Fig. 7. Model for the two lineages in the mouse spermatogenesis. In the first postnatal week, gonocytes directly give rise to Kit-positive differentiating spermatogonia and Ngn3-positive undifferentiated spermatogonia in parallel. This process is closely related to the presumptive seminiferous epithelial cycle pre-pattern, which initiates before birth. Kit-positive spermatogonia are specifically generated in the galectin 1-high segments (pink arrow 1). These cells do not pass through a Ngn3-positive, undifferentiated spermatogonia stage and differentiate in the first round of spermatogenesis, resulting in the formation of fertile spermatozoa (Ngn3^- lineage). By contrast, Ngn3-positive undifferentiated spermatogonia are generated preferably at galectin 1-medium segments (green arrow). They subsequently act as a self-renewing stem cell population, while also providing cells that transform into differentiating spermatogonia. Thus, these cells support steady-state spermatogenesis following the first round of spermatogenesis (Ngn3⁺ lineage). The transformation of undifferentiated spermatogonia into differentiating spermatogonia is tightly related to the seminiferous epithelial cycle, and Kit-positive differentiating spermatogonia are established in stages of high galectin 1 expression (stage IX-X), indicated by pink arrow 2. See Results for more details. PGC, primordial germ cells.