Failure of the testis to descend completely into the scrotum is a common problem in children, occurring in 2% to 4% of baby boys at birth and 1% to 2% of boys by 6 months of age. The embryology of testicular descent is complex but now reasonably well understood, although there are a number of issues still unknown.

The testis forms on the front of the urogenital ridge on the posterior abdominal wall in the early embryo. At <8 weeks of development the ambisexual gonad develops into a testis in boys, with the mesonephric duct differentiating into the vas deferens and the paramesonephric or Müllerian duct regressing.

The testis is anchored to the future inguinal abdominal wall by the genitoinguinal ligament or gubernaculum, which controls descent of the testis in what is now recognized as two basic steps. In the first phase, which occurs between 8 and 15 weeks, the gubernaculum enlarges in response to testicular hormones, effectively anchoring the testis near the groin as the abdomen and pelvic cavity enlarge with fetal growth. This so-called transabdominal phase of testicular descent is controlled primarily by secretion of insulin-like hormone 3 (INSL3) from the Leydig cells of the developing testis. There is also some evidence that Müllerian inhibiting substance (also known as antimüllerian hormone) is an important subsidiary factor. By 15 weeks of gestation the testis is anchored in a position just inside the future internal inguinal ring, and is already in a different position from the developing ovary. Between 15 and 25 weeks of development there is very little change in testicular position. The reason for this remains unknown, but it has been speculated that it is because sexual dimorphism of the nervous system is required for the second phase of testicular descent, which commences at about 25 weeks of development.

The second or inguinoscrotal phase of descent is controlled by testosterone from the testis and this hormone appears to act mostly indirectly on the gubernaculum by the genitofemoral nerve, which releases a neurotransmitter, calcitonin gene-related peptide (CGRP), from its sensory fibers in the inguinoscrotal region. CGRP appears to provide a concentration gradient as well as a growth stimulus to enable the gubernaculum to elongate toward the scrotum. Simultaneously, the gubernaculum is hollowed out by a diverticulum of the peritoneum— the processus vaginalis—which enables the intra-abdominal fetal testis to descend to a subcutaneous position while still remaining within the peritoneal cavity.

The second phase of descent requires a significant migratory step for the gubernaculum, the regulation of which remains unknown, but is likely to be regulated by a complicated series of trophic factors. Not surprisingly, this complex mechanical process is more frequently deranged so that palpable undescended testes following failure of the second
phase of descent are very common. By contrast, intra-abdominal, impalpable testes, where there has been failure of the first phase of descent, is relatively uncommon and usually occurs in <10% of patients. The fact that the mechanical processes required in the first phase are relatively simple probably accounts for its less frequent derangement producing undescended testes.

Congenital undescended testes or cryptorchidism (Ξ hidden testis) is usually a failure of the second phase, and the testis is located in the inguinal region, just outside the external inguinal ring. An important anatomical point is that the primary abnormality is failure of gubernacular migration, but the inguinal testis is still located within its peritoneal cavity (the tunica vaginalis), and is hence on a mesentery (the mesorchium). Therefore, an undescended testis remains surprisingly mobile on physical examination. On rare occasions, congenital undescended testes are caused by the gubernaculum migrating in the wrong direction, thereby taking the testis to an ectopic position in the perineum or occasionally the femoral region.

While congenital cryptorchidism is caused by failure of gubernacular migration to the scrotum, some testes develop acquired cryptorchidism after birth. This is because the distance from the external inguinal ring to the scrotum doubles from 4 to 5 cm in a baby to 8 to 10 cm in a prepubertal boy. For the testis to remain in the scrotum postnatally, the spermatic cord must elongate in proportion to growth of the boy. Failure of the spermatic cord to grow at the same speed as the child causes the testis to be left behind as the scrotum grows further away from the groin as the pelvis enlarges. Failure of spermatic cord elongation is now recognized as a common cause of what is now known as acquired cryptorchidism. Although the concept of acquired cryptorchidism still remains controversial in some quarters, it is becoming accepted as a common cause of cryptorchidism in boys between 5 and 10 years of age. In some series it accounts for nearly half the number of boys undergoing orchidopexy.

