The male urethra is a 20-cm (aprox.) canal extending from the bladder neck to the external meatus. Its function is to serve as a conduit for the passage of urine and semen
The male urethra is not an omogeneous tubular structure but is a canal made up of different parts. Each of these parts has its own typical structural and functional anatomy: this is the reason for the different origin of stricture pathologies affecting the various urethral parts and the different surgical implications.
Firstly the urethra is divided into
two anatomically, functionally and surgically distinct areas that present
entirely different problems: the Posterior prostatomembranous sphincter-active
urethra and the Anterior bulbopenile spongy urethra. Subsequently these two
principal sections should be further considered as subdivided into others
segments each of which has specific anatomical, pathological, surgical features(fig. A) [1, 2]:
Posterior urethra(5 cm)
Anterior urethra(15 cm)
•penile(6 cm)navicular(2 cm)
The anterior urethra is 15 cm long and extends from the end of the membranous urethra to the external meatus. It consists of three segments: bulbar, penile and navicular urethra.
The anterior urethra is a tubular structure which has its own wall, thecorpus spongiosum, whichat each end, expands, forming the glans distally and the bulb proximally. The corpus spongiosum is a highly vascular elastic spongy structure made up of a network of large irregular venous spaces lined by endothelium and separated from each other by trabeculae of fibroelastic tissue containing numerous smooth muscle fibers. This is where the pathological scarring processes which cause the stricture take place: namely it is the progressive thrombosis-fibrosis response to the damage of this unique spongy urethral structure that predisposes not only to the formation of strictures but also to its particular tendency to restenosis after surgical repair. The understanding of the pathogenesis and the surgical significance of spongio-fibrosis is fundamental to understand the stenotic urethral disease and essential to successful urethral reconstruction.
The spongiosum vascular structure is more consistent in the bulbar segment. This accounts for the different vascular blood supply (greater in the bulbar urethra and minor in the penile urethra) and for the different tendency to ischemia which explains the etiological, anatomical and pathological stenotic differences. It is also responsible for the different reconstructive concepts and results for the penile and bulbar urethra.
The spongiosum is enclosed in a connective tissue capsule namedtunica albuginea. Spongiosum and both corpora cavernosa are contained within two fascial layers: Buck’s fascia and dartos fascia.
Buck’s fasciais the deeper of the two and it lies adjacent to the tunica albuginea. It is an elastic layer which envelopes the erectile bodies in the penis and extends into the perineum, separately enveloping each crus of the corpora cavernosa. It is split around the corpus spongiosum and in the perineum it envelops the bulb. Proximally, it inserts onto the pubis and ischium, and distally it is attached to the glans penis at the corona. Dorsally, in the penis, on the undersurface of the Buck’s fascia is the neurovascular bundle.
Surrounding Buck’s fascia is thedartos fasciathat envelops the penis and connects (dorsally, at the base of the penis) with the Scarpa’s fascia of the anterior abdominal wall. Ventrally, in the scrotum and perineum this dartos fascia continues as Colles’ fascia which attaches to the central tendon of the perineum, and laterally to the ischium and the ramus of the pubis.The dartos fascia is the loose connective subdermal penile layer that allows mobility of the penile skin on the shaft of the penis. This fascia is of critical importance in providing a blood supply to all the penile skin flaps; indeed, the flaps use the fascia dartos as a conduit to carry the vascular pedicle supplying the flap.
The urethra is lined by mucosa that is an extension of the bladder transitional epithelium. The prostatic and membranous segments are lined with transitional epithelium. It gradually changes to pseudostratified columnar epithelium in the bulbar and penile segments. The navicular urethra is lined with nonkeratinizing stratified squamous epithelium. Mucus-secreting glands drain into the anterior urethral lumen: these are the two Cowper’s glands in the bulbous segment, and the Littre’s glans and the lacunae of Morgagni in the penile segment.
·The urethra is usually described as a tube withaventralanda dorsalsurface(fig. B).
