Although there was an observable effect on cremaster muscle development in these mutant mice, gubernacular development and testicular descent were otherwise normal, and thus there must be other reasons for the observed differences in humans with persistent Müllerian duct syndrome.

Objective To determine, in mice with disrupted Müllerian inhibiting substance (MIS) receptor genes, whether MIS affects gubernacular development; MIS causes Müllerian duct regression and is proposed to be involved in the first stage of testicular descent, because gubernacular development is abnormal in humans with persistent Müllerian duct syndrome. Materials and methods Ten wild‐type, 11 heterozygotic and 12 homozygotic mice for MIS receptor mutations were killed at 17.5 or 18.5 days after conception or at birth, to provide serial sagittal sections of the pelvis. The amount of cremaster muscle, mitotic bodies in the gubernacular bulb, and gubernacular size were quantified by computer analysis (four mice/group). Results Müllerian ducts were present in the homozygous mutants, partially present in the heterozygotes and absent in the wild‐type controls. All mice had descended testes. The cremaster muscle was significantly less developed in homozygous mutants than in wild‐type controls (P < 0.001) and heterozygotes (P < 0.01) at birth. The mitotic index between the gubernacula of all groups was indistinguishable. There was no statistical difference in gubernacular area amongst the groups. Poor cremaster muscle development in homozygous mutants gave the muscle a loose mesenchymal appearance. Conclusions Although there was an observable effect on cremaster muscle development in these mutant mice, gubernacular development and testicular descent were otherwise normal, and thus there must be other reasons for the observed differences in humans with persistent Müllerian duct syndrome.