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Maintenance a functional lifelong tissue requires a continual cellular turnover. Rate of turnover is thus carefully balanced with malfunctioned and dying cells. Physiological cell turnover predicates mostly on the resident terminally differentiated or stem cells. Terminally differentiated may have a replication tendency enable them to propagate proliferating precursors. Most organs and tissues have their own stem or progenitor cells which retain capacity for differentiation. Directing Stem cells to the target destination depend on their niche and microenvironment. Certain factors exert their effect on stem cells to guide specific gene expression which in turn promote cellar propagation and differentiation (Sanchez Alvarado and Yamanaka, 2014).

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Soliman, Soha
Dr. Soha Mohamed Abdel-lateif Soliman, Department of Histology, Faculty of Veterinary MedicineSouth Valley University, Egypt.

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