Cryptorchid Testis: Anatomy, Descent, and Pathology

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

A cryptorchid testis is a testis that has failed to complete its normal route from the caudal pole of the embryonic kidney to the scrotum, so it remains somewhere along the line of descent, most often inside the abdomen or within the inguinal canal. Cryptorchidism (from the Greek kryptos, hidden, and orchis, testis) is therefore a positional defect of a testis that may otherwise be structurally normal at birth.

This matters because the retained testis does not stop developing. It keeps producing cells that can transform, and it does so at a temperature several degrees warmer than the scrotum. Across species, the undescended testis carries a markedly increased risk of seminoma, Sertoli cell tumor, and Leydig (interstitial) cell tumor, and the descended scrotal testis in the same animal is not protected from the same risk. In dogs, cryptorchidism is considered the most common congenital testicular abnormality of companion animals and a recognized predisposition factor for testicular neoplasia [1]. Understanding the anatomy and the embryology is what lets a clinician predict where a retained testis will be found, and understanding the pathology is what explains why it should be removed.

This article covers the normal anatomy of the testis and its adnexa, the two-phase embryologic descent and its hormonal control, the anatomic locations of retained testes, a species comparison, and the tumor biology of the cryptorchid testis.

Normal Anatomy of the Testis and Its Supporting Structures

The testis is a paired, ovoid gonadal organ suspended in the scrotum by the spermatic cord. Each testis is covered by the tunica albuginea, a dense fibrous capsule, and internally divided by septa into lobules containing seminiferous tubules. The tubules converge on the mediastinum testis and drain into the epididymis, which has a head (caput), body, and tail (cauda). The tail of the epididymis continues as the ductus deferens toward the pelvic urethra.

Three structures govern the position of the testis:

• The gubernaculum testis, a gelatinous mesenchymal cord that connects the caudal pole of the testis to the future scrotal region. It is the principal anatomical driver of descent.
• The cranial suspensory ligament, which anchors the testis near the kidney in early fetal life and must regress for descent to proceed.
• The processus vaginalis, an outpouching of peritoneum that follows the gubernaculum through the inguinal canal and forms the serous covering of the testis.

The cremaster muscle develops in continuity with the internal oblique and transversus abdominis muscles and provides motor control of testicular position within the scrotum [2]. The genitofemoral nerve (GFN) supplies the gubernaculum and cremaster, descending from the L1-L2 segments, and is the conduit for the second phase of descent [3].

Embryology of Testicular Descent

Sexual differentiation begins with bipotential gonads that develop into either testes or ovaries. The fetal testis secretes hormones that direct the differentiation of internal and external genitalia, while the fetal ovary remains hormonally quiet [4]. Testicular descent is best described as a continuum with two main phases, the transabdominal phase and the inguinoscrotal phase [4].

Phase 1: Transabdominal Migration

In the transabdominal phase, the testis moves from the lower pole of the kidney toward the bladder neck and the future internal inguinal ring [5]. This phase depends on insulin-like peptide 3 (INSL3), a Leydig cell product that acts on the gubernaculum through its receptor RXFP2 (also called GREAT or LGR8) [4][6]. INSL3 induces the gubernaculum to swell and elongate, anchoring the testis near the groin as the fetus grows. In humans this phase occurs between roughly 8 and 15 weeks of gestation [7].

Testosterone from the fetal Leydig cells augments this phase by causing regression of the cranial suspensory ligament [7]. The transabdominal phase is rarely disrupted in humans [4], which is why pure abdominal retention is less common than inguinal retention in boys.

In the dog, INSL3 is immunolocalized to fetal Leydig cells, and RXFP2 is expressed in the muscle and connective tissue cells of the gubernaculum as well as in the epithelium of the developing excurrent ducts. Canine gubernacular RXFP2 immunoreactivity is limited to fetuses at approximately 35 to 45 days of gestation, the same window in which the fetal testis is endocrinologically active [6].