·By“urethral plate”we mean the emicircumferential surface of the urethral duct which appears after the longitudinal opening of the urethra(fig. C).Having common terms to describe urethral anatomy is essential to understand the reconstructive urologic literature.
Bulbar urethrais the segment located between the membranous and the penile urethra. It is divided from the penile urethra at the level of the penoscrotal junction below and the suspensory penile ligament above. It is shorter than the peno-navicular segment and lies in the midline between both penile crura.
The corpus spongiosum is thick: the complete structure is made of wide lacunae with a highly vascularized system of trabeculae.
The urethral lumen is dorsally eccentric within the bulk of the bulbospongiosum expansion: the spongy tissue in the dorsal, 12 o’clock, sector is only 3 to 4 mm thick, whilst the ventral spongiosus wall is 10 to 15 mm thick(fig. D).
Externally the corpus spongiosum is enveloped in the ischial cavernosus-bulbospongiosus musculature(fig. E).
•The following anatomical features of the bulbar urethral account for the higher success rate and the lower complication rate for the surgery of this segment compared with the urethral penile surgery:
1.the thick spongiosum guarantees a satisfying vascular and mechanical support to the substituting tissues in both dorsal and ventralaugmentationurethroplasties;
2.the bulbar urethra does not lengthen during erection and is not subject to mechanical erectile extensions which, conversely, penile urethral undergoes after a urethroplasty;
3.the perineal tissuesguarantee a further cover and vascular support for urethroplasties.
•The bulbar urethra is emptied after micturition by its closure mechanism created combiningthe elasticity of the spongy tissue with the voluntary contraction of the bulbospongiosus muscle; the efficiency of this urethral emptyingmechanism diminishes with age.
In a similar way an efficient urethral emptying during ejaculation is achieved by the combination of the bulbopenile urethral closure-pressure with the intermittent reflex contractions of the spongiosus muscle. Particularly, erection of the spongy tissue not only increases greatly the passive urethral closure-pressure but it increases the bulk of the bulb which augments the effect of the reflex ejaculatory contractions of the surrounding bulbospongiosus muscle. Thus, orgasmic emission is a function of the posterior urethra by a simultaneous contraction of the bladder neck, sustained contraction of the seminal vesicles and opening of the distal sphincter. After the emission has passedthe distal sphincter, the continuous flow of the semen is converted in a forceful intermittent ejaculation by the anterior bulbopenile urethral structure.
The bulbar-cavernous muscles together with theperineal central tendon on which they pivot, take part in the ejaculatory and mictural emission; for this reason, during the urethroplasty, it is advisable to try to spare the tendon and to reconstruct the muscles.
Some Authors supposed that the hypothetic surgical damage during bulbar urethroplasty to the branches of the perineal nerves innervating the bulbospongiosus muscles and the corpus spongiosum may explain the loss of rhythmic bulbar urethral contractions, causing difficulty in expelling semen and urine.In reality, urethral reconstructive procedures inevitably impair the natural emptying mechanism of the spongy tissue, because they cause an interruption of the structural muscle-elastic circularity of the urethral tube which may stop the voiding rhythmic urethral contractions. Furthermore the urethral elasticity is often already impaired to some extent by the spongio-fibrotic disease.
The bulbar urethra is ideally divided into two parts, proximal and distal.
The proximal bulbar urethra is the section which borders with the
membraneous urethra. The corpus spongiosum reaches its greatest thickness in
the “bulb” and then it disappears in the membraneous urethra: in this area , at 5 and 7 o’clock, there are the two bulbar arteries
which are principally responsible for the vascularization of the corpus spongiosum(fig. F).
•The thick spongiosum wall and the proximity of the bulbar arteries makes the surgical approach to this area particularly bloody.
•The urethral curve towards the prostate and the proximity with the area where the erigentes nerves are located (at 11 and 1 o’clock) make it difficult and dangerous to detach the urethra from the corpora cavernosa and to perform a urethrotomic dorsal approach.
