35 Tusshyenthan Seevagan1, Stephen Hulligan1, Jaspal Phull2, and Omar M. Aboumarzouk3, 4 1 Department of Urology, University Hospital of Wales, Cardiff, UK 2 Department of Urology, Royal United Hospital, Bath, UK 3 Glasgow Urological Research Unit, Department of Urology, Queen Elizabeth University Hospital, Glasgow, UK 4 University of Glasgow, School of Medicine, Dentistry & Nursing, Glasgow, UK The testes are the primary reproductive endocrine organs in males. Anatomically they are suspended outside the body in the scrotum. They weigh between 10 and 14 g and are usually approximately 5 cm in length, 2.5 cm in breadth, and an antero‐posterior diameter of 3 cm. In the majority the left testicle lies more distally compared to the right. They lie obliquely in the scrotum with an anterolateral tilt of the upper pole and a corresponding posterior‐medial tilt of the lower pole. They are contained within the tunica vaginalis, the most distal remnant of the processus vaginalis, which separates them from the other components contained within the scrotum. Through hypothalamic or pituitary mediation, they produce testosterone and spermatozoa. Keywords testicular anatomy; testicular embryology; testicular physiology; spermatogenesis; testosterone In many mammals, the testes enlarge and shrink with the seasonal rise and fall in testosterone. In man, there is a rapid increase in testicular size at puberty and a slower decline in old age. The location of the testes varies widely in different mammals. In elephants, the testes are located in the abdomen and are just caudal to the kidneys; in whales they are near the bladder; the internal ring in hedgehogs; the superficial inguinal pouch in pigs. In sheep and man, they hang at the bottom of a pendulous scrotum (Figure 35.1). Figure 35.1 The testicle in varying stages of descent in different mammals: in elephants they are near the kidney; in whales, near the bladder; in hedgehogs at the internal ring; in pigs in the superficial inguinal pouch; and in sheep and man at the bottom of a pendulous scrotum. Most mammalian testes reside intra‐abdominally for the majority of the year, descending into the scrotum only during mating season. Although the testicles lie in the scrotum in the majority of primates, the testicles are able to move freely in and out of the scrotum in some species [1, 2]. The epididymis is only found in mammals. It was once thought to be involved in the fertilisation of the ovum at the time of ovulation; however, immunochemical studies have been unable to establish a difference between sperm found in the ejaculation and sperm in the vasa efferentia [3, 4]. The normal adult testis hangs in the scrotum, the left usually hanging lower than the right. The left testis is often larger than the right. The epididymis is located behind the testis. The testis consists of a multitude of convoluted tubules which empty into the rete testis at the hilum, where about 12 vasa efferentia cross into the beginning the epididymis (Figure 35.2) [1]. Figure 35.2 Diagram of the anatomy of the testicle (a) lateral and (b) cross section. Each testicular tubule has a basement membrane, lined by spermatogonia and Sertoli cells. The outside of the tubules consists of connective tissue spaces with blood, lymphatic vessels, and Leydig cells. In the foetal testis, the tubules contain germ cells and immature Sertoli cells between the tubules the Leydig cells become particularly abundant just before birth, but disappear afterwards to be replaced by fibroblasts. At puberty luteinizing hormone secreted by the pituitary gland stimulates the Leydig cells to reappear, the Sertoli cells to become mature, and the dormant germinal cells to spring into activity. The spermatogonia divide by mitosis; some progeny will become spermatocytes, and others continue as stem cells. Each spermatocyte divides by meiosis to produce haploid secondary spermatocytes, which eventually become spermatozoa. The changes pass along the testicular tubules in waves so that a biopsy of the adult testis will show several different phases in the sequence of division and maturation from spermatogonia to mature sperms (Figure 35.3). A testicular biopsy must therefore include several tubules to give a useful picture of spermatogenesis [5]. Figure 35.3 Spermatogenesis. The testes are surrounded by three layers of tissues, the most superficial layer being the tunica vaginalis, then the tunica albuginea, and the deeper tunica vasculosa. The median raphe separates the right from the left. The tunica vaginalis is the distal continuation of the processus vaginalis which is the evagination of the abdominal peritoneum into the scrotum. In the foetus it is formed prior to the descent of the abdominal testes into the scrotum. Once the descent is complete, the proximal part of the tunica vaginalis is obliterated to form the sac in which the testes reside. It contains two layers, the visceral and parietal layer which are formed as a result of the reflection of the tunica from the testis onto the internal surface of the scrotum. Most