Phase 2: Transinguinal Passage and Inguinoscrotal Migration

In the inguinoscrotal phase, the gubernaculum bulges through the external inguinal ring and migrates across the pubic region to the scrotum [7]. This phase is androgen dependent, but androgens act mostly indirectly through the GFN, which releases calcitonin gene-related peptide (CGRP) to control the direction of migration [7][3]. In humans the inguinoscrotal phase occurs between roughly 25 and 35 weeks of gestation [7].

The gubernaculum does not simply get pulled. It actively everts and migrates. In the rat, the gubernacular core regresses postnatally except for a cranial proliferative zone, and by postnatal day 2 the gubernaculum has everted, creating the processus vaginalis [2]. Matrix metalloproteinases, particularly MT1-MMP, appear to remodel the gubernaculum for eversion and provide the collagenolysis needed for migration through the inguinal fat pad [8]. The cremaster muscle develops inside the gubernacular tip, supporting active elongation during the inguinoscrotal phase [9].

The following flowchart summarizes the two-phase mechanism and the hormonal and neural control at each step.

flowchart TD
    [Bipotential gonad] --> [Fetal testis]
    [Fetal testis] --> [Leydig cells]
    [Leydig cells] --> [INSL3 secretion]
    [INSL3 secretion] --> [Gubernaculum swelling]
    [Gubernaculum swelling] --> [Transabdominal phase]
    [Leydig cells] --> [Testosterone secretion]
    [Testosterone secretion] --> [Cranial ligament regression]
    [Testosterone secretion] --> [Genitofemoral nerve activation]
    [Genitofemoral nerve activation] --> [CGRP release]
    [CGRP release] --> [Inguinoscrotal migration]
    [Inguinoscrotal migration] --> [Scrotal position]

Timing Across Species

The timing of descent is a high-yield comparative point. In most domestic species, both phases are complete before birth. In the dog, descent continues after birth and is typically complete by a few weeks of age. The clinical consequence is that a puppy is not definitively cryptorchid until the testes have had time to arrive. In the horse, descent is usually complete before or around birth, and in the pig it is complete before birth.

Anatomic Locations of Retained Testes

Retained testes are classified by where they stop along the normal path of descent. Two categories matter clinically:

• Inguinal retention: the testis sits within the inguinal canal, between the internal and external inguinal rings. It is palpable as a firm, mobile structure in the groin.
• Abdominal retention: the testis remains inside the peritoneal cavity, anywhere from near the kidney to the internal inguinal ring. It is not palpable on external examination.

A third, less common category is ectopic retention, in which the testis deviates from the normal path and lodges in an abnormal location such as the perineum or the femoral region. Ectopic testes are rare and are usually identified at surgery or imaging.

Unilateral retention is more common than bilateral retention. In a survey of 4,924 male dogs examined by ultrasound over a 12-year period, 403 (8.2%) were cryptorchid. Of those, 70% were unilateral and 30% were bilateral. Among the unilateral cases, the right testis was affected more often than the left (59.5% of all cryptorchid dogs), and inguinal localization was the most common site (59.5%). In bilateral cases, the symmetrical form was most common, at 86.8% [1].

Body size influences the site. In the same study, cryptorchidism was more common in the inguinal region of small-breed dogs and in the abdominal region of medium- and large-breed dogs [1]. This is a useful clinical heuristic: a retained testis in a Chihuahua is more likely to be inguinal, while a retained testis in a Labrador is more likely to be abdominal.

In a colony study of Miniature Schnauzers, all unilateral retained testes were on the right side, and right-sided bilaterally retained testes were always smaller than their left-sided counterparts. With one exception, ectopic testes were in the abdominal position [10]. The anatomic appearance of the epididymis differed by laterality: it was primitive in bilateral cases and nearly normal in unilateral cases [10].

Species Comparison

The table below compares typical retention sites and descent timing across four species. It is intended as a study reference, not a diagnostic algorithm.

Pathology of the Cryptorchid Testis

The retained testis is not a static structure. It undergoes progressive degenerative and neoplastic change, and the risk applies to the descended testis as well.