•For the right exposure of this urethral tract sometimes it is necessary to cut the perineal central tendon: by weakeningthe perineal muscular structure and the distal sfincter, this procedure can cause a reduced ejaculatory contraction or a partial urinary incontinence in patients without proximal sphinteric function (bladder neck) following prostatic surgery.
The distal bulbar urethra is the part which borderswith the penile uretra. The copus spongiosum become progressive thin and finally similar in structure to the penile urethra.
•Due to its easy access, this area allows the detachment of the urethra from the corpora cavernosaand a dorsal urethrotomical approach, better than all the other segments.
The penile urethra extends from the suspensory ligament to the corona of the glans penis.
The penile spongiosum is thinner and more compact compared with the bulbar spongiosum:the structure is made of a tightersystem of trabeculae and less wide lacunae.
The urethral lumen is concentrically located within the corpus spongiosum that forms its walls; these walls are circumferentially (both dorsally and ventrally) 3 to 4 mm thick.
•The spongiosum wall is thinner (and consequently with a minor vascular supply) compared with the bulbar urethra, thus explaining the ischemic tendency to penile urethral lesions: namely the penile stenotic etiology is mainly ischemic and there is a high rate of post urethroplasty ischemic redos.
•The shortage of tissueswhich envelop the penile urethra ventrally and the thinness of the spongiosum ventrally make the covering of grafts unsatisfactory in ventral augmentation urethroplasties: this explain the tendency to prefer dorsal augmentation urethroplasties where the graft is covered by the corpora cavernosa.
•The penile urethra actively takes part in the erection, following the two cavernous cylinders in the longitudinal extension: the constant erection stress is an important concomitant cause of penile urethroplasty failure.
•The penile-bulbar urethra lies in a dorsal groove juxtaposed with the two corpora cavernosa. Such groove is deeper in the penile area than in the perineal one: consequently the penile urethra is embedded in this groove more deeply than the bulbar urethra. The anatomical location of the penile urethra accounts for its more difficult dorsal detachment from the corpora cavernosa compared with the bulbar urethra.
The navicular or glanular urethra is the tract between the balanic sulcus and the glans tip. It is the portion of the urethra invested by the spongy erectile tissue of the glans penis.
The luminal calibre widens to form the fossa navicularis, narrowing again at the external meatus; the sagitally oriented slit-like meatus is paramount in maintaining the compact configuration of the exiting urine stream.
·The mucosa is not easily detachable from the underlying layers and the refluent blood from the glans tissue is not easily controllable.
·The ventral balanic sulcus is the area where the urethra is more superficial and is covered by a very thin corpus spongiosum, the penile fasciae and by a particularly thin skin: therefore this tract has the highest risk of fistulization after a urethral reconstruction.
·The reconstruction of a navicular urethra with a glans-meatus that is cosmetically and functionally perfect requires meticulous attention to detail:
1.an aesthetically unacceptable meatus could condition the overall appearance of the genitalis, since the glans is considered the aesthetical spot of the penis par excellence.
2.an imperfect meatus leads to a sprayed stream instead of a compact stream.
The posterior urethra is the sphincter-active urethral section, extending from the bladder neck to the bulbomembranous urethral junction. It consists of two segments: prostatic and membranous urethra.
The Prostatic urethra is the section of the urethra invested by the prostate and muscolature of the bladder neck.
The Membranous urethra (about 1-2 cm long) extends from the prostatic apex to the bulbar urethra. Its thin wall (4-5 mm thickness) is formed by the smooth intrinsic musculature of the distal sphincter mechanism. The prostate gland is located deep in the pelvis and is well protected by the anterior bony pelvis. The prostatic apex is anchored to the pubic symphysis by the puboprostatic ligaments located anteriorly. Similarly, the proximal bulbar urethra is anchored deep in the perineum, especially in the midline posteriorly by a fibrous septum (central tendon of the perineum). The thin and unprotected membranous urethra is positioned between these two fixed points (prostatic apex and proximal bulbar urethra): in pelvic fractures the urethral disruptions occur at this location because of shearing forces that avulse the urethra just distal to the prostatic apex and just proximal to the corpus spongiosum, where the membranous urethra is unsupported and particularly vulnerable.
