Mouse Spermatogenic Stem Cells Continually Interconvert between Equipotent Singly Isolated and Syncytial States

Abstract

SummaryThe identity and behavior of mouse spermatogenic stem cells have been a long-standing focus of interest. In the prevailing “As model,” stem cell function is restricted to singly isolated (As) spermatogonia. By examining single-cell dynamics of GFRα1+ stem cells in vivo, we evaluate an alternative hypothesis that, through fragmentation, syncytial spermatogonia also contribute to stem cell function in homeostasis. We use live imaging and pulse labeling to quantitatively determine the fates of individual GFRα1+ cells and find that, during steady-state spermatogenesis, the entire GFRα1+ population comprises a single stem cell pool, in which cells continually interconvert between As and syncytial states. A minimal biophysical model, relying only on the rates of incomplete cell division and syncytial fragmentation, precisely predicts the stochastic fates of GFRα1+ cells during steady state and postinsult regeneration. Thus, our results define an alternative and dynamic model for spermatogenic stem cell function in the mouse testis.

Paracrine Wnt/β-catenin signaling mediates proliferation of undifferentiated spermatogonia in the adult mouse testis

“…4C). These data are consistent with previous studies that showing that SSCs follow a neutral competition model (6,31,33,34), although additional, detailed analyses are needed to validate this model further.…”

“…4C). These results are consistent with a neutral competition model in which tissue stem cells are routinely lost and replaced in a stochastic manner, as previously demonstrated by the use of specific Cre lines of GFRα1, Ngn3, and Bmi1 in the testis (6,31,33,34).…”

“…GFRα1 primarily marks A s and A pr as a marker for SSC-enriched subpopulations of undifferentiated spermatogonia (6,22,23), whereas PLZF marks A s , A pr , and A al cells as a pan-undifferentiated spermatogonia marker (24,25). LacZ signals were present together with GFRα1 and PLZF immunoreactivity, indicating that undifferentiated spermatogonia are Wnt-responsive cells ( Fig.…”

“…This model postulates that stem cell function is restricted to A s spermatogonia that can produce A pr and A al spermatogonia as progenitor cells (4). On the other hand, recent studies have suggested the possibility of contributions of A pr and A al spermatogonia, as well as A s , cells to the SSC pool, based on live imaging and mathematical modeling (6,7). In those studies, the investigators observed the fragmentation of A pr and A al cells to provide A s spermatogonia, and proposed that GFRα1 + spermatogonia form a single SSC pool that constantly interchanges between A s and syncytial states.…”

Microscopy lights up stem cells in action

“…Yoshida filmed the cells by first extracting the testes from inside the mouse's body without cutting off the blood supply. His footage suggests that the strings of stem cells regularly fragment and replenish the stem cell pool (6). "I was totally excited to see them being fragmented for the first time," he says.…”