The testes (testicles) are anatomically designed to hang outside the body in the scrotum. This location serves an important purpose, temperature regulation for optimal sperm production ( spermatogenesis) but also makes them highly vulnerable to external forces that can jeopardise fertility, hormonal function, and the psychological well-being of the injured male and his partner.

Why are the testes vulnerable to trauma? This has to do with the delicate structures that make up the testes. This article will discuss the anatomy (structure) of the testes and how these delicate structures, responsible for the production of sperm and testosterone, are susceptible to injury (vulnerable to trauma). 

Anatomy of the testes

The testis(single testicle) is a crucial organ essential for the survival of the human species. The testicles (testes) are a paired, egg-shaped, hormone-producing organ located slantwise inside the lower portion of the scrotum (i.e. within the tunica vaginalis). They are suspended in the scrotum by the spermatic cord. The testes are 5 cm long, 2.5 cm wide, and measure 3 cm from front to back. It weighs around 10g to 14g; 10g is equivalent to two sheets of standard A4 paper. In most males, the left testicle is lower than the right testicle in the scrotal sac, and usually bigger than the right. A midline raphe separates the left and right testis. During puberty, the testes quickly increase in size but gradually reduce in old age. The testes are the main reproductive hormone-secreting glands in males, testosterone. Testosterone plays a key role in sexual function, muscle health, bone density, and the proper functioning of the prostate and central nervous system. Under the influence of the hypothalamus and pituitary gland, the testes produce testosterone (the main hormone) and spermatozoa. In men, the testes are situated outside the body because for healthy sperm to be produced, the temperature must be lower than your body temperature.^1,2,3

Blood supply to the testis

The testes receive their blood supply majorly from the testicular arteries (split into two branches, the central and side branches) and secondarily from the long artery of the vas, also known as the artery to the ductus deferens. This artery provides additional blood to the epididymis and vas deferens. The long artery of the vas joins with the testicular arteries or one of its divisions from the testicular artery. These connections help maintain blood supply to the testis if the testicular artery is cut. ²

Blood drainage system of the testes

Deoxygenated blood from the testes and epididymis are collected primarily through the pampiniform venous network (a network of veins leaving the testis). Each pampiniform network of veins joins together and drains into the testicular veins. The testicular vein is made up of the right testicular vein and the left testicular vein. The right testicular vein returns deoxygenated blood from the testes to the heart through the largest vein called the inferior vena cava; in contrast, the left vein into the vein of the kidney before returning to the heart via the inferior vena cava. Additionally, a smaller alternate vein called the cremasteric vein collects deoxygenated blood from the testes. It drains into the deep epigastric vein (located in the upper middle part of the abdomen). The deoxygenated blood will ultimately return to the heart via the inferior vena cava.²

Lymph from the testes is drained into pre-aortic lymph nodes. ² 

How is the testes' temperature regulated?

The pampiniform plexus ensures that the temperature of the testes is 2-6 degrees below your normal body temperature by lying next to the testicular artery. This setup likely creates a heat-exchange system, where the cooler blood leaving the testis helps cool the warmer arterial blood flowing into it. The cremaster muscle regulates the testes’ temperature by pulling the testes closer to the body for warmth when it's cold. It relaxes and lets the testes drop away from the body to cool when it is warm. Also, the dartos is a muscle in the skin of the scrotum that regulates the testes’ temperature by tightening the scrotum to reduce heat loss when it's cold and relaxing the scrotum to cool down the testes when it's warm.⁴

Nerves that control and provide sensory and motor function to the testes are the testicular network of nerves and the inferior and superior spermatic nerves. These nerves are autonomic, which means they carry signals from the brain and spinal cord to the testicles to control involuntary functions such as blood flow, ejaculation, hormone secretion, and sperm movement in the ducts.^2,5 

This testicular network of the nerve is made up of:

• Sympathetic fibres: This nerve comes from the 10th and 11th thoracic spinal sections and the lumbar (L1-L2) part of the spinal cord
• Parasympathetic fibres: These fibres come from the vagus nerve
• Sensory fibres: These fibres carry pain and other sensory signals from the testicles to the spinal cord and brain (visceral afferent fibres)^2,5 

Layers of the testes 

The outer part of each testis is covered by three layers of membranous tissues namely tunica vaginalis, followed by the tunica albuginea and the tunica vasculosa.^2,5,6

The tunica vaginalis is a sealed peritoneal sac (derived from membranes that cover the organs in the abdomen) consisting of two layers, which are the visceral and parietal layers.

The visceral layer is the inner layer of the tunica vaginalis covering the surface of the testis (except the area where the testis joins the epididymis and spermatic cord), the epididymis, and the lower portion of the ductus deferens.

The parietal layer is the outer layer of the tunica vaginalis and lies next to the internal spermatic fascia. The parietal layer lines the inner wall of the scrotum. It covers not just the testis and epididymis but also extends to cover the end part of the spermatic cord, making it a more vast layer compared to the visceral layer. This layer continues with internal spermatic fascia, ensuring that the protective covering around the testis and spermatic cord is uninterrupted.

Between the visceral and parietal layers is a small space known as the cavity of the tunica vaginalis, which contains a small amount of fluid that allows the testis to move within the scrotum.