The sphincter mechanism is located in the posterior urethra and comprises two independent structural components: the proximal sphincter mechanism (bladder neck) and the distal sphincter mechanism (area of the membranous urethra)(fig. G).
The proximal sphincter mechanism consists of the smooth muscle collar at the bladder neck level. It is primarily formed by the extension of the detrusor muscle and extends downwards both on the inside and on the outside of the prostate.It is responsible for the passive continence and its occlusive function takes place from the internal meatus to the level of the verumontanum. Function of the bladder neck sphincter is coupled with detrusor activity, and involuntary bladder contractions will lead to bladder neck opening.
The distal sphincter mechanism is located in the area of the membranous urethra, extending from the prostatic apex to the proximal bulbar urethra. It is made of an intrinsic and an extrinsic component. Theintrinsic componentis made of a circular layer of smooth slow-twitch muscle fibers residing within the wall of the membranous urethra and extending upwards, on the inside of the prostate, to the verumontanum. It is capable to sustain tonic contraction which is responsible for the continence: its effective occlusive function is confined to the distal half of the posterior urethra. The intrinsic component is morphologically quite different from the extrinsic striated muscle of the pelvic floor, from which it is separated by a continuous layer of collagen.
The extrinsic component (elevator muscles of the pelvic floor) is made of an outer layer of striated muscle fibers that abuts the membranous urethra posteriorly and laterally only. This posterolateral periurethral muscle forms a“pubourethral sling”that inserts into the perineal body attached to the posterior surface of the bulbomembranous urethra. The traditional description of a complete (encircling the membranous urethra externally and totally) and sphincterically competent urogenital diaphgram is inaccurate. Indeed, at this level there is no periurethral external muscle fibers anteriorly and a potential “pubo-urethral space”lies between the pubic bone and the urethra, exactly in the supposed area of the anterior urogenital diaphgram (see below). The extrinsic muscular component comprises fast-twitch striated fibers capable of brief periods of contraction; although it does contribute to the resting closing pressure of the urethra, its main function is for the voluntary and momentarily interruption of the urinary stream by compressing the urethra from behind. Therefore, continence of the distal sphincter is entirely dependent upon the tonic occlusive function of the smooth intrinsic musculature within the thickness of the wall of the urethra.
In conclusion, the entire posterior urethra is sphincter-active, and any injury in this area will alter sphincter function to some degree. The proximal sphincter (bladder neck)and the distal sphincter (membranous urethra) may be considered to function independently. However, if the proximal sphincter is made non-functional by prior injury or surgery, perfect continence can be maintained by the distal sphincter mechanism. Conversely, continence is maintained by a functional bladder neck even after distal sphincter ablation such as may occur following pelvic fracture membranous urethral injury or its subsequent repair.
The sphincteric mechanism of the bladder neck is innervated by the sympathetic system through the alfa-1 receptors. The innervation of the distal sphincteric mechanism is more complex and has not been well defined yet: the somatic system (pudendal nerve) is the principal element which governs the extrinsic striated musculature but also the sympathetic and parasympathetic systems appear to take part in the sphincteric innervation.
Because of the proximity of the nervi erigentes to the subprostatic urethra, any operation in this area carries some risk of damaging the neural erectile mechanism, particularly if it involves dissecting the tissue planes behind the apex of the prostate. However, we must bear in mind that the most membranous urethral injuries are already associated with some degree of post-traumatic damage to the neuro-vascularmechanism of potency[11, 12].
