Self-renewal of spermatogonial stem cells (SSCs) is the foundation for maintenance of spermatogenesis throughout life in males and for continuation of a species. The molecular mechanism underlying stem cell self-renewal is a fundamental question in stem cell biology. Recently, we identified growth factors necessary for self-renewal of mouse SSCs and established a serum-free culture system for their proliferation in vitro. To determine whether the stimulatory signals for SSC replication are conserved among different species, we extended the culture system to rat SSCs. Initially, a method to assess in vitro expansion of SSCs was developed by using flow cytometric analysis, and, subsequently, we found that a combination of glial cell line-derived neurotrophic factor, soluble glial cell line-derived neurotrophic factor-family receptor ␣-1 and basic fibroblast growth factor supports proliferation of rat SSCs. When cultured with the three factors, stem cells proliferated continuously for >7 months, and transplantation of the cultured SSCs to recipient rats generated donor stem cell-derived progeny, demonstrating that the cultured stem cells are normal. The growth factor requirement for replication of rat SSCs is identical to that of mouse; therefore, the signaling factors for SSC self-renewal are conserved in these two species. Because SSCs from many mammals, including human, can replicate in mouse seminiferous tubules after transplantation, the growth factors required for SSC self-renewal may be conserved among many different species. Furthermore, development of a long-term culture system for rat SSCs has established a foundation for germ-line modification of the rat by gene targeting technology.germ-line stem cell ͉ spermatogenesis ͉ testis ͉ glial cell line-derived neurotrophic factor

The proliferation and differentiation of a stem cell are regulated intrinsically by the stem cell and extrinsically by the stem cell niche. Elucidation of regulatory mechanisms of spermatogonial stem cells (SSCs), the stem cell of the postnatal male germ line, would be facilitated by in vitro studies that provide a defined microenvironment reconstituted ex vivo. We analyzed the effect of in vitro environment on the maintenance of adult and immature SSCs in a 7-day culture system. Although the number of adult and immature SSCs decreased in a time-dependent manner, nearly one in four stem cells (24%) could be maintained in vitro for 7 days. Stem cell maintenance was enhanced by coculture with OP9 bone marrow stroma or L fibroblast cell lines, addition of glial cell line-derived neurotrophic factor, or utilization of specific culture medium. In contrast, coculture with TM4 or SF7 Sertoli cell lines and addition of activin A or bone morphogenetic protein 4 (BMP4) reduced stem cell maintenance in vitro. Only 4% of the stem cells remained when cultured with TM4 cells or activin A, and 6% remained when cultured with SF7 cells or BMP4. These results lead to the hypothesis that suppression of germ cell differentiation improves in vitro maintenance of SSCs by interrupting the unidirectional cascade of spermatogenesis and blocking stem cell differentiation.

Aging is evident in most tissues and organ systems, but the mechanisms of aging are difficult to identify and poorly understood. Here, we test the hypothesis that aging results in uncorrected defects in stem cell and/or niche function, which lead to system failure. We used the spermatogonial stem cell (SSC) transplantation assay to determine the effect of aging on testis stem cell/niche function in mice. Between 12 and 24 months of age, male mice experienced a declining level of fertility associated with decreased testis weight, level of spermatogenesis, and total stem cell content. However, when stem cells were consecutively passaged at 3-month intervals to testes of young males, these stem cells continued to produce spermatogenesis for more than 3 years. Thus, SSC self-renewal continues long past the normal life span of the animal when the stem cell is continually maintained in a young niche/microenvironment. Moreover, these data suggest that infertility in old males results from deterioration of the SSC niche and failure to support an appropriate balance between stem cell self-renewal and differentiation. STEM CELLS 2006;24:1505-1511

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The xenoestrogen bisphenol-A (BPA) is a widespread environmental contaminant that has been studied for its impact on male fertility in several species of animals and humans. Growing evidence suggests that xenoestrogens can bind to receptors on spermatozoa and thus alter sperm function. The objective of the study was to investigate the effects of varying concentrations of BPA (0.0001, 0.01, 1, and 100 μM for 6 h) on sperm function, fertilization, embryonic development, and on selected fertility-related proteins in spermatozoa. Our results showed that high concentrations of BPA inhibited sperm motility and motion kinematics by significantly decreasing ATP levels in spermatozoa. High BPA concentrations also increased the phosphorylation of tyrosine residues on sperm proteins involved in protein kinase A-dependent regulation and induced a precocious acrosome reaction, which resulted in poor fertilization and compromised embryonic development. In addition, BPA induced the down-regulation of β-actin and up-regulated peroxiredoxin-5, glutathione peroxidase 4, glyceraldehyde-3-phosphate dehydrogenase, and succinate dehydrogenase. Our results suggest that high concentrations of BPA alter sperm function, fertilization, and embryonic development via regulation and/or phosphorylation of fertility-related proteins in spermatozoa. We conclude that BPA-induced changes in fertility-related protein levels in spermatozoa may be provided a potential cue of BPA-mediated disease conditions.