of the testes is encompassed by the visceral layer except for the posterior part. When it reaches the posteriomedial aspect of the testes, it is reflected anteriorly to form the parietal layer, and the posterolateral aspect passes the epididymis and reflects to form the parietal layer on the lateral side. The parietal layer also continues below the testes and connects to the anteromedial aspect of the spermatic cord. The tunica albuginea, which is located within the tunica vaginalis, is a fibrous connective tissue. It facilitates the entry of the blood vessels and nerves at the epididymal head and tail and the posterior part of the testes, where it is not covered by the visceral layer of the tunica vaginalis. The tunica albuginea continues into the interior of the testis to form the mediastinum testis, an incomplete fibrous septum containing the testicular vessels. It also contains within it the tunica vasculosa, which contains blood vessels and soft connective tissue. The testes receive their arterial supply from the testicular (also called spermatic of gonadal) arteries. They originate from the anterior of the aorta just below the renal arteries. Both arteries cross anterior to the genitofemoral nerve and the ureter before taking its path along the deep inguinal ring and enters into the spermatic cord which runs through the inguinal canal to reach the scrotum. The testicular artery divides into two branches, the medial and lateral branches, first inserting into the tunica albuginea and then the tunica vasculosa. An additional minor arterial supply is provided by the long artery of the vas which anastomoses with a branch from the testicular artery to the epididymis (Figure 35.4). Figure 35.4 Blood supply to the testis. The testicle has a profuse venous drainage arranged in three layers between the external spermatic fascia, the cremasteric muscle, and the internal spermatic fascia. They arise from the posterior aspect of the testes and merge to form the pampiniform plexus. The plexus ascends through the spermatic cord anterior to the vas deferens. The different tributaries then coalesce in the abdomen to form two distinct veins which ascend either side of the testicular artery. The two veins then conjoin to form the testicular vein which inserts directly into the inferior vena cava on the right and the renal vein on the left (Figure 35.5). A minor collateral drainage exists through the cremasteric vein into the deep epigastric vein. Figure 35.5 The veins of the testicle and the coverings of the cord. The lymphatics of the testes arise in the spaces between the tubules and flow through the testicular hilum into the cord. They retrace the testicular artery to the para‐aortic lymph nodes around the origin of the testicular and renal arteries. The testicular nerves originate from the tenth and eleventh thoracic spinal segments via the autonomic plexuses. The testis has visceral afferent and efferent fibres and lacks somatic nerves [1]. The superior spermatic nerve arises from the spermatic ganglion, which receives fibres from the coeliac and intermesenteric plexuses, as well as the lumbar and thoracic sphlanchnic nerves and the vagus. From the spermatic ganglion the nerves run with the internal spermatic artery as a discrete nerve which accompanies the artery to the testis. The tunica albuginea has an abundant sensory nerve supply which is quite distinct from the innervation of the scrotum. The role of the autonomic motor nerves to the testicle is unknown [1]. The epididymis is a structure located in the posterior aspect of the testis and lateral to the vas deferens. It consists of a single convoluted ductus epididymis which arises from the joining of the efferent ducts of the testis. The tube of the epididymis is said to be 3–4 m in length when unravelled [3, 6, 7]. It is arranged in three parts the expanded head (globus major), body (corpus), and tail (cauda or globus minor). In most mammalian species as well as man, it has the shape of a dumb‐bell with a distinct waist (Figure 35.6). Between the epididymis and testis is a sulcus which forms a pocket facing laterally, which is a useful guide for the surgeon when replacing the testicle after scrotal operations. The epididymis is lined by columnar ciliated epithelium. The cilia of the epididymis resemble the cilia of the bronchioles, with which they share the structure of a dynein arm as well as a susceptibility to poisoning with mercury salts [7–10]. The lumen of the epididymis becomes progressively wider as it goes from head to tail, where muscle begins to surround the tubule which continues on as the vas deferens [11]. Figure 35.6 Anatomy of the epididymis. The blood supply to the epididymis comes from a branch of the testicular artery which enters the caput epididymis, runs down the epididymis, and anastomoses with the terminal branch of the artery of the vas, which runs alongside the vas inside its connective tissue sheath. The vas deferens is a 45 cm long firm cord with a small lumen and a thick wall of smooth muscle, which is convoluted at