsic signals can be largely excluded. Although there is no clear genetic evidence for intrinsically asymmetric cell divisions in vertebrates, the observation of putative stem cells in intact tissues has revealed several examples for asymmetrically segregating proteins such as Numb and the Notch receptor.

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