Testes

Development

As they descend, the testes also push layers of the anterior abdominal wall (internal and external oblique muscles and the transversalis fascia) and extend them into the scrotum. These extensions of the anterior abdominal wall create a musculofascial pouch that protects the testes within the scrotum.

The descent of the testes outside the body cavity is necessary because effective spermatogenesis requires temperatures that are 2-3°C lower than body temperature. If the testes fail to descend, the production of testosterone persists, but spermatozoa cannot be produced.

Scrotum and coverings of the testes

Scrotum is a cutaneous (skin) sac that protects the testes. It consists of two layers: most superficially is the skin, and deeper is the dartos fascia. The dartos fascia contains muscle fibers that contract when it is cold, which results in wrinkling of the scrotal skin and brings the testes closer to the body. The result is a reduction of heat loss when the outside temperature is too low.

Inside the scrotum are the coverings of the testes. They are continuous with the anterior abdominal wall, and going from superficial to deep, these layers are:

• External spermatic fascia (from the external oblique muscle)
• Cremaster muscle (from the internal oblique muscle)
• Internal spermatic fascia (from the transversalis fascia)

The tunica vaginalis is the peritoneal sac that partially encloses the testes. It is derived from the embryonic vaginal process. This process is the outpouching of the parietal peritoneum, which follows the testes during descent and then encloses them. It has parietal and visceral layers. The visceral (internal) layer covers the testis, the head of epididymis, and the inferior part of ductus deferens. The parietal (external) layer is larger and superiorly covers the distal part of the spermatic cord, then continues over the visceral layer of tunica vaginalis, and covers the duct of epididymis before blending with the visceral layer. Between the layers is a small amount of serous fluid that prevents friction and allows the testis to move in the scrotum.

Anatomy

On the testis, we can observe two sides (medial and lateral) that are separated by two edges (anterior and posterior). We can also observe superior and inferior poles since it is an ovoid organ. On the posterior edge and superior pole of the testis is a structure called the epididymis. Also on the inferior pole is the scrotal ligament (a remnant of the gubernaculum testis) and serves to fix the testis to the bottom of the scrotum.

The most superficial layer of the testis is a capsule made of dense fibrous connective tissue called the tunica albuginea, which perpendicularly gives rise to the septa that divide the tissue of the testis into lobules. The tunica albuginea thickens along the posterior surface of the testis and projects into it as the mediastinum testis, a connective tissue compartment through which all vessels and ducts of the testis pass on they way in or out. Fibrous septa extend from the mediastinum testis, creating the borders of the lobules which contain the seminiferous tubules.

Each of the 200-300 lobules of the testis are filled with one to four highly convoluted seminiferous tubules which each course towards the mediastinum testis. Before entering the mediastinum, they change to a straight course, so in this segment, each convoluted tubule becomes a straight seminiferous tubule. Straight tubules enter the mediastinum, and by interconnecting they form a collecting chamber called rete testis.

Histology

The tubules are surrounded by the connective tissue stromal cells which contain testosterone secreting Leydig (interstitial) cells. The tubules are lined with a layer of seminiferous epithelium, which contains supporting Sertoli (sustentacular) cells, and spermatogenic cells. The spermatogenic cells constantly multiply and through several phases of spermatogenesis differentiate into mature sperm, while the Sertoli cells nourish them.

Sertoli cells are tightly connected with one another and are placed between a layer of spermatogonia (most immature spermatogenic cells) and more mature forms of the spermatogenic cells. In this way, Sertoli cells form a barrier between the immature and mature spermatogenic cells called the blood-testis barrier. The name of the barrier itself suggests that even arterial blood cannot reach more mature of spermatogenic cells. Because of this, the immune system never actually gets to meet those cells and recognize them as domestic, which is why the integrity of the blood-testis barrier is so important. If it gets broken, the immune system will start a response towards the unfamiliar spermatogenic cells and destroys them, sometimes causing sterility. So, if blood doesn’t reach the mature spermatogenic cells, how do they survive? They do so, again, with the help of Sertoli cells which transport nutrients from their basal surface through the apical to the higher layers.

Duct system

From its superior end, the rete testis gives rise to 12-20 efferent ductules that penetrate the capsule and connect with the epididymis where sperm cells become mobile and gain the ability to fertilize an egg. The epididymis is a large structure that extends along the entire posterior margin of the testes from the superior pole to the inferior pole. It has two components:

• The head of the epididymis is where the efferent ductules merge.
• The true epididymis is a coiled tubular continuation of the head. It extends inferiorly as the body of the epididymis and at the inferior pole of the testis enlarges and forms the tail of the epididymis.

As spermatozoa travel through the epididymis, they mature to be able to move and fertilize eggs. Mature sperm cells are stored within the epididymis until ejaculation, upon which they empty into the ductus deferens.

The ductus deferens, (also called the vas deferens) is a continuation of the epididymal duct and travels through the spermatic cord on its way to the pelvis. Its function is to transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. Anterior to the rectum and along the base of the bladder, it is joined by the duct of the seminal vesicle with which it creates the ejaculatory duct. This duct penetrates through the prostate to connect with the prostatic urethra into which it empties.

A quick recap: the duct system of the testes begins with the convoluted seminiferous tubules filled with spermatogenic cells. These cells mature through the initial phases of spermatogenesis and then move into the straight seminiferous tubules which eventually empty into the rete testis found within the mediastinum testis. There are 12-20 efferent ductules that arise from the rete testis and connect with the epididymis where the final phases of spermatogenesis occur. The ductus deferens drains sperm from the epididymis. Sperm then passes into the ejaculatory duct, which transports it to the urethra from which it is ejaculated from the body.

If you want to learn more about the male reproductive system and what happens when the epididymis becomes inflamed, take a look below!

Arterial supply

The testes are supplied by a pair of the testicular arteries (branches of the abdominal aorta inferior to the renal arteries) which descend to scrotum through inguinal canal. On the other hand, the scrotum is supplied by the internal pudendal artery (branch of the internal iliac artery) just like the rest of the external genitalia.

Besides the testicular arteries, the testes also have a collateral blood supply formed by the cremasteric artery (branch of the inferior epigastric artery) and the artery to ductus deferens (branch of the inferior vesical artery). This is essential in cases of obstruction of the testicular arteries, because this collateral flow will enable the testes to survive.

Venous drainage

Deoxygenated blood from the testes drains into the small spermatic veins that intercommunicate and form a venous network called the pampiniform plexus. This plexus surrounds the branches of the testicular artery, which is very important for temperature regulation. The veins of the plexus cool the arteries that carry warm arterial blood before it enters the testis, thus acting as a countercurrent heat exchanger.

The veins of the plexus ascend through the inguinal canal, anterior to the ductus deferens, and just below the superficial inguinal ring they merge into three or four veins that enter the abdomen. These veins unite to form a single testicular vein at each side. The right testicular vein drains into the inferior vena cava at an acute angle. The left testicular vein on the other hand drains into the left renal vein at a right angle. These angles are important because the obstruction of either of these veins can occur (especially to the right one), which then leads to a stoppage or slowdown in blood flow of the blood within the testes.

Lymphatic drainage 

The testicles are drained by the pre-aortic and lateral aortic lymph nodes–something which persists from their retroperitoneal development. The scrotum is drained by the inguinal lymph nodes.

Innervation

The ductus deferens, seminal glands, ejaculatory duct, and prostate are supplied by sympathetic branches of the lumbar splanchnic nerves and the superior and inferior hypogastric plexuses. Parasympathetic supply comes from the pelvic splanchnic nerves (S2-S3).