Fig. 1.1
Male genital system (scheme)
Fig. 1.2
Testis and epididymis (right lateral aspect)
The testis is surrounded by the tunica albuginea, a thick capsule of connective tissue and smooth muscle fibers. The innermost layer, known as the tunica vasculosa, is composed of loose connective tissue interspersed with numerous blood vessels. The tunica albuginea is thickest at the posterior margin, where it invaginates to form the mediastinum testis. The testis is secured to the scrotum by the scrotal ligament. Numerous connective-tissue septa arising from the mediastinum divide the testis into around 250 pyramid-shaped lobules, which are broad based at the surface of the capsule and become narrower as they converge to the mediastinum. Each lobule contains one to four highly-coiled seminiferous tubules (150–250 µm in diameter and 30–80 cm long). Towards the apex, straighter tubules known as tubuli recti connect the coiled tubules to the rete testis (rete of Haller) located in the mediastinum testis.
The seminiferous tubules are composed of a seminiferous epithelium lined by a lamina propria or peritubular tissue layer.
Lamina propria: This layer is formed by connective tissue containing numerous fibroblasts and contractile myoid cells, whose rhythmic contraction facilitates the transport of spermatozoa.
Seminiferous epithelium: This is a stratified epithelium composed of two main cell types: Sertoli cells and germ cells at different stages.
Sertoli cells are columnar cells that rest on the basement membrane and extend to the tubular lumen. They contain an oval nucleus, highly developed smooth endoplasmic reticulum, lysosomes, and Golgi complexes. Sertoli cells, which are bound to each other by tight junctions, have several functions:
Promotion of nutrient exchange for germ cells
Phagocytosis of degenerated germ cells and residual spermatid cytoplasm
Production and secretion of testicular fluid
Production and secretion of androgen-binding protein (ABP) that binds specifically to the testosterone required for spermatogenesis
Production and secretion of inhibin
Regulation of germ cell movement in the epithelium, facilitating cell differentiation and the release of spermatozoa into the lumen
Formation of the blood–testis barrier
Germ cells at various stages of maturity are found in the testis: spermatogonia, spermatocytes, spermatids, and spermatozoa.
Spermatogonia:
These lie adjacent to the basement membrane of the tubular epithelium and are divided into two subtypes. Type A spermatogonia are stem cells which replicate and also divide by mitosis to produce Type B spermatogonia. The latter divide mitotically to produce primary spermatocytes. Due to incomplete cytokinesis during mitotic division, cells remain bound by cytoplasmic bridges during spermatogenesis.
Spermatocytes:
Primary spermatocytes are the largest of the germ cells. They are formed in the basal compartment and migrate to the adluminal compartment. They are diploid cells which undergo meiosis I to produce secondary spermatocytes.
Secondary spermatocytes are haploid cells rarely seen under the microscope because they are present for only 8 h of the 64-day spermatogenic cycle. They divide by meiosis II to produce spermatids.
Spermatids:
These are haploid cells found near the lumen of the seminiferous tubule. Rather than dividing, they undergo a process of differentiation to become spermatozoa.
Spermatozoa:
These are fully mature haploid cells but are incapable of fertilization.
As referred to briefly above, the tight junctions between Sertoli cells form the histological basis of the blood–testis barrier, whose functions are to create a microenvironment for the correct development of spermatozoa and prevent contact between maturing (haploid) gametes and the immune system.
The barrier divides the tubule into two distinct sections:
1.
Basal compartment: This compartment, which is in contact with the underlying connective vascular tissue, contains spermatogonia and primary spermatocytes.
2.
Adluminal compartment: This compartment contains secondary spermatocytes, spermatids, and spermatozoa, and it is not in contact with underlying connective vascular tissue.
The final portion of the seminiferous tubules, containing a columnar epithelium composed entirely of Sertoli cells, opens into the rete testis, an anastomosing network of tubules lined by a cuboid epithelium, some of whose cells may contain flagella. Seminiferous tubules are separated from each other by peritubular interstitial tissue composed of loose connective tissue containing blood vessels, lymph vessels, nerves, and Leydig cells (also referred to as interstitial cells).
Leydig cells may occur singly or in clusters and are characterized by a euchromatic nucleus and eosinophilic cytoplasm. Their appearance is typical of steroid-secreting cells, that is, abundant smooth endoplasmic reticulum, lipid vacuoles, and numerous mitochondria with tubular cristae. These are testosterone-producing cells [4–7].