Temperature and the Retained Testis

The scrotum exists to keep the testis cooler than core body temperature. When a testis remains in the abdomen or inguinal canal, it is exposed to a temperature several degrees higher than normal. This thermal environment is one proposed mechanism for the increased tumor risk, though the relationship is not fully resolved. A review of the dog and human literature notes that the question of whether the location of the undescended testis directly causes testicular cancer, or whether both conditions share a common origin, remains debated [11]. The same review emphasizes that dogs and humans differ significantly in reproductive development, in the histologic and molecular features of testicular tumors, and in the prevalence of specific tumor types, with Sertoli cell tumors predominating in cryptorchid dogs and germ cell tumors predominating in humans [11].

Tumor Types

Three tumor types are classically associated with the cryptorchid testis:

• Seminoma: a germ cell tumor. It is the most common testicular tumor in humans and is also seen in dogs.
• Sertoli cell tumor: a sex cord-stromal tumor. It is the most common testicular tumor in cryptorchid dogs [11]. Sertoli cell tumors can produce estrogen, leading to feminization, gynecomastia, and attraction of other males.
• Leydig (interstitial) cell tumor: also a sex cord-stromal tumor. It arises from the interstitial cells that produce testosterone.

The retained testis is at markedly increased risk for all three. The scrotal testis in the same animal is not protected, which is a point students frequently miss. A unilateral cryptorchid dog with a normal scrotal testis still has a testis that can develop neoplasia, and the scrotal testis may be the first to show a tumor.

Other Pathologic Changes

Beyond neoplasia, the retained testis shows impaired spermatogenesis. In a case report of a 7-month-old dog with right-sided cryptorchidism, the retained right testis lacked spermatogenesis but contained normal Sertoli cells [12]. The epididymis of an abdominal testis is often developmentally primitive, particularly in bilateral cases [10]. The gubernaculum itself shows altered extracellular matrix composition in cryptorchid patients. In human fetal and cryptorchid gubernacula, collagen concentration increases with fetal age while glycosaminoglycan concentration decreases, and in cryptorchid children the collagen and GAG values do not correlate with age. When the testis is located more proximally, collagen content is lower [13]. These matrix changes are consistent with a failure of the normal remodeling that permits descent.

Genetic and Molecular Contributors

The genetic basis of cryptorchidism is polygenic and incompletely understood. In the Miniature Schnauzer colony study, the degree of inbreeding was greater for bilateral cases than for unilateral cases, and the morphologic observations suggested a multiple gene defect [10]. In dogs, only three DNA variants have been associated with cryptorchidism: variants in the RXFP2, HMGA2, and KAT6A genes. DNA polymorphism in KAT6A, associated with changes in global H3K9 acetylation, and DNA methylation patterns in INSL3 suggest that epigenetic modifications are an important area for further research [14]. The development of the epididymo-testicular junction is also an active area of investigation [14].

Comparative genomics has clarified the evolutionary context. Both RXFP2 and INSL3 are lost or nonfunctional in four afrotherian lineages (tenrec, cape elephant shrew, cape golden mole, and manatee) that completely lack testicular descent. The presence of remnants of once-functional orthologs in these species shows that the gene losses happened after the split from the placental mammal ancestor, providing strong evidence that testicular descent is the ancestral condition in placental mammals [15].

Clinical Relevance, Limitations and Common Mistakes

The retained testis is a surgical and oncologic concern. The standard recommendation is castration of both the retained and the descended testis, because the scrotal testis is not protected from neoplasia. The timing and technique of surgery are outside the scope of this article.

Diagnostic localization matters. Ultrasound is a valuable tool for identifying and localizing a retained testis and for detecting parenchymal changes, which supports safe clinical treatment [1]. In the survey of 4,924 dogs, ultrasound provided precise localization and identified parenchymal changes [1]. Palpation alone can miss abdominal testes and can mistake other structures for an inguinal testis.