The “pubo-urethral space”
There is a dearth of accurate textual anatomic descriptions of this area. The anterolateral surface of the membranous urethra relates directly to the pubo-urethral space. This space is somewhat horseshoe shaped: it is bounded anterolaterally by the medial margins of the crura, the inferior pubic rami and the pelvic floor elevator muscles, and posteriorly by the membranous urethra. Posterolaterally, it extends to the insertion of the levator muscles into the perineal body which is densely adherent to the posterior surface of the membranous urethra.
The empty pubo-urethral space is surgically important because it is possible to develop during the posterior urethroplasty[1, 13]. After mobilization of the urethra, a finger tip can be inserted into the developable pubo-urethral space between the membranous urethra and the subpubic arch, up to the level of the prostatic apex and its roof is formed by the puboprostatic ligaments. It contains loose adventitial tissue and a few blood vessels and proximal-dorsal fascicles of the bulbospongiosis muscle. Its surgical development is relatively simple unless it is involved in post-traumatic fibrosis.
The blood supply to the prostatic and the membranous urethra is via the direct branches from the hypogastric arteries.
The blood supply of the bulbo-penile urethra is dependent on the terminal branches of the internal pudendal artery. The pudendal artery courses to the perineum (via Alcock’s canal) and here takes the name of perineal artery; shortly after arriving in the perineum, the perineal artery continues as the common penile artery. The latter pierces the muscular pelvic floor and continues along the medial margin of the inferior ramus of the pubis. In proximity with the urethral bulb, outside the crus of the penis, the common penile artery divides into the terminal branches: the bulbar artery, the urethral artery, the cavernosal artery(also named deep artery) and the dorsal artery of the penis(fig. H) .
The cavernosal arteries enter the hilum of the penis adjacent to the junction of both cavernous bodies and continue distally in the center of the corpora cavernosa almost to their tips. Along their intracavernous course, they give off terminal branches known as the helicine arteries that empty into the sinusoidal spaces.
The branches of the dorsal artery of the penis are the circumflex arteries.
The cavernosal arteries and the dorsal arteries of the penis terminate as arborizations in the spongy erectile tissue of the glans penis. Thus, the corpus spongiosum has a dual blood supply: a distal retrograde arterial flow via the glans penis and a proximal arterial inflow via the arteries of the bulb and the circumflex arteries. This dual blood supply has important surgical implications: it allows the urethra to be detached from the corpora, transected and mobilized as a “flap” on its distal blood supply, avoiding the ischemic necrosis of the distal urethra. Regarding this matter, we must bear in mind that the retrograde blood supply to the distal urethra, derived from distal collaterals in the penile area, may be impaired by: (a) excessive mobilization of the penile urethra (which cuts many distal collateral vessels); (b) a hypospadiac deformity (in which there is no continuity of the spongy tissue of the penile urethra with that of the glans); (c) an incidental distal spongiofibrosis. However, also in presence of a normal dual blood supply, postoperative neurovascular disorders after anastomotic procedures have been reported such as decreased sensitivity or vascular deficiency of the glans penis.
In addition to the internal pudendal artery,anaccessory pudendal artery travels along the anterolateral surface of the bladder and prostate to the root of the penis.
The venous drainage from the corpora cavernosa and corpus spongiosum originates in venules leading from the peripheral sinusoids immediately beneath the tunica albuginea. These venules form the subtunical venular plexus before exiting as theemissary veins.
The glans penis empties into large and small veins, which form a retrocoronal plexus, the origin of the deep dorsal vein. The deep dorsal vein receives circumflex and emissary veins that provide the main venous drainage of the glans penis and distal two thirds of the corpora cavernosa and corpus spongiosum. The deep dorsal vein runs upward behind the symphysis pubis to become the periprostatic venous plexus draining into the internal pudendal veins.
Emissary veins draining the proximal part of the corpora cavernosa and corpus spongiosum join to form the cavernous,urethraland bulbar veins running medial to the cavernous artery and nerves, and draining into the internal pudendal veins.
Lymphatic drainage is by means of the superficial and deep inguinal nodes.
