Immunohistochemical study on androgen receptors in the anterior cruciate ligament in dogs.

Androgen is closely involved as the cause of rupture of anterior cruciate ligament (ACL) in human. In dogs, however, factors contributing to rupture of ACL remain unknown. In this study, expression of androgen receptor (AR) and histological distribution of blood vessels in ACL, and serum testosterone concentration were investigated in relation with age and sex to confirm whether canine ACL is an androgen-responsive tissue. Materials of ACL were obtained from 26 dogs: 12 young female Beagles, 2 old female mixed breeds, 9 young male Beagles, and 3 old male mixed breeds. In all canine ACL, positive AR expression was recognized in the nuclei of the fibrocytes, fibroblasts, synovial cells, and vascular endothelial cells of ACL. Expressions of AR were lesser in old males compared to the young males; however, females had no age difference in expression. Distributions of blood vessels in the synovial membrane of the ligament were fewer in old dogs both of males and females than youngs. Although distributions of vessels in the interstitium were apparently fewer in young females. Serum testosterone concentration was significantly higher in young males. Females had no age difference in the levels. From these results, it is suggested that canine ACL is an androgen-responsive tissue, and this consideration seems to closely relate to the epidemiological background that the incidence of rupture of ACL of dogs is higher in females than in males.