Testicles are involved in sperm production as they contain certain types of cells which function in sperm development. Any of these types of cells can develop a tumor. Cryptorchidism is when testicles do not descend into the scrotum and remain in the abdomen. It is a major cause of malignant tumors of testicles. In pets with testicular tumors, the affected testis becomes enlarged. In the case of Sertoli cells tumor, the hormonal imbalance leads to the development of feminization symptoms which include production of milk by mammary glands and pattern hair loss. interstitial cell tumors develop without any obvious sign and are accidentally diagnosed during an ultrasound procedure. Testicular tumors are slow to propagate and can be cured by surgical removal of the testes.