Testes and Epididymis Anatomy

The epididymis and the testes are integral components of the male reproductive system, performing both reproductive (spermatogenesis) and endocrine functions (hormone production or steroidogenesis). ^[1] The testes are responsible for producing spermatozoa through spermatogenesis in the seminiferous tubules and synthesizing testosterone (by the interstitial endocrine cells [Leydig cells]), the primary male sex hormone. ^[2] The testes are encapsulated by a dense fibrous layer called the tunica albuginea, which extends inward to form fibrous septa that divide the testis into lobules. ^[3]

The epididymis is a highly coiled tubular structure located posterior and slightly lateral to the testis. ^[2] It serves as a site for sperm maturation, storage, and transport to the vas deferens. Spermatozoa entering the epididymis from the seminiferous tubules are immature and nonmotile. During their transit through the epididymal duct, they acquire motility and fertilization capacity due to biochemical changes induced by epididymal secretions. ^[4]

Initially, there is an undifferentiated gonad in the retroperitoneal area. Transcription of the SRY gene (sex-determining region Y gene), which is the testis-determining factor region on the Y chromosome, ultimately leads to sex differentiation. Without the SRY gene, the gonad would develop into an ovary. As the fetus develops, the functioning testis produces the male hormone testosterone to allow development of the male genitalia. Over the last 3 months' gestation, the testis must course its way down from its original retroperitoneal position to its final destination in the scrotum. During its journey, it must pass through the peritoneum, abdominal wall via the inguinal canal, and into the scrotal pouch.

An image depicting the testes and epididymis can be seen below.

Male reproductive organs, sagittal section.

View Media Gallery

The testis is a paired, ovoid male reproductive organ that sits in the scrotum, separated from its mate by a scrotal septum. Described by some as being shaped and sized like a large olive or a small plum, the average volume of the adult testis is approximately 15-25 mL. Typically, it measures 3.5-5 cm in length, 2.5-3 cm in width and 3-4 cm in depth (anteroposterior diameter). ^{[5, 2]}

Smooth to palpation, the testis sits obliquely with its long axis mostly vertical with a slight anterior and lateral slant to the superior pole. Superiorly, it is suspended by the spermatic cord, with the left testis often sitting lower than the right testis. Inferiorly, the testis is anchored to the scrotum by the scrotal ligament, a remnant of the gubernaculum. ^[6]

The tunica vaginalis testis (a remnant of the processus vaginalis) envelopes the testis in a double layer, except at the superior and posterior borders, where the spermatic cord and epididymis adhere to the testes. ^[7]

The visceral layer of the tunica vaginalis testis is closely applied to the testis, epididymis, and ductus deferens. On the posterolateral surface of the testis, this layer invests a slit-like recess between the body of the epididymis and the testis called the sinus of epididymis. ^[8]

The parietal layer of tunica vaginalis is adjacent to the internal spermatic fascia, is more extensive, and extends superiorly into the distal part of the spermatic cord.

Deep to the tunica vaginalis, the tunica albuginea is a tough, fibrous outer covering of the testis. On the posterior surface, it is reflected inwardly to form an incomplete vertical septum called the mediastinum testis.

The mediastinum testis extends from the superior to near the inferior portion of the gland. It narrows in width as it travels inferiorly. Anteriorly and laterally, numerous imperfect septa are given off, which radiate to the glands surface and are attached to the tunica albuginea. These divide the interior of the testis into numerous, cone-shaped spaces that have a wide base at the gland's surface and narrow as they converge to the mediastinum. In these spaces, the numerous lobules of glandular structures (the minute but long and highly coiled seminiferous tubules) are housed. The mediastinum supports the ducts and vessels as they pass to and from the glandular substance.

Internally, the tunica albuginea gives rise to septa that divide the testis into approximately 250 lobules. Each lobule contains 1-4 highly coiled seminiferous tubules, where spermatogenesis occurs. These tubules are lined by Sertoli cells and surrounded by interstitial tissue containing Leydig cells responsible for testosterone production. ^[9]

The seminiferous tubules are lined with germ cells that produce sperm and nutrient fluid. The seminiferous tubules converge into straight tubules that lead to the rete testis, a network of interconnected channels located within the mediastinum testis. From here, sperm are transported via efferent ductules to the epididymis. ^[9]

The epididymis is a comma-shaped, elongated structure composed of a single, fine tubular structure estimated up to 6 m (approximately 20 ft) in length. This tube is highly convoluted and tightly compressed (average size is ~5 cm) to the point of appearing solid. A deep groove, the sinus of the epididymis, is present laterally and it marks the boundary between the testis and epididymis. ^[2] Located on the posterior border of the testis, it is composed of three parts, including: ^[4]

The head (caput): About 8-12 efferent ductules from the superior pole of the testis drain into and form the head of the epididymis.

