HOMOLOGOUS TRANSPLANTATION OF PREPUBERTAL TESTES IN THE DOG1

Abstract

The current standard of care for anorchia, the absence of both testes, is testosterone replacement therapy. Although transplantation of a single testis has been successful in a case of an identical twin (1), transplants in nonidentical siblings have not been performed. Homologous testis transplantation has been performed in sexually mature dogs, however, only one testis was transplanted, and histopathology alone was used to evaluate sperm production (2). The donor and recipient dogs in our experiment were two prepubertal (18-week-old) Beagle/Brittany littermates; the sire and dam of which shared a common ancestor. Based on results of a previous study (2), major histocompatability com- plex testing was not performed. The testes were harvested from the donor and transplanted into the recipient within 1 hour, via a prescrotal incision, by end-to-end anastamosis of the testicular artery, vein, and vas deferens. Immediately before anastamosis of the first testis, the recipient received 1000 U of heparin i.v. to reduce the risk of thrombosis. After anesthetic recovery, the dog was maintained on a course of previously successful immunosuppressive therapy and anticoagulants (2). The recipient lacked signs of graft-versus-host disease, or deleterious effects of the cyclosporine and prednisone therapy. Serum testosterone levels were monitored weekly by radioimmunoassay (Diagnostic Systems Laboratories, Webster, TX) from 18 to 39 weeks of age (see Fig. 1). Testosterone levels may fluctuate by as much as 2.5 ng/ml during a 24hr period in an individual dog, however, levels in castrated animals are consistently <0.20 ng/ml (3). A gradual increase in baseline testosterone, with mean levels approaching the normal range for a sexually mature animal (0.5–5.0 ng/ml), signals the onset of puberty (4). Figure 1: Longitudinal serum testosterone levels (ng/ml) from the day of transplantation, 18 weeks of age, through 39 weeks of age. Minimum testosterone level for a sexually mature dog is 0.20 ng/ml (indicated by dashed line).When the recipient reached 7 months of age, a human chorionic gonadotrophin (hCG) stimulation test was performed in order to determine the responsiveness of Leydig cells, which produce testosterone. The dog received 44 μg/kg hCG i.m. and serum was collected 4 and 24 hr later. Baseline testosterone was 0.83 ng/ml and increased to 1.35 ng/ml at 4 hr and 2.46 ng/ml at 24 hr, consistent with functional Leydig cells (3, 4). At 8 months of age, serum testosterone remained consistently within the normal range for a sexually mature dog. Semen was collected at 8 months of age, using an artificial vagina, in the presence of a bitch in estrus. A total of 300×106 sperm/ejaculate were present, 80% of sperm were progressively motile, and 85% were morphologically normal. All parameters are known indicators of fertility and were within normal limits for fertile dogs (3). Lack of graft-versus-host disease did not guarantee normal sperm production, as many factors and functional interactions are involved in organ maturation and spermatogenesis. At 17 months of age, the dog was bred to a bitch in estrus, which was confirmed by ultrasound to be pregnant with a litter of 4 pups. This case is the initial report of bilateral transplantation of prepubertal canine testes with subsequent maturation, semen evaluation, and proven fertility. These findings suggest that the dog may be a suitable animal model to evaluate microsurgical techniques and immunosuppressive therapies for use in prepubescent humans receiving a testis transplant. If successful, homologous transplantation among siblings would provide stable testosterone levels and restore fertility. Jennifer K. Pullium2 Peter H. Lin, and Martin J. Pinter