Hidden in plain sight: the mammary line in males may be the missing link regulating inguinoscrotal testicular descent

Inguinoscrotal testicular descent is controlled by androgens and the genitofemoral nerve, but the trigger for what makes the gubernaculum become a migratory organ like a limb bud remains unknown. Recent observations in the flutamide-treated rat suggested a link with the mammary line. We aimed, therefore, to reassess histologic anatomy in 2 different rodent models of androgen blockade, the testicular feminisation mouse (TFM) and the flutamide-treated rat.

Neonatal TFM mice and fetal and neonatal rats after pretreatment of dams with an antiandrogen, flutamide (75 mg/kg; sunflower oil; days 16-19), were prepared for histologic analysis of the inguinal region and compared with fetal and neonatal controls.

Fetal control rats (E15.5 days) showed a mammary bud just outside the future inguinal canal adjacent to the gubernaculum. Neonatal TFM mice showed persistence of the inguinal breast bud supplied by the genitofemoral nerve. Flutamide-treated rats (D2) showed the gubernaculum surrounded by a persisting breast bud.

The inguinal mammary line is adjacent to the gubernaculum in fetal rodents, and after androgen blockade, the gubernaculum becomes connected to the breast. The male mammary line, which is hidden in plain sight outside the inguinal canal, is made visible by androgen blockade. It may be the missing link in testicular descent, regulating gubernacular migration.