Embryologically, the genital ridge becomes bipotential gonads, which in males differentiate into testicles based on influences from the sex-determining region of the Y chromosome (SRY gene) among other genes and hormones. Sertoli cells secrete anti-Mullerian hormone to cause Mullerian structure regression, and Leydig cells begin secreting testosterone to stimulate Wolffian duct differentiation and external virilization.¹³

Testicular descent occurs in two phases: a transabdominal phase and a transinguinal, or inguinoscrotal, phase. Transabdominal descent is largely passive and appears closely related to the descent of the diaphragm with some regulation by insulinlike peptide (INSL3).¹⁴ Within this phase, the gubernaculum, which extends from the caudal pole of the testis through the inguinal canal to the scrotum, increases in volume to expand the inguinal canal. During transinguinal descent, the peritoneum forms the vaginal tunic, and the testis is drawn through the inguinal canal into the scrotum by regression and contraction of the gubernaculum under the influence of androgens and INSL3.¹³ This migration is generally completed by 10 days after birth, and a presumptive diagnosis can be made by 8 weeks of age.^15-17 However, because of high testicular mobility in young animals and breed variations, an animal is not typically defined as cryptorchid until 6 months of age, when the inguinal rings of most dogs have closed.^{4,10,15,18,19}

Within each testicle, convoluted seminiferous tubules are composed of three different cell types—spermatogenic, Sertoli, and Leydig cells—which are supported by a connective tissue support network. Spermatogenic cells facilitate the production and maturation of spermatozoa. Sertoli cells, or sustentacular cells, control the release of spermatozoa, support their development, and aid in forming the blood–testis barrier. They are stimulated by follicle-stimulating hormone. Leydig cells, or interstitial cells, produce testosterone through stimulation by interstitial cell–stimulating hormone and are inhibited by luteinizing hormone.

If testicular descent was successful, each cavity of the scrotum contains a testis that has passed through the inguinal canal, with its associated epididymis and distal spermatic cord, and is covered by a vaginal tunic and spermatic fascia. The spermatic cord contains the ductus deferens as well as testicular vessels and nerves. The ductus deferens is the continuation of the tail of the epididymis and ascends the spermatic cord, enters the abdomen through the inguinal canal, crosses ventral to the ureter at the lateral ligament of the bladder, and enters the prostate to open into the urethra. It is accompanied by the artery and vein of the ductus deferens, arising from the prostatic artery, a branch of the internal iliac artery, and internal iliac vein, respectively. The testicular artery arises from the aorta near the fourth lumbar vertebra and anastomoses with the artery of the ductus deferens within the spermatic cord. The testicular vein follows a similar course, but it forms into the pampiniform plexus within the spermatic cord, surrounding the internal spermatic artery, lymphatics, and nerves to create a countercurrent thermal exchange. The right testicular vein drains into the caudal vena cava, and the left drains into the left renal vein. Testicular lymphatics drain into the lumbar lymph nodes. Nervous supply to the testis is largely sympathetic, supplied by the testicular plexus from the fourth to sixth lumbar ganglia.²⁰ If the testicle is still present within the abdomen, these structures will not have been bundled into the spermatic cord by the vaginal tunic and so will instead course independently from their respective origins.

Cryptorchidism is a heritable trait and is suspected to have a sex-linked, autosomal recessive inheritance^9,18,21; however, its exact etiology is multifactorial and incompletely understood. Its incidence in dogs has been reported between 1.2% and 12.9%.^{1,4,5,8,22,23} Although widely reported in purebred and mixed-breed dogs, cryptorchidism is more prevalent in purebred and inbred dogs and more common in small breed compared with large breed dogs.²³ Specifically, a high prevalence has been reported in Chihuahuas, miniature Schnauzers, Pomeranians, poodles, Shetland sheepdogs, Siberian huskies, and Yorkshire terriers.^9,10,24 Cryptorchid dogs also have a higher incidence of concurrent congenital defects such as inguinal and umbilical hernias, patellar luxation, and preputial or penile abnormalities.^10,24 Cryptorchidism has been reported in 1.3% to 3.8% of cats, with an increased prevalence in Persians.^2,3,5

