Spermatogonial stem cells: updates from specification to clinical relevance

Human spermatogonia are target for exploration of adult stem cell characteristics and potential source for the development of therapeutic applications. Almost 50 years ago, Yves Clermont stated with regard to the nature of the true stem cells: there is the possibility that other classes of spermatogonia exist beside the three classes (A(dark), A(pale) and type B)...; ...we still know too little about the human spermatogonial stem cells'... This review seeks to provide current knowledge, focusing on different aspects of human spermatogonia, and novel information based on species comparisons with regard to the adaptation of their proliferative potential. Moreover, the objective is to provide an update on the state of the art concerning the potential use of human spermatogonia for clinical applications. Germ cell specification mechanisms and epigenetic as well as transcriptional features of primordial germ cells (PGC) and adult spermatogonia at the single-cell level are reviewed. Studies on single-cell analyses have been included as they provide hitherto unequaled resolution of the transcriptional profiles of unselected human testicular cells and, thereby, new insights into the molecular aspects of germ cell differentiation. Datasets on models of spermatogonial expansion were identified and spermatogonial turnover and lifetime sperm production rates in various species were calculated, based exclusively on studies employing the optical dissector approach. Finally, the state of the art concerning causes of impaired spermatogonial function and fertility preservation were comprehensively reviewed. RNA sequencing data from PGC and spermatogonia indicate that transcriptional heterogeneity is a feature of germ cells prior to differentiation. Based on these data as well as lineage-tracing studies it is now debated whether spermatogonia are a rather plastic population of undifferentiated germ cells with the stem cell niche being the regulatory unit for cell fate decisions. Based on our novel calculations we suggest that spermatogonia are adapted to the individual reproductive lifespan and that the life-long sperm output from a spermatogonium is balanced against the duration of a generation. Thereby, the risk of jeopardizing genome integrity is balanced against a maximized sperm output. With reference to Yves Clermont's statement, and based on recent datasets, we suggest that the question that needs to be answered is: Is there a true stem cell?' or better Is there a population of various cells with distinct features serving as a stem cell pool?'. This review provides an update including novel views on various aspects of spermatogonial biology (from embryonic to adult stages). We consider this review relevant for all research scientists and clinicians dealing with fertility, spermatogenesis and fertility preservation.