A revised Asingle model to explain stem cell dynamics in the mouse male germline

Abstract

Spermatogonial stem cells (SSCs) and progenitor spermatogonia encompass the undifferentiated spermatogonial pool in mammalian testes. In rodents, this population is comprised of A_single, A_paired and chains of 4-16 A_aligned spermatogonia. Although traditional models propose that the entire A_single pool represents SSCs, and formation of an A_paired syncytium symbolizes irreversible entry to a progenitor state destined for differentiation; recent models have emerged that suggest that the A_single pool is heterogeneous, and A_paired/A_aligned can fragment to produce new SSCs. In this review, we explore evidence from the literature for these differing models representing SSC dynamics, including the traditional 'A_single' and more recently formed 'fragmentation' models. Further, based on findings using a fluorescent reporter transgene (eGfp) that reflects expression of the SSC-specific transcription factor 'inhibitor of DNA binding 4' (Id4), we propose a revised version of the traditional model in which SSCs are a subset of the A_single population; the ID4-eGFP bright cells (SSC_ultimate). From the SSC_ultimate pool, other A_single and A_paired cohorts arise that are ID4-eGFP dim. Although the SSC_ultimate possess a transcriptome profile that reflects a self-renewing state, the transcriptome of the ID4-eGFP dim population resembles that of cells in transition (SSCtransitory) to a progenitor state. Accordingly, at the next mitotic division, these SSC_transitory are likely to join the progenitor pool and have lost stem cell capacity. This model supports the concept of a linear relationship between spermatogonial chain length and propensity for differentiation, while leaving open the possibility that the SSC_transitory (some A_single and potentially some A_paired spermatogonia), may contribute to the self-renewing pool rather than transition to a progenitor state in response to perturbations of steady-state conditions.

Figure 1. Different models to explain the dynamics of the SSC pool

The traditional model proposes that all A_single spermatogonia comprise the SSC pool. These A_single cells can either self-renew, or transition to a progenitor state; as represented by the formation of an A_paired syncytium. A_paired progenitors undergo further mitotic divisions to produce longer chains of spermatogonia (A_aligned) that will respond retinoic acid (RA) by transitioning to a differentiating state. In the traditional A_single model, formation of A_paired signifies irreversible commitment to a differentiating pathway. In contrast to the A_single model, the fragmentation model proposes that the self-renewing SSC pool is comprised of interchanging A_single, A_paired and A_aligned spermatogonia, all of which express GFRA1. In this model, the transition to progenitor is signified by loss of GFRA1 expression and gain of NGN3 expression. Finally, the revised A_single model proposes that a specified subset of the A_single pool (i.e. cells with high levels of ID4 expression) represent the SSC_ultimate population that serve as the self-renewing reservoir in steady-state conditions. From the SSC_ultimate pool, transitory cells (i.e. spermatogonia with lower levels of ID4) arise. The continuum of transition to a progenitor state involves declining expression of key self-renewal genes in SSC_transitory that will allow for the next mitotic division to result in a true A_paired syncytium. In all models, RA drives A_paired and A_aligned progenitors into a differentiating state at periodic intervals, characterized by attainment of Kit expression. Following this, spermatogonia transition through A1, A2, A3, A4, intermediate and type B stages, before entering into meiosis as spermatocytes, then producing haploid spermatids, and finally spermatozoa.