Excretion is an essential process of an organism's removal of the waste products of metabolism to maintain a constant chemical composition of the body fluids despite changes in the external environment. Excretion is performed by the kidneys in vertebrates and by Malpighian tubules (MTs) in Drosophila. The kidney serves as an excellent model organ to investigate the cellular and molecular mechanisms underlying organogenesis. Mammals and Drosophila share common principles of renal development. Tissue homeostasis, which is accomplished through self-renewal or differentiation of stem cells, is critical for the maintenance of adult tissues throughout the lifetime of an animal. Growing evidence suggests that stem cell self-renewal and differentiation is controlled by both intrinsic and extrinsic factors. Deregulation of stem cell behavior results in cancer formation, tissue degeneration, and premature aging. The mammalian kidney has a low rate of cellular turnover but has a great capacity for tissue regeneration following an ischemic injury. However, there is an ongoing controversy about the source of regenerating cells in the adult kidney that repopulate injured renal tissues. Recently, we identified multipotent stem cells in the MTs of adult Drosophila and found that these stem cells are able to proliferate and differentiate in several types of cells in MTs. Furthermore, we demonstrated that an autocrine JAK-STAT (Janus kinase–signal transducers and activators of transcription) signaling regulates stem cell self-renewal or differentiation of renal stem cells. The Drosophila MTs provide an excellent in vivo system for studying the renal stem cells at cellular and molecular levels. Understanding the molecular mechanisms governing stem cell self-renewal or differentiation in vivo is not only crucial to using stem cells for future regenerative medicine and gene therapy, but it also will increase our understanding of the mechanisms underlying cancer formation, aging and degenerative diseases. Identifying and understanding the cellular processes underlying the development and repair of the mammalian kidney may enable more effective, targeted therapies for acute and chronic kidney diseases in humans
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