The tunica albuginea is a fibrous connective tissue found between the tunica vaginalis. It allows the passage of blood vessels and nerves into the testis primarily through the epididymal head and tail and behind the testis. The tunica albuginea continues inward and forms a structure called the mediastinum testis, which is an incomplete fibrous wall (septum) that divides the testis into many hundreds of testicular cone-shaped lobules. Each lobule contains 1-4 seminiferous tubules. 

The tunica vasculosa is made up of a network of blood vessels and soft connective tissue that surrounds the seminiferous tubules and supply the tissues of the testes.

Tissues and cells that make up the testis

The testes are made up of the following tissues and cells:^3,5,7,8

Seminiferous tubules

The seminiferous tubules are long, twisted tubules that produce sperm. They are the main functional unit of the testes. They are made up of seminiferous epithelium lined by germ and Sertoli cells. Germ cells produce sperm cells while Sertoli cells are big columnar cells that assist: 

• In the production of sperm cells
• Secrete molecules that control the function of Leydig cells and peritubular cells
• Secretes inhibin, which controls hormone production, and secrete seminiferous tubular fluid
• Swallow up and destroy the cytoplasm of the spermatid that has been removed 
• Form the blood-testis barrier, which protects developing sperm from immune attacks

The interstitium

This is a layer of connective tissue found outside the seminiferous tubules that helps maintain the shape and integrity of the testis. It consists of Leydig cells, lymphocytes, and macrophages. Leydig cells make and secrete male sex hormones, mainly testosterone, under the direction of luteinising hormones and other substances not derived from steroid compounds.

Peritubular myoid cells 

These are muscle-like cells that surround the seminiferous tubules. They contract irregularly, helping move the fluid and sperm through the tubules and into the rete testis, a network of ducts in the testes. They also produce several growth factors, like activin A and platelet-derived growth factor, which may affect the function of other cells in the testes.

Peritubular myoid cells have long been recognised for their impact on Sertoli cell function, protein expression, and normal peritubular myoid cell development and function depending on the presence of Sertoli cells. 

Rete testis

Rete testes are made up of a single layer of cube-shaped cells with small hairlike projections on the surface of the cells called cilia and a layer of smooth muscle. Together, these enable the rete testis to transport sperm to the ductuli efferentes and the epididymis. The seminiferous tubules drain into the rete testis, a network of channels within the mediastinum testis.

Mediastinum testis

This is a thickened layer of connective tissue located behind the testes that provide structural support and contains blood vessels, lymphatics, and the rete testis. 

Ductules efferents

Sperm drains from the rete testes into the epididymis via the ductuli efferentes. The ductules are lined by a single layer of epithelial cells that are either tall, column-shaped, and ciliated or are short and lacking cilia. The cells with cilia push the immobile sperm to the epididymis, while the cells that lack cilia re-ingest the fluid made by the testes. In addition, each ductulus is encircled by a narrow band of smooth muscle arranged like a ring. This enables the sperm to move towards the epididymis.

Diagram showing the labelled structures and layers of the testes¹² https://commons.wikimedia.org/wiki/File:Figure_28_01_03.JPG .

Testes vulnerability to trauma 

Testes are very vulnerable to trauma because they are placed outside the body in a thin scrotal sac, and as such, they may be prone to injuries or assaults that may be intentional or unintentional. Testicular trauma can result in rupture, twisting, and blockage of the delicate structures of the testes. ^{9, 10, 11}

Types of Injuries

Direct injuries occur when force or trauma directly impacts the testicles. Direct Injuries to the testes may be blunt or penetrating. Blunt injuries involve force applied to the testicles, such as from a kick, fall, or accident from sports or motorcycle injuries, without breaking the skin. On the other hand, penetrating injuries occur when an object punctures the skin and directly damages the testicles, like with a stab wound or a gunshot. Blunt injuries happen twice as often as penetrating injuries. In blunt injuries, usually just one testicle is hurt, while in about a third of penetrating injuries, both testicles are damaged.⁹

Another reason the testes are prone to trauma is that the testes have a rich supply of blood vessels. Testicular trauma can occur during groin surgery (inguinal hernia repair or varicocele surgery) when the blood supply to the testes is disrupted, which may lead to testicular atrophy which is the shrinkage or reduction in the size of the testicles. This condition may lead to decreased sperm production and lower testosterone levels.⁹

Also, the pathways that carry sperm from the testicles (epididymis, vas deferens) can become blocked, either due to injury or complications from surgery. This is especially concerning in surgeries done in infancy or childhood, like inguinal hernia repairs or surgeries for undescended testicles. If these pathways are blocked, it can affect sperm flow and fertility later in life.¹⁰

Hence, testicular trauma should be treated promptly and carefully because delayed or prolonged management increases the risk of infection, testicular atrophy, blockage and tissue death (necrosis).^9,10,11

Summary

The testes are a paired egg-shaped hormone-secreting organ located in the scrotum outside the body. The testes are made up of delicate structures. The main function of the testes is to produce sperm and testosterone. The testes are vulnerable to trauma due to their location and rich supply of blood vessels and as a result, their function can be jeopardised. Testicular trauma can lead to infertility, and hormonal dysfunction, which in turn can affect the psychological well-being of the injured individual and his partner. Testicular trauma should be treated promptly to avoid damage to the testes.