Molecular Mechanisms Of Tubulogenesis

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Abstract
As organisms increased in size and complexity, tubular systems evolved to transport substances into and out of the body with maximum efficiency. Although the final structure of tubular organs is markedly different, common elements of cell behaviour underlie their formation. A fundamental feature of all tubular organs is that they are made up of cells that have apical–basal polarity. The cells have a clearly defined apical membrane facing the central lumen and a basal surface that is attached to a layer of extracellular matrix, both of which are connected by specialized intercellular junctions. Acquiring this polarity is a defining event in tubulogenesis. Different strategies for elaborating tubes are discussed, as well as mechanisms for elongating tubes, connecting them together, and remodelling tubular networks during development. The special role of tips cells in controlling the elongation of tubes is also discussed. Genetic analysis of tubular systems in model organisms has given important insights into the basic mechanisms that underlie their formation and remodelling. Examples are given from Drosophila (salivary gland and tracheal system), Caenorhabditis elegans (excretory cells), zebrafish (gut and vascular system) and mouse (lung, mammary gland and neural tube). In vitro culture systems have also given insights into the basic mechanisms of tubulogenesis, including the role of intracellular signalling through receptor tyrosine kinases in directing apical polarity, cytoskeletal organization and formation of cell contacts. In the future, we need to know more about the mechanisms that have evolved to co-ordinate the interdependent growth and differentiation of the different components of tubular organs, such as the epithelial tubes, the surrounding smooth muscle cells and the vascular system. These mechanisms presumably increase the efficiency of tubular systems and their response to physiological needs.