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Seminal colliculus

The seminal colliculus, also known as the verumontanum or colliculus seminalis, is a prominent, rounded elevation located on the posterior wall of the prostatic urethra in the male urogenital system. It forms the apex of the urethral crest, a midline ridge that runs along the posterior aspect of the prostatic urethra, and serves as a critical anatomical landmark for the entry of seminal fluid into the urethra during ejaculation.^[1] At the summit of the seminal colliculus, the openings of the bilateral ejaculatory ducts are situated, through which semen from the seminal vesicles and vas deferens is discharged into the prostatic urethra. Additionally, the prostatic utricle, a small blind-ending diverticulum approximately 4-6 mm in length, opens at this site and represents an embryological remnant of the paramesonephric (Müllerian) ducts. Flanking the urethral crest on either side are the prostatic sinuses, into which numerous prostatic ducts empty, contributing prostatic secretions to the seminal fluid.^[2]^[1] Structurally, the seminal colliculus is composed of thickened submucosa covered by urethral epithelium, and it plays a role in the glandular functions associated with the prostate, potentially aiding in the regulation of semen flow to prevent post-ejaculatory oozing. In clinical practice, it is visualized endoscopically during procedures such as urethroscopy or transurethral resection of the prostate (TURP), where it serves as a surgical landmark to avoid injury to the ejaculatory ducts and preserve fertility.^[3]^[4]

Anatomy

Location

The seminal colliculus, also known as the verumontanum, is a midline elevation situated on the posterior wall of the prostatic urethra, forming a prominent mound within the midportion of this segment.^[5] The prostatic urethra extends approximately 3 to 4 cm from the bladder neck to the membranous urethra, positioning the seminal colliculus roughly 1.5 to 2 cm distal to the bladder neck.^[6] This location places it centrally within the prostate gland, surrounded by prostatic glandular and stromal tissue.^[7] In relation to surrounding structures, the seminal colliculus lies proximal to the external urethral sphincter at the distal end of the prostatic urethra and forms the elevation of the urethral crest.^[7] The opening of the prostatic utricle is positioned on its summit, while the ejaculatory ducts open adjacently on either side.^[5] On magnetic resonance imaging (MRI), the seminal colliculus appears as a distinctive rounded prominence in the mid-prostatic urethra, aiding in anatomical delineation.^[8] During cystoscopy, it is readily visualized as a bulbous elevation protruding into the urethral lumen, serving as a critical landmark for urological procedures.^[9]

Structure

The seminal colliculus, also known as the verumontanum, is an elevation in the prostatic urethra formed by the widening of the urethral crest into a mound-like structure.^[6] It is lined by transitional epithelium, or urothelium, which provides a protective barrier consistent with the histology of the prostatic urethra.^[6] Key structural features include the openings of the two ejaculatory ducts located on its lateral margins within the prostatic sinuses, as well as a central slit-like opening for the prostatic utricle, a remnant of the Müllerian ducts measuring approximately 4-6 mm in length.^[10] These openings are positioned along the posterior wall of the prostatic urethra.^[10] Histologically, the seminal colliculus consists of underlying fibromuscular stroma, though it is surrounded by the glandular and stromal elements of the prostate's periurethral zone, which is closely associated with the central zone.^[6] It is enclosed by fibers of the internal urethral sphincter muscle and adjacent to periurethral glands.^[10] In adults, the seminal colliculus typically measures 5 to 6 mm in length and 3 to 5 mm in width, varying slightly by morphological type such as volcano or lighthouse forms.^[11]

Development

Embryology

The seminal colliculus, or verumontanum, originates from the endodermal epithelium of the urogenital sinus during gestational weeks 9-12, forming as a prominence on the urethral crest within the primitive urethra.^[12] This structure emerges during the pre-bud and initial budding stages of prostate development, where multilayered urogenital sinus epithelium expresses key markers such as Foxa1, androgen receptor, and various keratins, confirming its endodermal derivation.^[12] Prostatic buds begin to outgrowth from the urogenital sinus near the verumontanum site around weeks 10-11, with the prominence becoming visible by week 12 as epithelial cords elongate and branch.^[12] Androgen signaling plays a pivotal role in this process, with androgen receptors present in the urogenital sinus mesenchyme by approximately week 11.5 and in the epithelium by week 15, stabilizing prostatic buds and promoting their morphogenesis under the influence of dihydrotestosterone.^[12] The prostatic utricle, a remnant of the Müllerian ducts, develops within the seminal colliculus, opening at its apex, while the Wolffian (mesonephric) ducts contribute to the ejaculatory ducts that incorporate into the structure near the verumontanum.^[12] These ducts are evident by week 9 and integrate as the urethral crest elevates.^[12] Canalization of the prostatic ducts and associated structures progresses from week 12 onward, with lumens forming in the solid epithelial cords by the late fetal period, resulting in a fully canalized seminal colliculus by birth.^[12]