Sperm Ducts
A whole set of tubular structures are involved in transporting spermatozoa from their origin in the testis to the urethra, through which both semen and urine are removed. These structures, to be subsequently described, are the epididymis, ductus deferens, and ejaculatory ducts.
Epididymis
The epididymis is an elongated organ located over the posterosuperior border of the testis . It is roughly 50 mm in length and contains a highly coiled tube known as the epididymal duct, which is around 4 m long and 0.3 mm in diameter. The epididymis can be divided into three portions: the head or caput (wider upper portion), the body or corpus (middle portion), and the tail or cauda (lower portion), which continues into the ductus deferens. The head of the epididymis is attached to the testis by efferent ducts and fibrous tissue and the tail by fibrous tissue alone; the middle portion of the epididymis is not attached to the testis (Figs. 1.1 and 1.2). The visceral layer of the tunica vaginalis forms a recess between the testis and the body of the epididymis, known as the sinus of the epididymis. The functions of the epididymis include sperm storage, resorption of most of the testicular fluid produced in the seminiferous tubules, and secretion of the enzymes required for the maturation of spermatozoa.
As indicated previously, the two main structures of the epididymis are the efferent ducts and the epididymal duct:
Efferent ducts:
Around 15–20 ductules connect the rete testis with the epididymis, across the tunica albuginea. They are composed of a simple epithelium containing columnar ciliated cells which facilitate the transport of spermatozoa, and cuboid non-ciliated cells whose function is to absorb testicular fluid.
Epididymal duct:
This is a single, coiled duct around 4 m long, located mainly in the body and tail of the epididymis. It is here that spermatozoa acquire motility and the ability to fertilize. The epididymal duct is surrounded by a layer of smooth muscle which becomes thicker towards the tail. The lumen is lined by a pseudostratified columnar epithelium containing stereocilia; the major cell types are principal cells and basal cells.
Principal cells:
These are columnar and contain a basal nucleus, with rough endoplasmic reticulum and abundant Golgi complexes, lysosomes, and micropinocytotic vesicles. The apical surface contains long, nonmotile microvilli (stereocilia).
Basal cells:
Ductus (Vas) Deferens
The ductus (vas) deferens, a duct around 40 cm long, is the continuation of the tail of the epididymis. It traverses the scrotal sac, the inguinal canal, the pelvic cavity below the peritoneum, and the rear lateral portion of the bladder, where it enlarges to form the ampulla. It is cylindrical in shape, with a diameter of around 2 mm and a caliber of 0.5 mm; this thick wall ensures great consistency.
The ductus deferens is divided into various anatomical portions: scrotal, funicular, inguinal, iliac, and pelvic (Fig. 1.1).
Scrotal portion: Here, the ductus deferens ascends along the medial side of the epididymis, from which it is separated by the testicular veins.
Funicular portion: From the anterior surface of the body of the epididymis, the ductus deferens continues upwards to the superficial inguinal ring. Here, it is accompanied by blood vessels supplying the testis and epididymis, nerves, pampiniform and testicular venous plexuses, lymph vessels, and a vestige of the peritoneum known as the processus vaginalis. All these structures are sheathed by the external spermatic fascia and together form the spermatic cord (Fig. 1.2).
Inguinal portion: This portion of the ductus deferens traverses the inguinal canal to the superficial inguinal ring. At this stage it is accompanied not only by the elements forming the spermatic cord but also by the genitofemoral and ilioinguinal nerves and the cremasteric artery.
Iliac portion: After passing through the deep inguinal ring, it enters the pelvic cavity (below the peritoneum) and moves downwards across the internal iliac vein and artery. Here, it separates from the other structures of the spermatic cord except for the artery of ductus deferens.
Pelvic portion: Here, the ductus deferens runs first to the side and later to the rear of the bladder. From the side of the bladder, it is directed backwards (below and adhering to the peritoneum) towards the rearmost portion of the lateral border of the bladder, where it crosses in front of and above the ureter. Thereafter, it descends along the posterior wall of the bladder and then changes direction, moving downwards, forwards, and inwards until it reaches the base of the prostate. Over this last stretch, the ductus deferens becomes enlarged and tortuous, forming the ampulla, which is separated from the fundus of the bladder first by the peritoneum and later by the rectovesical septum or retroprostatic fascia.
