Video: Testis and epididymis
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Presenting the crown jewels, of the male species? Yep, we're talking, of course, about the most prized possession of every man – the testes. When it comes to learning the anatomy of these little or... Read more
Presenting the crown jewels, of the male species? Yep, we're talking, of course, about the most prized possession of every man – the testes. When it comes to learning the anatomy of these little organs, it can almost be as painful as a hit below the belt. But don't worry, we'll be gentle. It's time to delicately learn about the anatomy of the testis and the epididymis.
So, during this tutorial, we're going to cover the structures seen in this illustration. We'll be exploring the entire structure and function of the testes including the numerous fascial layers surrounding the testis. We’ll then explore the different parts of the epididymis followed by the ductus deferens, and we'll discuss why these organs function just like a factory assembly line for the maturation of sperm. Next, we'll remind ourselves about the neurovascular supply of the testes and we'll then conclude with some clinical notes.
Right, let's get started, beginning with the structure and function of the testes.
Okay, so we all know that the testes are a pair of primary sex organs of the male and are located in the subpubic region of the pelvis along with the rest of the male reproductive components. The testis are ovoid-shaped bilateral organs roughly four centimeters long with a diameter of three centimeters in an adult male. They have two functional roles – sperm production and testosterone secretion.
The testes reside inside the scrotum – a cutaneous fibromuscular sac which hangs outside of the body and contains the testes, the epididymis, and distal parts of the left and right spermatic cords. These are contained within several fascial layers which act to protect and regulate the testes beginning most superficially with the scrotal skin, the dartos fascia, the external spermatic fascia, the cremaster muscle and fascia, and the internal spermatic fascia.
The testis itself is also invested in a number of layers, which are known as tunicae. These are the tunica vaginalis, the tunica albuginea, and the tunica vasculosa. Tunica means clothing in Latin and is often used in biology to describe the layers which immediately surround an organ.
The outermost tunica is the tunica vaginalis, which consists of two layers called the visceral and parietal layers. This layer is actually derived from the peritoneum and is carried with the testes as they descend from the abdomen. If we look closely at our illustration, we can see that although it looks like there are two separate layers, in fact, both layers are continuous with each other.
The visceral layer is innermost and is tightly adhered to most of the testes except at its hilum. The parietal layer is the outermost and is separated from the visceral layer by the cavity of the tunica vaginalis. This cavity is filled with a small amount of lubricating fluid which is secreted by serous membranes in both layers and functions to give cushioning to the testis.
The next layer, the tunica albuginea, is a thick collagen-rich layer which lies underneath the visceral layer of the tunica vaginalis. The collagen gives it a whitish color and provides strength. The tunica albuginea surrounds the testis only and does not encapsulate the epididymis.
The last layer is the tunica vasculosa, which is often overlooked as it is very tightly adhered to the tunica albuginea. Because of this, it is difficult to illustrate and it's best viewed under the microscope using a thin cross-sectional slice of a testis and a trichrome stain. The tunica vasculosa is a layer rich in vascular tissue and contains a plexus of blood vessels which extends over the internal aspect of the tunica albuginea providing nutrients to the cells of the testis.
Now, I think we can lay off these layers for a bit and turn our focus to the internal structure of the testis. Along the posterior border of the testis, the tunica albuginea and vasculosa project internally to form the mediastinum testis. This is a massive fibrous connective tissue and acts as a sort of gateway for the entrance and exit of blood vessels. The mediastinal testis also houses the rete testes, which we'll come to later.
If we look a bit closer at the mediastinum testis, can you see that there appears to be a fibrous network radiating into the testis? Well, this fibrous network is called the septa testis. Just like the mediastinum, the septa testes are derived from the tunica albuginea and vasculosa. Consequently, they are composed of collagen fibers and are embedded with vascular tissue. These septa have a really important job in defining and bringing nutrients to the testicular lobules. In reality, there are roughly two hundred fifty to three hundred of these lobules in each testis, which those in the center being generally larger than those at the superior and inferior extremities.
But what is the point of these lobules and what are these tubules contained within them? Each lobule is typically composed of three convoluted seminiferous tubules. There are also straight seminiferous tubules, but these appear further on. The convoluted seminiferous tubules have a job to provide a comfortable environment for spermatogenesis, which is just a fancy word for the differentiation of a germ cell into a spermatozoa.
There are two male specific cells to help with this job – Sertoli or nurse cells which make up the majority of the tubule epithelium and interstitial or Leydig cells which reside in the interstitium. And we can have a quick look at these cells if we zoom into the histological slide.
Sertoli cells or nurse germ cells develop through the spermatogenic process and Leydig cells synthesize and secrete a number of androgens including testosterone which is important in regulating spermatogenesis. We can see both these cell types in this histological slide, but if you want more information on them, please check out our content on the histology of the testis.
Now just imagine for a second that you're a spermatozoa. You'll start your journey in these convoluted seminiferous tubules and have approximately seven meters to travel between this point and the urinary meatus. If we scale a sperm up to human size that's the equivalent of traveling approximately two hundred and sixty kilometers. In travelling such a distance, energy conservation is essential and whilst we might jump on a bus, such luxuries are unavailable to a sperm. Therefore, other means of energy conservation must be available.
Consequently, sperm are only motile after they have passed through the epididymis. Until they are motile, sperm rely on fluid flow caused by peristaltic contractions from smooth muscle in the convoluted tubes for transportation. Fluid flow first transports sperm to the straight seminiferous tubules. No spermatogenesis occurs here and these tubules serve as an intermediary between the convoluted seminiferous tubules and the rete testes. Like we saw previously, the straight seminiferous tubules are surrounded by smooth muscle which aids fluid flow.
