Hypothesis to account for testicular weight increase in the αERKO mouse. The αERKO mouse testis was shown to increase in weight from day 40 to 75 days of age, and then the weight declined until the testis was atrophied by day 185 . Two hypotheses were proposed to account for mechanisms that could explain the transient increase in testis weight prior to regression. In the normal testis, efferent ductules receive low concentrations of sperm from the rete testis. Approximately 95% of this fluid is reabsorbed by the efferent ductule epithelium, which increases the concentration of sperm that enter the epididymis. Disruption of ERα causes testicular swelling through one of two possible mechanisms: A. the efferent ductules become occluded, or B. the fluid reabsorption pathways are inhibited. Both mechanisms will result in fluid accumulation in the seminiferous tubules and backing up of fluids into the testis. Atrophy occurs by an unknown mechanism that inhibits spermatogenesis.