Toshinori Nakagawa

Summary: Germ cells secure the continuity and evolutionary potential of species. Yet how reproductive success is allocated among the founder cells of the mammalian male germline, the primordial germ cells (PGCs), remains unknown. Here, we quantitatively traced individual PGCs, non-invasively barcoded in mouse embryos, across their establishment, lifelong maintenance within the testis, spermatogenesis, and transmission to the next generation. We found that highly skewed clonal contributions arise from a very early stochastic bottleneck during migration to the developing testes and remain stable throughout adult reproductive life. Reproductive success during adulthood was proportional to the embryonically established PGC clone size. Mathematical modeling shows that the patchy compartmentalization of PGC clones within the extended thin seminiferous tubules maintain clonal diversity and safeguard against the expansion of clones that gain a selective advantage. These findings uncover fundamental principles governing the development, stability, and evolutionary transmission of the mammalian germline.

Sperm Production-Line Maintenance: The average man makes upwards of 1500 sperm per heartbeat. Such a feat requires a robust stem cell system. Using mice, Nakagawa et al. (p. 62 , published online 18 March) shed light on some of the properties of the murine germline stem cell system that contribute to its robustness. During steady-state spermatogenesis, the majority of the stem cell population lies within a subset of cells called type A spermatogonia. However, during regeneration (for example, during recovery of the stem cell pool after drug exposure) the system in essence hijacks early-differentiating cells back into the stem cell compartment. Lineage analysis and live-imaging also suggests that during sperm production there is more than one path from a stem cell to differentiation.

In order to assess the behavior of the spermatogonia that contribute to the stem cell system in steady‐state mouse spermatogenesis, a series of pulse‐labeling experiments of a subset of “undifferentiated spermatogonial” was performed using transgenic approaches. The results suggest that, in addition to the spermatogonial population that actually acts as the stem cells (actual stem cell compartment), a second set of undifferentiated spermatogonia also exists that possesses the potential to self‐renew, but does not normally do so (potential stem cell compartment). The potential stem cells turn over rather rapidly, suggesting that they act as the transit amplifying cells in the normal situation. The potential stem cells seem to change their mode from transientamplification to self‐renewal upon transplantation, regeneration, or loss of actual stem cells that occurs during the long reproduction period. Such a heterogeneous composition of the stem cell system may be important for the robustness of overall spermatogenesis.

Mammalian spermatogenesis is maintained by a continuous supply of differentiating cells from self-renewing stem cells. The stem cell activity resides in a small subset of primitive germ cells, the undifferentiated spermatogonia. However, the relationship between the establishment of this population and the initiation of differentiation in the developing testes remains unclear. In this study, we have investigated this issue by using the unique expression of Ngn3, which is expressed specifically in the undifferentiated spermatogonia, but not in the differentiating spermatogonia or their progenitors, the gonocytes. Our lineage analyses demonstrate that the first round of mouse spermatogenesis initiates directly from gonocytes, without passing through the Ngn3-expressing stage (Ngn3^- lineage). By contrast, the subsequent rounds of spermatogenesis are derived from Ngn3-positive undifferentiated spermatogonia, which are also immediate descendents of the gonocytes and represent the stem cell function (Ngn3⁺ lineage). Thus, in mouse spermatogenesis, the state of the undifferentiated spermatogonia is not an inevitable step but is a developmental option that ensures continuous sperm production. In addition, the segregation of gonocytes into undifferentiated spermatogonia (Ngn3⁺ lineage) or differentiating spermatogonia (Ngn3^- lineage) is topographically related to the establishment of the seminiferous epithelial cycle, thus suggesting a role of somatic components in the establishment of stem cells.