Plasma levels of both INSL3 and testosterone have been shown to be significantly lower in bilateral cryptorchid dogs than normal dogs, although still higher than in castrated dogs. This suggests impairment of Leydig cell function as a contributing factor as well as consequence of cryptorchidism.^25-27 Retained testes are generally smaller with up to 60% reduction in seminiferous tubules compared with a scrotal testis,²⁷ and even unilaterally affected dogs have decreased testosterone levels and semen quality.^26,27 Although still producing hormones, cryptorchid testicles are sterile because of the adverse effects of core body temperatures on spermatogenesis.^1,3,19 However, a unilaterally affected male remains fertile, albeit less so than a normal male.^1,10 In prepubescent male dogs, hormone treatments with human chorionic gonadotropin (hCG) or gonadotropin-releasing hormone (GnRH) have been used in an effort to encourage testicular descent with only anecdotal reports of success.¹⁰

On physical examination, if one or both of the testes cannot be palpated within the scrotum, it should be considered to be undescended. Testicular aplasia, resulting in monorchism or anorchism, is rare.^2,7 Palpation of the inguinal region, including the inguinal fat pad, should be performed to assess if the testicle has exited through the inguinal canal or if it remains within the peritoneal cavity. This should be performed with the animal standing as well as in dorsal recumbency and may require sedation or general anesthesia because the retained testicle is often smaller and can be difficult to palpate. If the animal is a unilateral cryptorchid, the descended testicle should be manipulated cranially into the prescrotal tissue; during this maneuver, the testis will deviate to one side of the penile shaft, indicating the side from which it has descended and allowing more focused investigation of the contralateral inguinal region. If the testicle cannot be palpated inguinally, it should be assumed to be present within the peritoneal cavity; it is uncommon for the testis to be lodged within the inguinal canal.

Testicular torsion should be considered upon presentation of a cryptorchid male with peracute abdominal pain but also in those with more vague clinical signs (lethargy, anorexia, vomiting) and should be considered a surgical emergency.^12,28 A palpable inguinal or caudal abdominal mass may be found during evaluation, and on ultrasound examination, the mediastinum testis, a cord of connective tissue running lengthwise through the middle of the testicle, may be poorly defined in either a torsed or neoplastic testicle.^29,30 Given the risk of neoplastic transformation within retained testicles, testicular neoplasia should be considered in any abdominal or inguinal masses in middle-aged to older males. Retained testes in dogs have a 9.2 to 13.6 times higher incidence of neoplasia, with Sertoli cell tumors and seminomas being most common.^11,24,31 Inguinal testes are at twice the risk of neoplasia than intraabdominal testicles.^8,32 The risk of neoplasia also increases 4.7-fold when inguinal hernias are present.¹¹ Estrogen production, most commonly seen with Sertoli cell tumors, causes feminization syndrome in 16% to 39% of dogs, including bilateral alopecia, prostatic hyperplasia, penile atrophy, gynecomastia, galactorrhea, and myelotoxicosis (anemia, leucopenia, thrombocytopenia).^33-37

In the case of suspected bilaterally retained testes with an unknown castration history, other historical and physical examination findings can be used to support presumption of an intact cryptorchid male. This would include behavior typical of an intact male or the typical odor of tom cat urine.³ In dogs, a well-developed prostate and, in cats, spines along the penile shaft are both androgen-dependent structures that are indirect evidence of the presence of testicular tissue. Plasma testosterone levels can be measured and a stimulation test performed if confirmation of the diagnosis is needed. Basal blood testosterone levels are assessed, and the patient is administered either GnRH (2 or 50 mcg/dog GnRH) or hCG (50 IU/kg hCG intramuscularly), with a second blood sample drawn 60 minutes (GnRH) or 120 minutes (hCG) after injection.³⁸ A substantial increase in plasma testosterone levels after injection is indicative of the presence of testicular tissue.^39,40

Diagnostic ultrasonography has been shown to be highly effective at localizing retained testicles in both the inguinal region and intraabdominally. In one study, it carried a 100% positive predicative value compared with surgery and had a 96.6% sensitivity for abdominal testicles and 100% sensitivity for inguinal testicles in the hands of experienced evaluators.³⁰ Intraabdominal testicles were found anywhere between the kidneys and scrotum, so scanning of the entire ventral abdomen is recommended. Testicles generally maintained their normal sonographic appearance of a coarse, homogenous texture with central hyperechoic mediastinum testis unless they had undergone neoplastic transformation or torsion.^29,30 Magnetic resonance imaging has been reported in humans to search for cryptorchid testicles; however, even though it is more accurate than computed tomography, it had a low diagnostic accuracy.¹³