Diagnosis of undescended testes is sometimes quite difficult, because of the mobility of the testis and existence of retractile testes. The latter are ones which are pulled back into the groin by a very active cremasteric reflex. The cremasteric reflex is a normal physiological response to touch and temperature, to maintain the homeostasis of the testis, which is programmed to function at the low temperature of the scrotum (33ºC). Some boys, however, have very retractile testes, and in many of these it is quite likely that this is an early stage of development of acquired cryptorchidism, secondary to failure of the spermatic cord to elongate in proportion to growth of the boy. In children with cerebral palsy this is caused by an upper motor neuron lesion in the cremaster muscle. In normal boys it is caused by persistence of the processus vaginalis with either a clinical hernia or persistence of a fibrous remnant of the obliterated processus vaginalis, which fails to grow postnatally and is inelastic. Failure of the processus vaginalis to obliterate completely, preventing elongation of testicular vessels and vas deferens, is probably related to inadequate postnatal androgen stimulation.

Because the testis is mobile within its tunica vaginalis, it is important to palpate the inguinal region very carefully, aiming to milk the testis out of the inguinal region into the scrotum. By traction on the testis by the examiner’s other hand, once the testis can be delivered into the scrotum, the lower limit of the tunica vaginalis can be appreciated. When the testis is released, it should remain in the scrotum. However, if it immediately retracts out of the scrotum, this suggests a pathologically retractile testis that is probably becoming secondarily undescended as an acquired undescended testis.

At birth some testes have not yet reached the scrotum, either because the baby was born prematurely or descent is delayed. At birth about 4% or 5% of boys have cryptorchidism, but in approximately half of these the testis descends spontaneously in the first 3 months of life. Therefore, by 3 to 6 months of age the frequency of cryptorchidism is about 1% to 2%. Where the testis is cryptorchid at birth, the boy should be reexamined at 3 to 6 months of age; if the testis is still not descended I would proceed to orchidopexy. The current recommended age of operation for congenital undescended testes, based on our knowledge of postnatal germ cell development, is about 6 months.

In some children (-10%) the testes cannot be palpated in the inguinal region at all, as any testis within the canal or abdomen is impalpable. The examiner is then left with the possibility that the testes are either intra-abdominal or absent. Even more importantly, there is a very rare group of children with disorders of sex development where the external anatomy looks male, but the child actually has congenital adrenal hyperplasia and the testes are impalpable because the child actually has ovaries inside the abdomen. A useful tip on physical examination when the testes are impalpable is to determine whether or not the external inguinal ring is palpable as a V-shaped defect in the external oblique aponeurosis. If the external ring feels open, it usually indicates that the testis is inside the inguinal canal. When the testis is truly intra-abdominal or absent because of perinatal torsion and atrophy, the external inguinal ring usually feels closed.

In some centers ultrasound is occasionally done to locate the testis, but this is often an unnecessary investigation, as testes outside the external ring should be palpable on physical examination, while testes inside the abdomen are often too difficult to see on ultrasound if there are loops of bowel in front of the testis.

Laparoscopy is recommended to determine the presence and location of a testis that is truly impalpable. Torsion of the testis during descent (causing atrophy postnatally) is believed to cause the so-called vanishing testis. When a boy is found with impalpable undescended testes, there is a 30% to 50% chance that the testis is absent secondary to a prenatal vascular accident. Torsion is thought to occur during testicular descent as the gubernaculum does not become fixed to the inside of the scrotum (preventing torsion) until after descent is complete.

The testis normally resides in the scrotum, which provides a specialized, low-temperature environment at 33ºC. When the testis does not reside in the scrotum, it is insulated by the inguinal fat pad, which increases the ambient temperature of the undescended testis and this abnormal temperature interferes with testicular physiology. In the postnatal period, the germ cells normally undergo a series of developmental steps to prepare for subsequent spermatogenesis at puberty [Fig. 1]. It is now thought that the key developmental step is transformation of the neonatal gonocyte into an adult dark spermatogonium, the putative stem cell for spermatogenesis. This transformation normally occurs between 3 and 9 months of age, but is significantly inhibited in undescended testes; hence, the recommended age of orchidopexy is 6 months (after spontaneous postnatal descent is complete). This is based on the hope that the germ cell abnormality is secondary to the higher temperature and therefore preventable or reversible by relocating the testis into the cooler scrotum. Of course, in those testes with an intrinsic developmental defect causing maldescent, orchidopexy may not correct
the problem. There is little long-term follow-up evidence to suggest that early orchidopexy will prevent germ cell abnormalities, but there are some early encouraging trials, which suggest that this is likely. Certainly, without surgery it is well recognized that boys with unilateral undescended testes have about a 30% chance of infertility, and in boys with bilateral undescended testes the infertility risk for is <70%.