An unusual case of a monorchid horse with an abdominally retained testicle.

Introduction Monorchidism is a rarely described condition in the horse and is not to be confused with cryptorchidism. The diagnosis is challenging and confirmed by surgery and histology in combination with hormonal assays. This report describes, to the best of the author's knowledge, the first case of monorchidism and abdominal cryptorchidism of the developed testicle in a horse. Methods An Irish Cob underwent laparoscopic castration for removal of bilateral cryptorchid testicles. At surgery the horse was diagnosed as a monorchid with the testicle retained intra-abdominally. Histopathological, hormonal, molecular and cytogenetic analysis was performed. This included measuring testosterone and anti-mullerian hormone (AMH) in serum blood, isolating genomic DNA from EDTA- and heparin treated blood, PCR amplification of the SRY gene, metaphase chromosome preparation and DAPI-banding before metaphase analysis with Fluoresence in situ hybridisation (FISH) analysis. Results The horse was positive for the SRY gene and had a mosaic 63,X/64,XY karyotype with the aneuploid cells being present in only 2% of metaphases. Fluorescence in situ hybridization showed that the missing sex chromosome of the aneuploid cell line was the Y chromosome embedded in micronuclei. An abnormal high rate of micronuclei (6.6%) was observed indicating genotoxic events and/or genome instability. Hormonal assay results confirmed that AMH was not significantly increased, suggesting that no further testicular tissue was present. Histopathology was consistent with testicular tissue displaying a Sertoli cell only pattern with bipolar ductal structures. Conclusion The exact causes of monorchidism and cryptorchidism is unclear, but the elevated rate of micronuclei is clear evidence for genome instability which might have been involved in the failure of normal testicular development and descent. Future cases could further clarify the disease mechanism based on this report.