each end (Figure 35.7). It is the distal continuation of the epididymis, and its main function is to transport sperm to the ejaculatory ducts. Its tall columnar epithelium is lined with ‘stereocilia’ which are not motile and resemble structures in the ependymal of the canal of the spinal cord, and the tympanic cavity [12]. Figure 35.7 Surgical anatomy of the vas deferens. The vas starts lateral to the epididymis and ascends along the posterior aspect of the testis and continues into the posterior part of the spermatic cord. It leaves the spermatic cord at the deep inguinal ring, curves around lateral to the inferior epigastric artery and follows the inside of the pelvis, crossing the external iliac vessels and the ureter, where it follows the cleft between the inner and outer zones of the prostate (Figure 35.6). Just before it enters the prostate, the vas gives off a diverticulum, the seminal vesicle. The artery of the vas is a branch of the superior vesical artery which arises from the internal iliac artery. Its venous drainage leads to the pelvic venous plexus along with the venous drainage of the seminal vesicles. The embryology of the seminal vesicle has been meticulously studies [13]. Starting as a modest pouch of the vas deferens, it enlarges in puberty and in the adult each may hold up to 2–10 ml of fluid in a convoluted hollow sac with a strong muscular coat and a columnar epithelium with folds like a honeycomb (Figure 35.8) [14]. The common ejaculatory ducts emerge in the prostatic urethra either side of the verumontanum. Interesting to note that its secretion were tasted and found to be sweet by John Hunter, due to the rich fructose content. Figure 35.8 The seminal vesicle develops as a diverticulum of the ejaculatory duct. The verumontanum contains spongy tissue resembling that of the corpora of the penis. Its summit contains the utriculus masculinis, the vestige of the lower ends of the müllerian ducts, on either side of which are the ejaculatory ducts (Figure 35.9). Figure 35.9 Structure of the verumontanum. Note the presence of erectile tissue. During early life the intra‐abdominal testes travel towards the scrotum carrying its blood supply, nerve supply, and the vas deferens along with it. The spermatic cord is formed when these reach the deep inguinal ring. It contains four layers, each representing a component of the abdominal wall (Figure 35.5). The core of the spermatic cord contains the vas deferens, the testicular artery, and the venous drainage. This is surrounded by peritoneum in infants, but in adults, the peritoneal layer has shrivelled to a thin strip into the processus vaginalis. The next layer of peritoneum is a thin layer derived from transversalis fascia, the internal spermatic fascia. This is surrounded by muscle fibres continuous with the internal oblique muscle of the abdominal wall, the cremasteric muscle, which covers the entire testicle. Overlying the cremaster, the external spermatic fascia is a continuation of the external oblique aponeurosis. The Latin ‘testiculus’ means ‘witness’ of virility and enlightens us to the primary function of these gonadal endocrine organs. The main function of the testes is spermatogenesis and androgen synthesis. Figure 35.10 Hypothalamic‐pituitary‐gonadal axis. FSH, follicle‐stimulating hormone; GnRH, gonadotropin‐releasing hormone; LH, luteinising hormone. The hypothalamus produces gonadotrophin‐releasing hormone (GnRH) which stimulate the anterior pituitary to release gonadotrophins: Luteinizing hormone (LH) and follicle‐stimulating hormone (FSH). GnRH is discharged in a pulsatile manner every 90–120 minutes, continuous GnRH release causes inhibition of LH and FSH release. LH controls Leydig cell function and FSH controls Sertoli cells. The secretion of GnRH is in a pulsatile manner, which dictates the corresponding pulsatile secretion of LH and FSH. LH at approximately 8–14 pulses per 24 hours, and FSH at a lower amplitude [15]. The secretion of LH and FSH is dependent on this pulsatile stimulation of the gonadotroph cells by GnRH, continuous administration or intermittent administration of GnRH equivalents suppresses the release of LH and FSH. This is due to GnRH receptors requiring replenishment once stimulated, when stimulated in a pulsatile manner replenishment is optimum; however, when stimulates in a nonpulsatile manner, the replenishment of GnRH receptors is inhibited so that insufficient receptors are available for function. This mechanism is used in the clinical treatment of prostate cancer, whereby GnRH analogues are administered intermittently in a nonpulsatile manner and therefore lower LH and FSH levels, reducing stimulation to testicular cells so the production of prostate‐stimulating androgens is reduced [15]. Leydig cells are located in the supportive connective tissue; their primary function is to secrete androgenic hormones, which are essential to the development of masculine sex characteristics and sperm production. LH stimulate G‐coupled receptors on Leydig cells membranes, and via cyclic adenosine monophosphate (cAMP) and kinase pathways activate gene transcription that increases enzymes necessary for the steroid synthesis of testosterone from cholesterol [16]. Sertoli cells are located in the testicular seminiferous tubules, these tubules make up 80% of the testis. FSH binds to receptors on Sertoli cells, leading to increase in protein synthesis. Several proteins are synthesised; In addition, during foetal life, Sertoli cells release the mϋllerian duct inhibitory factor (MIS) in the seventh week. Cholesterol is the essential precursor for androgen synthesis by Leydig cells, which use a series of 5 enzymes to produce testosterone, 3 of which are P‐450 enzymes. Multiple pathways exist for the formation of testosterone beginning in the mitochondria where the long side chain of cholesterol is removed by cytochrome P‐450 enzyme, this produces prognenolone, then at the smooth endoplastic reticulum (SER), 17a‐hydroxylase adds a hydroxyl group to from 17a‐hydroxy‐pregnenlone. A further P‐450 enzyme removes another side chain resulting in the steroid called dehydroepiandrosterone. A non‐P‐450 enzyme in the Leydig cells forms androstendiol, the hydroxyl group is finally oxidised to a ketone to form testosterone [17]. Several other tissues also produce testosterone: adipose tissue, brain, muscle, skin, and adrenal cortex. Peripheral organs and tissues can convert testosterone to the weaker hormone androstenedione, a hormone with different actions, estradiol, or through the microsomal enzyme 5α‐reductase to a more potent hormone‐dihydrotestosterone [18]. Testosterone travels in the circulation, 60% of which is bound to testosterone‐binding globulin also known as SHBG, 38% to albumin and corticosteroid‐binding globulin, approximately 2% circulates free in plasma; hence, 40% available for biological activity. In adults, testosterone maintains the male phenotype, sexual function, and exerts anabolic effects. While in the foetus, dihydrotestosterone is responsible for the differentiation of the external genitalia and its deficiency of absence leads to intersexual states. During embryological development, germ cells migrate into the testicles; they are immature germ cells and are called ‘spermatogonia’. These spermatogonia lie next to the basement membrane of the seminiferous tubules, and beginning at puberty, they divide mitotically so having the normal 2 pairs of 22 chromosomes, plus x and y (46) (Figure 35.3). Spermatogonia will then undergo meiosis in two stages. In stage 1 (prophase), primary spermatocytes are formed each containing a duplicated set of 46 chromosomes, 22 pairs of duplicated chromosome plus duplicated x and y (92). The secondary spermatocytes are formed, containing a haploid number of duplicated chromosomes and a duplicated x or y. The second meiotic division then occurs stage 2, resulting in smaller cells containing a haploid number of unduplicated chromosomes; these are called ‘spermatids’ and form the inner layer of the epithelium of the seminiferous tubules [18]. Spermatids convert into spermatozoa by spermatogenesis. Spermatogenesis takes approximately 74 days. The production rate is approximately 6.5 million sperm per testicular gram per day in a 20‐year‐old male, decreasing to 3.8 million per gram per day for 50–90‐year‐old men [18]. Sertoli cells are the support or ‘nurse’ cells of the spermatids. Sertoli cells form tight junctions forming the blood‐testis barrier. The barrier separates the testicular interstial blood from the lumen of the seminiferous tubules, thus creating an immuno‐privileged site. The Sertoli cell permits entry of nutrient and chemical mediators to spermatids. The processes of the Sertoli cell surround the spermatids. As spermatogenesis progresses and spermatids transform they progressively move closer to the lumen of the seminiferous tubules to eventually lose all contact with the Sertoli cell and are released as spermatozoa. The spermatozoa at this point, however, are still immature and are not motile, and so travel passively assisted by flowing secretions and luminal epithelial cilliary action into a network of tubules that the seminiferous tubules open up into the rete testes and then to the epididymis via the efferent ductules. The spermatozoa undergo maturation at the epididymis and are now motile able to fertilise. It takes 12–26 days from the release of spermatozoa to the full maturity present in the ejaculate [18].
Testes Structure and Function
Abstract
35.1 Comparative Anatomy
35.2 Topographical Anatomy
35.2.1 Testes
35.2.2 Coverings of the Testicle
35.3 Blood Supply and Lymphatic Drainage
35.3.1 Arterial Supply
35.3.2 Venous Drainage
35.3.3 Lymphatic Drainage
35.3.4 Nerve Supply
35.4 Epididymis
35.4.1 Blood Supply
35.5 Vas Deferens
35.5.1 Blood Supply
35.6 Seminal Vesicle
35.7 Verumontanum
35.8 Spermatic Cord
35.9 Testicular Physiology
35.9.1 Hypothalamic‐Pituitary‐Gonadal Axis (Figure 35.10)
35.9.2 Leydig Cells
35.9.3 Sertoli Cells
35.9.4 Production and Action of Testosterone
35.9.5 Spermatogenesis
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