The mucosa of the ductus deferens is lined by an epithelium similar to that of the epididymal duct, surrounded by a slender lamina propria of dense connective tissue. The star-shaped cross-sectional appearance of the lumen is due to a number of longitudinal folds. The thick muscularis of the ductus deferens is formed by inner and outer layers of longitudinal fibers and a middle layer of circular fibers. The longitudinal fibers shorten and widen the ductus deferens immediately prior to ejaculation, drawing in the content of the epididymis and expelling it towards the urethra. These muscle fibers receive abundant innervation from the sympathetic nervous system [4–7].
Ejaculatory Ducts
The right and left ejaculatory ducts, tubes of around 25 mm in length, run from the terminal portion of the ampulla of the ductus deferens and the start of the seminal vesicles (Fig. 1.3), passing through the prostate and opening into the posterior portion of the prostatic urethra, to either side of the prostatic utricle, at the right and left colliculi seminalis. The ducts are lined by simple columnar epithelium and have no muscle layer [4–7].
Fig. 1.3
Seminal vesicles, ejaculatory ducts, and prostate (posterior aspect)
Scrotum
The testes, epididymides, and a portion of the ductus deferens are sheathed in several layers of tissue which together form the scrotum or scrotal sacs. These sheaths derive from the abdominal wall, due to the descent of the testes from the abdomen into the scrotum. The two sacs are separated by a central septum (median raphe); the left sac hangs lower than the right and the two are attached to the perineum by the scrotal pedicle.
Working inwards from the outside, the layers of the scrotum are as follows:
Scrotal skin: The scrotal skin is thin, dark, and shows numerous folds.
Tunica dartos: This layer is composed of smooth muscle fibers interspersed with connective tissue and elastic fibers; at the midline, it contributes to the formation of the central septum (median raphe).
Subcutaneous cell tissue layer.
External spermatic fascia: This thin membrane is the scrotal prolongation of the lining of the abdominal external oblique muscle.
Cremaster muscle: This muscle is formed by inner and outer layers of striated muscle, derived from the internal oblique and transverse abdominal muscles, which eventually insert into the internal spermatic fascia.
Internal spermatic fascia: This layer, deriving from the abdominal fascia transversalis, extends downwards to line the spermatic cord, the epididymis, and the testis.
Tunica vaginalis: This serosa, deriving from the peritoneum, lines the anterior and lateral surfaces of the testis but not the posterior surface. It comprises two layers: the superficial or parietal lamina and the deep or visceral lamina (which lines the tunica albuginea of the testis). Both are composed of mesothelial cells and submesothelial connective tissue. The interval between the visceral and parietal laminae is known as the serous cavity of the scrotum [4–7].
Accessory Sex Glands
Seminal Vesicles
These two oval structures (50 mm long, 10 mm wide, and 10 mm thick) are located behind the urinary bladder at either side of the ampulla of the ductus deferens and are separated from the rectum by the rectovesical septum. The end of each seminal vesicle joins the end of each ampulla to form the ejaculatory duct (Figs. 1.1 and 1.3). The seminal vesicle consists of a coiled, glandular tube around 15 cm in length, covered by fibrous tissue. It produces a yellowish, gelatinous secretion which accounts for 60–80 % of seminal fluid or semen. This secretion is particularly rich in fructose, citrate, amino acids, and prostaglandins.
Seminal vesicles do not store sperm. Contraction of the smooth muscle during ejaculation pushes this secretion into the urethra. The single tube, of which each vesicle is composed, comprises three concentric layers. The outermost layer of loose connective tissue surrounds two layers of smooth muscle tissue, a longitudinal outer layer, and a circular inner layer.
The innermost layer of the vesicle is a mucosa arranged into numerous folds, giving the lumen a highly irregular appearance. The mucosa comprises a pseudostratified columnar epithelium and a lamina propria of connective tissue. Epithelial cells are of two types: principal and basal. Principal cells are columnar, and their cytoplasm contains secretion vesicles, lipofuchsin granules, and lipid droplets, while basal cells are stem cells (progenitors of principal cells) [4–7].
