Sertoli cell and germ cell numbers are increased in follistatin-like 3 (FSTL3) knockout (KO) mice without affecting spermatogenic program. Stereological analysis of testes from 16-mo-old WT (n = 5) and FSTL3 KO (n = 5) mice presented as bar graphs showing increased Sertoli and germ cells, whereas Leydig cell numbers are unaltered (A). Profiling germ cell development in 16-mo-old FSTL3 KO mice by flow cytometry (B–E). Flow cytometric profiles of germ cell suspensions prepared from WT (top) and FSTL3 KO (bottom) testes (B). DNA content profiles of testicular cells from WT (top) or FSTL3 KO (bottom) (C). Percentages of total number of cells present in each of the populations from B and C (n = 3) are presented as column graphs in D and E, respectively. R1, premeiotic 2N spermatogonia; R2, 4N primary spermatocytes; R3, late-stage spermatocytes; R4, postmeiotic secondary spermatocytes; R5, round spermatid; R6, elongated spermatid; R7, spermatozoa. Error bars = SEM, *P < .05.