Androgen signaling is dispensable for fetal testis formation: decoupling testis organogenesis from fetal masculinization programming.

Androgen receptor (AR) signaling is central to male sexual differentiation. Notably, however, fetal testis formation and androgen-dependent masculinization are temporally and mechanistically distinct processes that are not always clearly differentiated in developmental and toxicological interpretations. Evidence from human androgen insensitivity syndromes (AIS) and rodent genetic models demonstrates fetal testis formation proceeds independently of AR, whereas downstream masculinization is AR-dependent. Across mammals, early testis differentiation is driven by intrinsic programs that establish Sertoli cells, organize testis cords, and specify fetal Leydig cells in the absence of AR activity. Consistently, mouse Ar-knockout models show persistent fetal testis formation, and individuals with complete androgen insensitivity syndrome (CAIS) develop structurally intact testes that produce normal or elevated testosterone despite absent virilization. These conserved outcomes reflect shared developmental timing, as key morphogenetic events occur before AR is expressed in Sertoli or fetal Leydig lineages in both rodents and humans. By contrast, AR-mediated androgen signaling becomes essential once androgens from the fetal testes begin to act on peripheral target tissues, governing masculinization of external genitalia, testis descent, and postnatal maturation of somatic testis compartments, as demonstrated by graded AIS phenotypes and cell-specific Ar ablation in mice. This developmental uncoupling explains why disruption of androgen signaling still allows establishment of early testis architecture yet predictably impairs downstream male reproductive development. Recognizing this distinction is crucial for interpreting human genetic disorders, aligning rodent models with human biology, but also to refine toxicological frameworks to target androgen-dependent windows of vulnerability.