The body (corpora): Extends along the posterolateral aspect of the testis; this region is where the sperm undergo further maturation.

The tail (cauda): Located at the inferior pole, it serves as a storage site for mature sperm and transitions into the ductus deferens. The tail of the epididymis progressively tapers and becomes continuous with the convoluted portion of the ductus deferens.

Due to its length, the epididymal duct allows space for storage and maturation of sperm. (vas deferens; see the image below).

The epididymal duct is lined by pseudostratified columnar epithelium with stereocilia that facilitate fluid absorption and secretion of factors essential for sperm maturation. The tail contains smooth muscle layers that contract rhythmically during ejaculation to propel sperm into the vas deferens. ^[10]

Male reproductive organs, sagittal section.

View Media Gallery

The arterial supply to both testes is primarily from the testicular arteries, which arise from the anterolateral aspect of the abdominal aorta just inferior to the renal arteries. They travel retroperitoneally, cross over the ureters and the inferior parts of the external iliac arteries to pass through the deep inguinal ring to enter the inguinal canal and become one of the components of the spermatic cord. The testicular artery enters the testis through the posterior midportion. The testicular artery or one of its branches anastomoses with the artery of the ductus deferens.

Venous drainage from the testis and epididymis forms a network of 8-12 veins, called the pampiniform venous plexus, lying anterior to the ductus deferens and surrounding the testicular artery in the spermatic cord. This plexus plays a crucial role in thermoregulation via counter current heat exchange. The counterflowing arteries and veins are separated only by the thickness of their vascular walls. This permits the exchange of heat and small molecules and facilitates the maintenance of lower testicular temperatures. ^[11] The veins converge superiorly, forming a testicular vein, after passing through the deep inguinal ring. The right testicular vein enters the inferior vena cava, and the left testicular vein drains into the left renal vein. This asymmetry in venous drainage has clinical significance, particularly in conditions such as varicocele, which is more common on the left side due to increased venous pressure. ^[11]

Lymphatic drainage of the testis follows the testicular vessels (in the spermatic cord) to the right and left lumbar (caval/aortic) and preaortic lymph nodes at the second lumbar level. This pathway is distinct from that of scrotal lymphatics, which drains into superficial inguinal lymph nodes. ^[2]

Autonomic innervations of the testis arise as the testicular plexus of nerves on the testicular artery, which contains vagal parasympathetic and visceral afferent fibers and sympathetic fibers from the T7 segment of the spinal cord.

The testis is composed of lobules of glandular tubules. These tubules are highly convoluted and held together by loose connective tissue with interspersed groups of "interstitial cells," which contain Leydig cells. The individual tubule consists of a basement membrane formed by laminated connective tissue with numerous elastic fibers with flattened cells between the layers and covered by an external layer of flattened epithelioid cells. Within the basement membrane are epithelial cells arranged in several irregular layers but may be separated into germ cells at the periphery and varying cells of spermatogenesis up to mature sperm cells as they advance toward the lumen. Also, interspersed in the layer are Sertoli cells, which project inward from the basement to the lumen and provide support to the developing sperm cells.

Testicular histology magnified 500 times. Leydig cells reside in the interstitium. Spermatogonia and Sertoli cells lie on the basement membrane of the seminiferous tubules. Germ cells interdigitate with the Sertoli cells and undergo ordered maturation, migrating toward the lumen as they mature.

View Media Gallery

In the apices of the lobules, the tubules are less convoluted and converge into 20-30 larger, straight ducts (tubuli recti). These ducts merge into anastomosing tubes in the fibrous stroma, lined with flattened epithelium (rete testis). The tubes terminate into approximately 15 ducts that are initially straight in their course.

After piercing the tunica albuginea at the superior mediastinum, they enlarge and become increasingly convoluted. These convolutions are held together by fine areolar tissue and bands of fibrous tissue. They form a series of conical masses, the conic vasculosi. This series forms the head of the epididymis. These ducts are thicker and lined by ciliated columnar epithelium. Below this epithelium is muscular tissue arranged in a mostly circular fashion. As the tail of the epididymis merges with the ductus deferens, the microscopic anatomy demonstrates a thickened duct with increased muscular material, increased in diameter, and still lined with ciliated columnar epithelium.

