Stem cells in various somatic tissues (bone marrow, skeletal muscle) can be identified by the `Side Population' marker based on Hoechst 33342 efflux. We show that mouse testicular cells also display a `Side Population' that express Bcrp1 mRNA, the ABC transporter responsible for Hoechst efflux in hematopoietic cells. Inhibition of Hoechst efflux by specific BCRP1 inhibitor Ko143 show that germinal `Side Population' phenotype is dependent on BCRP1 activity. Analysis of two well-defined models of altered spermatogenesis(W/Wv mutants and cryptorchid male mice) and RNA expression studies of differentiation markers demonstrate that germinal `Side Population' contains spermatogonial cells. In addition,α 6-integrin and Stra8 germinal stem cell markers, are expressed in the `Side Population'. In vivo repopulation assay clearly establishes that testis `Side Population' in adult mice is highly enriched in male germ stem cells.
Testis is one of the organs with the most telomerase activity in the adult. This activity protects chromosomes from telomere attrition and ensures the transmission of full-length chromosomes to progeny. Little is known about telomerase activity during adult germ cell differentiation, however. We demonstrate here that the telomerase activity of adult mouse testis resides in the alpha6-integrin-positive Side Population containing spermatogonia and enriched in spermatogonial stem cells. The telomerase activity of these cells fell upon entry into meiosis and during the subsequent spermiogenesis. In addition, the telomerase activity of cells in various stages of differentiation was unaffected by aging and, notably, remained high in the alpha6-integrin-positive Side Population.
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