Duct system

The Sertoli cells of the developing fetal testes produce masculinizing hormones, such as testosterone, which stimulate the mesonephric (old kidney) ducts to form male genital ducts. Some mesonephric tubules persist and are transformed into efferent ductules. These ductules open into the mesonephric duct which has become the ductus epididymis. Just beyond the epididymis the mesonephric duct develops a thick covering of smooth muscle and becomes the ductus deferens. Lateral outgrowths from the caudal end of each mesonephric duct give rise to the seminal vesicles, glands that produce a secretion that provides nourishment for the sperm. The part of the mesonephric duct between the seminal vesicle ducts and the urethra becomes the ejaculatory duct (Fig. 8.1).

Figure 8.1 Male embryology. The origin of the male genital tract from the mesonephric duct system and from the urogenital sinus.

The prostate develops from multiple outgrowths from the prostatic part of the urethra, which grow into the surrounding mesenchyme. The appendix testis is a vestigial remnant of the paramesonephric duct system and is attached to the head of the epididymis.

Descent of the testes

The testes need to descend into the scrotum so that viable spermatozoa can develop. The scrotum is outside the thermal regulation of the body and is a cooler environment. Failure to descend can result in infertility or even malignancy.

The inguinal canals are the pathways for the testes to descend from their intra-abdominal location through the anterior abdominal wall into the scrotum. A ligament called the gubernaculum is attached at its upper or cranial end to the inferior pole of each gonad. It descends on either side of the abdomen and passes obliquely through the developing anterior abdominal wall at the site of the future inguinal canal (Fig. 8.2). The gubernaculum attaches caudally to the internal surface of the labioscrotal swellings and later serves to anchor the testes in the scrotum. The processus vaginalis is an evagination of the peritoneum and follows the gubernaculum. It herniates through the abdominal wall along the path formed by the gubernaculum and also serves to help guide the testis into the scrotum. The processus vaginalis carries the layers of the abdominal wall before it and goes on to form the walls of the inguinal canal. In males these layers are the coverings of the spermatic cord and testes.

Figure 8.2 The path of descent of the normal testis into the scrotum.

Testicular descent is associated with enlargement of the testes and atrophy of the intermediate or mesonephric kidneys. This atrophy allows the movement of testes caudally along the posterior abdominal wall. Little is known about the cause of testicular descent through the inguinal canals into the scrotum, but the process is controlled by androgens produced by the fetal testes. Conditions associated with diminished gonadotropins are associated with cryptorchidism such as the Prader–Willi syndrome.

The initial descent of the testes in the transabdominal phase is androgen independent and usually takes place before the 26th week of intrauterine life. After that the testes enter the scrotum from the internal inguinal ring, and this phase appears to be androgen dependent. The inguinal canal then contracts around the spermatic cord.

During the first 3 months after birth most undescended testes descend into the scrotum. Spontaneous testicular descent does not occur after the age of 1 year.

During the perinatal period, the connecting stalk of the processus normally obliterates, isolating the tunica vaginalis as a peritoneal sac related to the testes.

Cryptorchidism, or undescended testes

Cryptorchidism occurs in up to 30% of premature males and in about 3–4% of full-term males. It is the commonest genitourinary abnormality in males. Cryptorchidism may be unilateral in about 65% of males or bilateral in about 10%, but in most cases the testes descend into the scrotum by the end of the first year. If testes remain in the abdominal cavity they fail to mature and sterility is common. Cryptorchid testes may be in the abdominal cavity or anywhere along the usual path of descent but they are usually in the inguinal canal.

The cause of cryptorchidism is unknown, but a deficiency of androgen is an important factor. There is a higher incidence of cryptorchidism in prune belly syndrome and abdominal wall defects. Men with a history of cryptorchidism have a 20–44% increase in risk of developing testicular cancer.

Ectopic testes

After traversing the inguinal canal, the testes may deviate from their usual path of descent and lodge in various abnormal locations (Fig. 8.3). They may be present in the contralateral scrotum, the perineum, the superficial inguinal pouch, the femoral canal or suprapubically. Ectopic testes occur when a part of the gubernaculum passes to an abnormal location and the testes follow it. Ectopic testes do not have a higher incidence of malignancy but should be placed in a normal position before the age of 2 years.

Figure 8.3 Common positions where an ectopic (right) or undescended testis (left, numbered) may be found.

Retractile testis

A retractile testis is one that is fully descended but can move freely from its intrascrotal position to the groin because of a hyperactive cremasteric reflex. It occurs in the older boy once the cremasteric reflex develops, and descends spontaneously at puberty. There is no increase in infertility in these boys.

Hydrocele

Occasionally the abdominal end of the processus vaginalis remains open but is too small to permit herniation of intestine. Peritoneal fluid passes into the patent processus vaginalis and forms a hydrocele of the testes. If the middle part of the processus vaginalis canal remains open, fluid may accumulate and give rise to a hydrocele of the spermatic cord (Figs 8.4 and 8.5).

Figure 8.4 Positions of hydroceles in the inguinal canal and scrotum. The diagram shows the positions of hydroceles and herniations related to the inguinal canal and testis, depending on the patency of the processus vaginalis.

Figure 8.5 Inguinal hydrocele. Longitudinal sonogram of the inguinal canal. There is a hydrocele within the canal, and the testis can also be seen lying ectopically high in the inguinal canal.

Transducer

The scrotum is best examined with a high-frequency linear 15L8 probe, with the testes being examined in both the longitudinal and transverse planes. If the scrotum is enlarged and swollen so that the testes cannot be fully examined using a linear probe, then a gel stand-off or intravenous saline water bag may be used in order to demonstrate the whole testis. Views of the epididymis can be obtained by scanning obliquely or laterally using the opposite testis as a window. It must be remembered that the scrotal contents are highly mobile, and it may be necessary to hold the testis in position or the technique should be varied according to the image seen.²

Examination of suspected varicoceles should start with the patient standing, as varicoceles are often empty when the patient is lying down.

The role of ultrasound in the evaluation of scrotal masses is to confirm whether or not there is a lesion present, localize its site of origin, establish whether it is intratesticular or extratesticular, and to characterize its contents. The distinction between intra- and extratesticular lesions is important since the vast majority of intratesticular lesions are malignant whereas extratesticular lesions are benign. Ultrasound is very good at detecting whether a lesion is solid or cystic, but it is not good at distinguishing whether a solid tumor is benign or malignant.