The rete testes are located in the mediastinum testes and consist of a network of anastomosing ducts, which are not surrounded by smooth muscle like the seminiferous tubules. The rete testis function to condense the numerous seminiferous tubules into approximately fifteen efferent ductules. These efferent ductules perforate the superior portion of the tunica albuginea and leave the testis. The efferent ductules help ensure absorption of water from fluid produced by the testis. The ductules are once again surrounded by a thin circular coat of smooth muscle, which contracts in a peristaltic manner to maintain fluid flow. These ductules are initially straight, but soon become enlarged and highly convoluted forming the head of the epididymis.
This leads us out of the testis and into section two of this video – the structure and function of the epididymis.
The epididymis is a curved or arc-shaped tubular structure which sits across much of the posterior aspect of the testis. It is divided into three main regions – the head, the body, and tail. As we discussed earlier, the head region of the epididymis is encapsulated by the parietal layer of the tunica vaginalis. The rest of the epididymis is not covered by a tunic.
The head region is attached to the upper end of the testis by the efferent ductules and consists of approximately fifteen ducts. These ducts merge to form the single duct of the epididymal body which is tightly coiled and held together by bands of fibrous connective tissue. You'll notice in this diagram that some efferent ductules flow directly into the body, and while this is true, the vast majority form the head.
The single duct of the epididymal body forms the tail at the inferior portion of the epididymis. The epididymal tail then continues as the ductus deferens. The epididymis is not just a tube for sperm to pass through. It also plays an essential role in the maturation of sperm. You see, freshly differentiated sperm require further development to become motile and fertile. Consequently, the epididymis acts as a sort of biological assembly line whereby immature sperm enter via the efferent ductules and mature sperm ready for ejaculation leave via the ductus deferens. The maturation process compacts the DNA within the head of the sperm and alters the plasma membrane of the head region causing the head to become smaller and more streamlined.
The epididymis, if unraveled, would measure more than six meters in length. Consequently, a sperm typically takes twelve days to pass through which is sufficient time for maturation. As the sperm is still immotile, the epididymal tubes are surrounded by smooth muscle which contracts in a peristaltic motion pushing the sperm towards the ductus deferens.
The ductus deferens is the continuation of the epididymal tail and it is roughly about forty-five centimeters long and acts to take sperm out of the scrotum and into the ejaculatory ducts. As this illustration shows, the ductus deferens starts off somewhat convoluted, but then straightens as it ascends within the spermatic cord behind the testis. It exits the scrotum through the inguinal canal and then leaves the spermatic cord at the deep inguinal ring.
Before we finish up, let's quickly refresh our memories of the neurovascular supply of the testes and epididymis.
The testes are supplied by the testicular arteries, which arise directly from the abdominal aorta just below the level of the renal arteries reminding us of the origins of the testes in the abdominal cavity. They are drained by a plexus of veins known as the pampiniform plexus which empty into the testicular veins. The right testicular vein drains into the inferior vena cava while the left testicular vein empties into the left renal vein. Lymphatic drainage from the testes run alongside the testicular arteries and drain into the lumbar lymph nodes. And, finally, the testes received their autonomic sympathetic innervation via the spermatic plexus which originates from the paraaortic ganglia.
And that concludes our whirlwind trip across the testes and the epididymis.
Let's wrap up our tutorial with some clinical notes.
Okay, so testicular torsion is a spontaneous condition occurring in males most commonly just after being born and during puberty. It occurs in about one to fifteen thousand males under the age of twenty-five each year and presents with sudden severe testicular pain. The testis itself may be swollen and displaced higher in the scrotum due to the subsequent shortening of the spermatic cord. It is usually diagnosed using ultrasound. Treatment involves physically untwisting the testis through the scrotum, or in severe cases, surgery whereby the scrotum is excised to gain immediate contact with the twisted cord.
Quick diagnosis and management are critical to protect the testes from infarction or permanent damage, and if it's corrected within six hours, the outlook is pretty promising with no lasting conditions. However, any longer than six hours can result in infertility and eventual tissue necrosis.
Wow, what a journey! Let's summarize what we covered today.
So, in this tutorial we covered the journey that a sperm takes from the seminiferous tubules to the ductus deferens and all the structures that have been put in place to make it possible. We started with the layers of the testis which were the tunica vaginalis, tunica albuginea, and the tunica vasculosa. We then had a look at the internal structure of the testis and how it is separated into lobes by the testicular septa which are continuations of the tunica albuginea. These lobes are filled with highly convoluted seminiferous tubules which is where the sperm differentiate – thanks to the help of Sertoli cells and Leydig cells. And from here, the sperm travel through the straight seminiferous tubules to the rete testes in the mediastinum testes.
The rete testes function to condense multiple tubules into roughly fifteen efferent ductules which transports sperm out of the testicle and into the epididymal head. In the epididymis, as the sperm passes through the head and body regions, the sperm mature and are modified to become motile. The sperm are then stored in the tail region ready to be pushed into the ductus deferens during ejaculation.
Finally, we looked at the neurovascular supply of the testes. We saw arterial supply comes from the testicular arteries with venous drainage occurring via the pampiniform plexus and the testicular veins. We then saw lymphatic drainage ending up at the lumbar lymph nodes, and finally, sympathetic innervation coming via the spermatic plexus.
And that brings us to the end of the video.
I hope you enjoyed this tutorial. Thanks again for watching. And if you're a real nerd, why not take this to the next level and check out our content on the histology of the testes.