The epithelial lining of the epididymal tubule contains principal, basal, apical, and clear cells. Principal cells are tall columnar cells with apical stereocilia that absorb excess fluid from testicular secretions and secrete glycoproteins essential for sperm maturation. Basal cells are small round cells located at the base of the epithelium that act as stem cells for epithelial renewal. Apical and clear cells are far less common than principal and basal cells. Apical cells have abundant mitochondria and are mainly found in the head of the epididymis. Clear cells are columnar and have few microvilli but numerous endocytic vesicles and lipid droplets . They are most abundant in the tail of the epididymis and facilitate the acidification of the luminal fluid. ^{[2, 12, 4]}

Beneath the epithelium lies a thin lamina propria followed by smooth muscle layers. In the head and body regions, smooth muscle is arranged circularly to facilitate gentle peristalsis. In the tail region, where sperm storage occurs, the muscular layer thickens to assist in powerful contractions during ejaculation. ^{[2, 13]}

Connecting the rete testis to the head of the epididymis are 10-15 efferent ductules. These ducts are lined with alternating patches of ciliated columnar cells (to propel sperm) and nonciliated cuboidal cells (for fluid absorption). ^[13]

Two vestigial embryonic structures of no known physiological function are often found on the testis and epididymis as follows:

• The appendix testis is a pear-shaped structure that is the vesicular remnant of the cranial end of the paramesonephric (Mullerian) duct. This is the embryonic genital duct that forms half the uterus in females. It is found in approximately 92% of all testes. Its typical location is at the superior testicular pole in the groove between the testis and the head of the epididymis.
• The appendices of the epididymis are remnants of the cranial end of the mesonephric (Wolffian) duct, the embryonic duct that forms part of the ductus deferens in males. This is located in approximately 23% of testes. Its location may vary, but it usually projects from the head of the epididymis. Testicular and epididymal appendices are collectively referred to as hydatids. These structures may be sessile or pedunculated. Pedunculated appendices are more prone to torsion, which can mimic testicular torsion clinically. ^[14]

In 6-7% of males, the epididymis is found on the anterior surface of the testis.

The testis may be arrested or delayed anywhere along its course of descent into the scrotum. When it is retained in the inguinal canal, it is often complicated by a congenital hernia. ^{[7, 8, 15]}

In premature infants, the testis may not be fully descended. This process is usually complete by 9 months' gestation. Special attention should be paid to an undescended testis, and this should be closely followed up. The testis may exit the superficial inguinal ring but slip down between the scrotum and thigh and come to rest in the perineum. This is known as perineal ectopia testis.

Androgen insensitivity may lead to the testes being found in the labia majora of a chromosomal male 46 XY but phenotypical female.

Another rare anomaly is polyorchidism, characterized by more than two testes, with triorchidism being the most common form. Management depends on factors such as drainage anatomy and associated complications such as torsion or malignancy. Preservation of functional supernumerary testes is often recommended when they share a common drainage system with normal testes and exhibit no malignancy on biopsy. ^[16]

The testis begins as a retroperitoneal organ in the lumbar region. Its descent outside the abdominal cavity is imperative for normal testicular function and reproductive viability. By resting in the scrotum, the testis is kept at a temperature 2-3º C lower than the core body temperature. The countercurrent of blood flow with the venous plexus surrounding the testicular artery allows heat transfer to cool the blood flow. Further, the cremasteric reflex allows the testis to be elevated toward the body core to control testicular temperature.

Because of its intraabdominal origin, the testis have lymph drainage to the lumbar lymph nodes. Thus, infection of the epididymis or testicular carcinoma does not typically cause enlarged inguinal lymph nodes.

Cryptorchidism is important to recognize, not just because of sterility, but due to an increased incidence of testicular cancer in these males. Unless it is able to be surgically brought into the scrotum, where testicular surveillance may be performed, the undescended testis should be excised. Also, understanding the increased risk is extended to the contralateral testis as well is important. So, the individual should be instructed in careful, regular self-examinations.

Serous fluid may collect between the layers of the tunica vaginalis. This is termed hydrocele. This may be due to trauma, inflammation, or congenital due to persistent communication with the general peritoneal cavity.

Epididymitis is inflammation of the epididymis. It may be due to infectious process, commonly in men ages 19-35. In this age group, the treatment should cover gonorrhea and chlamydia. In young children and older men, E coli is the most common pathogens. Still, other causes include pathogens such as ureaplasma, Mycobacterium tuberculosis, and the drug amiodarone, which is commonly used for cardiac rate control.

Testicular torsion may occur if the testis twists on the suspending spermatic cord. This is a surgical emergency because the blood supply needs to be restored within 6 hours of initiation of symptoms. After 12 hours, the testis may be so badly damaged that it cannot be salvaged. The testis is fixed in the scrotum with suture to prevent recurrence, and the contralateral testis should be sutured in place as well. This is due to the increased incidence of testicular torsion recurrence.

Most acute presentations of scrotal pain and swelling can be attributed to epididymitis, testicular torsion, or torsion of a testicular appendage. In many cases, torsion of a testicular appendage, although a benign condition, may present identically to testicular torsion, a true urologic emergency. Ultrasound may be used for defining the problem, but clinical examination of a normal, nontender testis with the presence of a paratesticular nodule at the superior pole may point more to appendical torsion. Classically, a blue-dot appearance (blue dot sign) may be seen in the area of the injury, but this is only present in approximately 20% of cases.