Prostate
This is a chestnut-shaped gland and resembles an inverted cone, its apex pointing downwards. It is located beneath the urinary bladder (the base of the prostate is fused to the fundus of the bladder) and therefore surrounds the initial portion of the urethra. The vertical, transverse, and anteroposterior diameters are 25–30, 40, and 25 mm, respectively. It consists of anterior and posterior surfaces, two lateral surfaces (down-facing and out-facing), a base, and an apex (Figs. 1.1 and 1.3).
Although there are no clear divisions within the prostate, it is traditionally divided into four lobes:
Anterior portion (in front of the urethra)
Posterior portion (behind the urethra and between the ejaculatory ducts)
Right and left lateral lobes (rest of the gland)
The anterior portion of the prostate, including the apex, is surrounded by fibers of the external sphincter muscle of the urethra and by fibrous laminae deriving from the periprostatic fascia of the pelvic floor. The posterior portion is lined by the rectovesical septum (the Denonvilliers prostatoperitoneal membrane or fascia); as indicated earlier, this posterior fascia also sheathes the ductus deferens and the seminal vesicles. Laterally, the prostate is surrounded by the prostatic fascia, which contains periprostatic venous plexuses. These fibrous fascia are attached in the anterior portion to the pubis, laterally to the elevator muscle of the anus and posteriorly to the sacrum. The prostate secretes a colorless fluid which makes up between 15 and 30 % of the volume of semen. This secretion contains acid phosphatase, citric acid for sperm nutrition, and fibrolysin for semen liquefaction. It should be noted that the prostate contains the prostatic urethra, the internal sphincter muscle of the urethra, the upper portion of the external sphincter muscle of the urethra, the ejaculatory ducts, and the prostatic utricle. The prostate is surrounded by a fibroelastic capsule containing bundles of smooth muscle fibers. It consists of 30–50 glands arranged in three concentric layers: an inner mucosal layer of glands surrounding the urethra, which is in fact invaginations of the urethral epithelium, an intermediate submucosal layer of tubuloalveolar glands, and a peripheral principal layer containing the main tubuloalveolar prostatic glands. The principal layer is the most susceptible to prostate cancer, while the mucosal and submucosal layers tend to be more affected by benign prostatic hyperplasia. The glandular parenchyma is surrounded by abundant connective tissue and bundles of smooth muscle fibers. During ejaculation, these muscles contract to expel the prostatic secretion into the urethra. The glandular mucosa is heavily folded, with a pseudostratified epithelium. It comprises principal cells containing numerous secretory granules and abundant acid phosphatase, and basal cells which are the precursors of principal cells. The alveoli of the prostatic glands often contain corpora amylacea, formed by precipitation of secretory material around cell fragments; over time, these may become calcified [4–7].
Bulbourethral Glands
These two lentil-sized glandular structures, also known as Cowper’s glands , are located at the upper right and left of the urethral bulb, in the deep perineal pouch alongside the deep transverse perineal muscle. A duct, around 4 cm in length, arises from each gland to finish in the spongy urethra (Fig. 1.1). The glands produce a clear, viscous secretion which acts as a lubricant during sexual stimulation. They are surrounded by a capsule of connective tissue, with a simple epithelium comprising columnar mucus-secreting cells [4–7].
Penis
The penis is the male copulatory organ. It is located above the scrotum and in front of the symphysis pubis. The flaccid penis measures roughly 10 cm and is cylindrical in form, while the erect penis assumes the form of a triangular prism with rounded angles and measures around 16 cm (Fig. 1.1). Its anterior end is expanded in the form of an obtuse cone. This expansion, termed the glans penis, contains the external urethral orifice or urinary meatus. At the neck (the boundary between the glans and the body of the penis) the sheath of the penis folds in upon itself to form the foreskin or prepuce. The posterior portion of the penis, known as the root, is attached to the front of the symphysis pubis by the suspensory ligament and to the ischiopubic rami through the corpora cavernosa penis. The penis is formed by three blood-storing structures known as erectile organs: two dorsal tissue masses and one ventral tissue mass. The dorsal erectile organs, known as the corpora cavernosa, are capped by the glans. Behind, the corpora diverge, forming the crura, which attach to the ischiopubic rami, and terminate in a conical structure known as the root of the penis. The ventral erectile organ surrounding the urethra, termed the corpus spongiosum, is located in the lower longitudinal groove formed by the two corpora cavernosa. At its anterior end, the corpus spongiosum broadens out to form the glans, while the posterior end assumes a bulb shape to form the bulb of corpus spongiosum.
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