Common mistakes students and clinicians make:

• Assuming a retained testis is sterile and harmless. It is neither. It is at high risk for neoplasia.
• Assuming the scrotal testis is safe. It is not. The descended testis in a cryptorchid animal can also develop tumors.
• Assuming a puppy with no palpable testes is cryptorchid. In dogs, descent continues after birth, so the diagnosis is not final until the testes have had time to arrive.
• Assuming laterality is random. In dogs, the right side is affected more often than the left [1], and in the Miniature Schnauzer colony all unilateral retained testes were right-sided [10].
• Assuming retention site is the same across breeds. Small breeds tend toward inguinal retention, medium and large breeds toward abdominal retention [1].
• Assuming the retained testis is always abdominal. Inguinal retention is at least as common, and in the dog survey it was the most common site overall [1].

Individual cases require veterinary assessment. The information here is a study framework, not a diagnosis.

Quick Review

• A cryptorchid testis is a testis that failed to complete its route to the scrotum, most often remaining in the abdomen or inguinal canal.
• Descent has two phases: transabdominal (INSL3 and RXFP2) and inguinoscrotal (androgens acting via the genitofemoral nerve and CGRP).
• In dogs, descent continues after birth. In most other domestic species, it is complete before birth.
• Unilateral retention is more common than bilateral, and the right side is more often affected in dogs.
• Small-breed dogs tend toward inguinal retention, medium and large breeds toward abdominal retention.
• Retained testes carry a markedly increased risk of seminoma, Sertoli cell tumor, and Leydig cell tumor.
• The scrotal testis in the same animal is not protected from the same tumor risk.

Frequently Asked Questions

What is a cryptorchid testis?

A cryptorchid testis is a testis that has not descended into the scrotum and remains in the abdomen or inguinal canal.

Is cryptorchidism more common on one side?

In dogs, the right testis is affected more often than the left, and unilateral retention is more common than bilateral retention.

Can a retained testis still produce sperm?

Spermatogenesis is impaired in the retained testis because of the higher temperature, and the retained testis may show no spermatogenesis at all.

Does the scrotal testis need to be removed too?

Yes. The descended testis in a cryptorchid animal is also at increased risk of neoplasia and is typically removed along with the retained testis.

When can I be sure my puppy is cryptorchid?

Because descent continues after birth in dogs, the diagnosis is not final until the testes have had time to arrive. Your veterinarian can assess this at a puppy visit.

What tumors are associated with a retained testis?

Seminoma, Sertoli cell tumor, and Leydig cell tumor are the three tumor types classically associated with the cryptorchid testis.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

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Sources

1. Is the right testis more affected by cryptorchidism than the left testis? An ultrasonographic approach in dogs of different sizes and breeds.
2. Development of the gubernaculum during testicular descent in the rat.
3. Role of Gubernaculum testis inervation during the process of testicular migration in human fetuses.
4. Embryology and physiology of testicular development and descent.
5. Embryology of the testicular descent.
6. An insight into testis and gubernaculum dynamics of INSL3-RXFP2 signalling during testicular descent in the dog.
7. Regulation of testicular descent.
8. Gubernaculum as icebreaker: do matrix metalloproteinases in rodent gubernaculum and inguinal fat pad permit testicular descent?
9. Cremaster muscle myogenesis in the tip of the rat gubernaculum supports active gubernacular elongation during inguinoscrotal testicular descent.
10. An anatomic and genetic study of canine cryptorchidism.
11. Cryptorchidism and testicular cancer in the dog: unresolved questions and challenges in translating insights from human studies†.
12. Right-sided cryptorchidism and contralateral intrascrotal testicular torsion in a young dog: a rare case report.
13. Extracellular matrix remodeling in the human gubernaculum during fetal testicular descent and in cryptorchidic children.
14. Exploring Testicular Descent: Recent Findings and Future Prospects in Canine Cryptorchidism.
15. Loss of RXFP2 and INSL3 genes in Afrotheria shows that testicular descent is the ancestral condition in placental mammals.