Cellular polarity: A comparison of animal and plant systems
Jong, J.M.A. de
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All living organisms, from ‘simple’ unicellular prokaryotes to plants and to mammals, show different degrees of spatial organization. In most multicellular organisms, this spatial organization is easily visible. This organization requires a sense of orientation: what is front and what is back, what is head and what is tail? All organisms, at some point, show a form of polarity: the distinction between two sides of the organism. This polarity can be observed at the level of the whole organism, but individual tissues also show polar organization. Even at the cellular and molecular level this polarity can be observed. The epithelial cells lining our gut are a great example of polar organization within individual cells. The apical side facing the lumen has a completely different membrane organization containing protruding microvilli and has a different function than the basal side of the cell, which has a completely different structural organization. This structural and functional distinction within a cell requires polar organization of cellular components. The polar orientation of cellular components enables a cell to have distinct structural and functional domains. Cell polarity, in turn, relies on the polarized distribution of molecules brought about by molecular polarity. Microtubules, for example, show polarity at the molecular level. Both ends have different characteristics resulting in a specified orientation. Such molecular intrinsic polarity contributes to polarity at the cellular level as will be described in the following chapter. This form of organization is of great importance during the development of organisms. It is a process that has intrigued researchers for decades. Some aspects of the underlying mechanisms have been identified. Intensive investigation of factors that are involved in the initiation and maintenance of polarity has resulted in some explanatory models, but these are still incomplete. The use of evolutionary distinct model systems allows for a comparison between the basic mechanisms that are employed to obtain polarity. In this thesis I will draw comparisons between the plant Arabidopsis thaliana and the nematode Caenorhabditis elegans and to some extent the fly Drosophila melanogaster. What do these organisms share in the sense of polarity establishment? What are the main differences and how can this specific knowledge contribute to the overall understanding of the phenomenon of cellular polarity? Using examples of cellular polarity in these different organisms, I will try to elucidate some of the similarities and differences. This may provide some new ideas of how knowledge of different organisms can lead to new insights of a common phenomenon: cellular polarity.