RESUMEN
STUDY QUESTION: Can human spermatogonia be detected in long-term primary testicular cell cultures using validated, germ cell-specific markers of spermatogonia? SUMMARY ANSWER: Germ cell-specific markers of spermatogonia/spermatogonial stem cells (SSCs) are detected in early (1-2 weeks) but not late (> 6 weeks) primary testicular cell cultures; somatic cell markers are detected in late primary testicular cell cultures. WHAT IS KNOWN ALREADY: The development of conditions for human SSC culture is critically dependent on the ability to define cell types unequivocally and to quantify spermatogonia/SSCs. Growth by somatic cells presents a major challenge in the establishment of SSC cultures and therefore markers that define spermatogonia/SSCs, but are not also expressed by testicular somatic cells, are essential for accurate characterization of SSC cultures. STUDY DESIGN, SIZE, DURATION: Testicular tissue from eight organ donors with normal spermatogenesis was used for assay validation and establishing primary testicular cell cultures. PARTICIPANTS/MATERIALS, SETTING, METHODS: Immunofluorescence analysis of normal human testicular tissue was used to validate antibodies (UTF1, SALL4, DAZL and VIM) and then the antibodies were used to demonstrate that primary testicular cells cultured in vitro for 1-2 weeks were composed of somatic cells and rare germ cells. Primary testicular cell cultures were further characterized by comparing to testicular somatic cell cultures using quantitative reverse transcriptase PCR (UTF1, FGFR3, ZBTB16, GPR125, DAZL, GATA4 and VIM) and flow cytometry (CD9 and SSEA4). MAIN RESULTS AND THE ROLE OF CHANCE: UTF1, FGFR3, DAZL and ZBTB16 qRT-PCR and SSEA4 flow cytometry were validated for the sensitive, quantitative and specific detection of germ cells. In contrast, GPR125 mRNA and CD9 were found to be not specific to germ cells because they were also expressed in testicular somatic cell cultures. While the germ cell-specific markers were detected in early primary testicular cell cultures (1-2 weeks), their expression steadily declined over time in vitro. After 6 weeks in culture only somatic cells were detected. LIMITATIONS, REASONS FOR CAUTION: Different groups attempting SSC culture have utilized different sources of human testes and minor differences in the preparation and maintenance of the testicular cell cultures. Differences in outcome may be explained by genetic background of the source tissue or technical differences. WIDER IMPLICATIONS OF THE FINDINGS: The ability to propagate human SSCs in vitro is a prerequisite for proposed autologous transplantation therapy aimed at restoring fertility to men who have been treated for childhood cancer. By applying the assays validated here it will be possible to quantitatively compare human SSC culture conditions. The eventual development of conditions for long-term propagation of human SSCs in vitro will greatly facilitate learning about the basic biology of these cells and in turn the ability to use human SSCs in therapy. STUDY FUNDING/COMPETING INTERESTS: The experiments presented in this manuscript were funded by a Project Development Team within the ICTSI NIH/NCRR Grant Number TR000006. The authors declare no competing interests. TRIAL REGISTRATION NUMBER: Not applicable.