guard cell

Asymmetric cell division

An asymmetric cell division produces two daughter cells with different properties. This is in contrast to normal cell divisions, which give rise to equivalent daughter cells. Notably, stem cells divide asymmetrically to give rise to two distinct daughter cells: one copy of themselves and one cell programmed to differentiate into another cell type.

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 interactions of cells with their environment it must rely on intrinsic asymmetry. The term asymmetric cell division usually refers to such intrinsic asymmetric divisions.

Intrinsic asymmetry

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 as 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 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 successfully studied in three animal models: the mouse, the nematode Caenorhabditis elegans and the fruitfly Drosophila melanogaster.

The role of asymmetric divisions in development

Animals are made up of a vast number of distinct cell types. During development these 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. However, asymmetric divisions often give rise to only one novel cell type in addition to a new copy of the mother cell. Such divisions are called self-renewing. Self-renewal is a hallmark of stem cells, and there is growing evidence that stem cells self-renew through asymmetric division. In this way the production of new cell types (differentiation) is precisely balanced by renewal of the stem cell population.

Asymmetric division of somatic cells also creates a drift in cell function through the human life span contributing to aging of the organism. It is due to the asymmetric distribution of DNA between daughter cells.

Asymmetric Cell Division, Progress in Molecular and Subcellular Biology, volume 45, A. Macieira-Coelho, Editor. Springer Verlag, Berlin, Heidelberg, New York (2007), ISBN-10: 3-540-69160-0

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