The blood supply to the penile skin is important in providing the nutrition to the penile skin flaps. This blood supply is from the external pudendal arteries bilaterally, extending medially from the femoral vessels. Branches from these vessels course dorsally and ventrally along the shaft of the penis towards the glans in an axial pattern. These arteries are contained within the dartos fascia and take part in creating the vascular pedicle of the penile skin flaps used in the flap repairs. The external pudendal axial system is the sole cutaneous blood supply to the penile skin exclusive of the preputial skin that is served by three sources: the axial penile skin vessels, perforators from the corporeal bodies, and branches from the dorsal arteries of the penis.
The understanding of the neuroanatomy of this area is a prerequisite for prevention or reduction of the neural injuries during the surgical urethral repairs.
Two sets of nerves control erectile function. They are designated as autonomic (motor) and somatic (sensory) innervation.The autonomic system consists of sympathetic (T10-L2) and parasympathetic fibers (known as pelvic nerves: S2-S4) that make the pelvic plexus. This latter is a 4-5 cm network lying on the posterolateral aspect of the bladder and the prostate. Its caudal fibers form the cavernous nerves (also named“nervi erigentes”) which travel posterolateral to the seminal vescicles towards the apex of the prostate. They then accompany the membranous urethra, located at the 3 and 9 o’clock positions external to the striated muscles. At the penile hilum, the cavernous nerves pierce the corpora cavernosa to innervate the helicine arteries and the erectile tissue: therefore they are located superolaterally (at the 2 and 10 o’clock positions) to the proximal bulbar urethra(fig. I).
Detailed anatomic studies on the prostatic apex and membranous urethra (relating to radical prostatectomy) have focusedthe location and importance of the neurovascular bundles, stressing the risk of damage to the erectile function due to urethral repair in this area[16, 17, 18].
The somatic system consists of the paired pudendal nerves which receive contributions from S2-S4 and innervate the pelvis and perineum. They accompany the internal pudendal vessels through Alcock’s canal to innervate the striated muscle of the pelvic floor (levator ani) and the penis (ischiocavernosus and bulbocavernosus muscles). The pudendal nerves also supply sensory perception to the pelvis and the perineum and terminate as the dorsal nerve of the penis.
The dorsal nerve of the penis is an important branch of the pudendal nerve: it travels underneath the levator ani and continues distally along the dorsolateral surface of the penis lateral to the dorsal artery, gives multiple branches, and terminates in the glans penis.
Anterior urethral innervation is by the urethrobulbar nerve, a branch of the perineal nerve[4, 14, 19, 20]: this latter is a branch of the pudendal nerve.
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URETHRA Definition ( WIKIPEDIA)
In anatomy, the urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both genders to pass urine to the outside, and also a reproductive function in the male, as a passage for sperm. The external urethral sphincter is a smooth muscle that allows voluntary control over urination.
The length of a male's urethra, and the fact it contains a number of bends, makes catheterisation more difficult.
The epithelium of the urethra starts off as transitional cells as it exits the bladder. Further along the urethra there are stratified columnar cells, then stratified squamous cells near the external meatus (exit hole). There are small mucus-secreting urethral glands, that help protect the epithelium from the corrosive urine.
Medical Problems of the urethra
- Hypospadias and epispadias are forms of abnormal development of the urethra in the male, where the opening is not quite where it should be (it occurs lower than normal with hypospadias, and higher with epispadias). In a severe chordee, the urethra can develop between the penis and the scrotum.
- Infection of the urethra is urethritis, said to be more common in females than males. Urethritis is a common cause of dysuria (pain when urinating).
- Related to urethritis is so called urethral syndrome
- Passage of kidney stones through the urethra can be painful and subsequently it can lead to urethral strictures.
Endoscopy of the bladder via the urethra is called cystoscopy.
Medical Problems of the urethra
The male urethra is the conduit for sperm during sexual intercourse. It also serves as a passage for urine to flow.