Congenital anomalies

The most common congenital anomalies affecting the seminal colliculus involve the prostatic utricle, a small sac-like structure that opens onto the colliculus via a slit-like orifice. Enlargement or cyst formation of the prostatic utricle results from incomplete regression of Müllerian duct remnants during embryonic development, leading to focal dilatation and potential diverticula formation.^[13]^[14] These cysts typically manifest as midline, fluid-filled lesions posterior to the prostatic urethra and are often lined by cuboidal or columnar epithelium.^[13] Such anomalies can produce diverticula measuring up to 1-2 cm in diameter, though most are smaller, commonly under 10 mm, and rarely extend superior to the prostate base.^[15]^[13] Prostatic utricle anomalies occur in approximately 1-10% of males, with autopsy studies reporting a 1% incidence and clinical series in urologic patients indicating about 5%; prevalence rises significantly to 11-14% in cases associated with hypospadias or intersex disorders and exceeds 50% in severe perineal hypospadias.^[16] These conditions are frequently asymptomatic, particularly when small, but may be identified incidentally on imaging modalities such as voiding cystourethrography, ultrasound, or MRI.^[13] Ectopic or accessory utricles are rare developmental variations characterized by abnormal positioning of the utricle opening relative to the seminal colliculus or the presence of duplicate structures. These may involve ectopic insertion of adjacent structures, such as the vas deferens or ejaculatory ducts, into an enlarged utricle, with fewer than 30 cases of ectopic seminal tract openings documented in long-term retrospective studies.^[17]^[18] These anomalies stem from disruptions in the normal embryologic processes of Müllerian duct regression, as outlined in the embryology section.

Function

Role in ejaculation

The seminal colliculus, also known as the verumontanum, serves as the primary conduit for semen during ejaculation, with the paired ejaculatory ducts opening directly onto its posterior surface within the prostatic urethra. This anatomical arrangement allows for the propulsion of semen into the urethral lumen as orgasm progresses, ensuring efficient delivery toward the external urethral orifice.^[7] During the emission phase of ejaculation, rhythmic contractions of the smooth muscle surrounding the ejaculatory ducts and seminal vesicles propel spermatozoa and seminal fluid to the level of the seminal colliculus, where they are deposited into a peri-verumontanum antechamber. Subsequent contractions in the peri-verumontanum region, combined with an anterior tilting action of the colliculus itself, facilitate the expulsion phase by directing antegrade semen flow through the urethra. This dynamic mechanism ensures coordinated semen transport without requiring direct muscular activity from the colliculus structure.^[19]^[20] Although the seminal colliculus does not produce secretions itself, its position at the confluence of the ejaculatory ducts and prostatic urethra enables the mixing of prostatic fluid—rich in enzymes and nutrients—with contributions from the seminal vesicles, forming the complete seminal plasma essential for sperm viability and motility.^[7]^[19]^[20]

Role in micturition

The seminal colliculus, also known as the verumontanum, forms a raised eminence on the posterior wall of the mid-prostatic urethra, resulting in a slight narrowing of the urethral lumen at this site. In healthy individuals, this anatomical feature does not significantly impede urine flow during micturition, as the urethra accommodates the protrusion without causing obstruction under normal physiological conditions.^[21] A hypothesis proposes that the posterior protrusion of the seminal colliculus contributes to a Venturi effect within the prostatic urethra, where the accelerated urine flow over the narrowed region creates a localized pressure drop, potentially enhancing the velocity of the urine jet and aiding complete bladder emptying. This mechanism is thought to facilitate the drainage of prostatic fluid into the urethral stream by generating negative pressure near the colliculus, with calculations indicating a pressure reduction of approximately -1058 Pa during voiding. The colliculus and associated urethral crest are hypothesized to regulate flow rates and pressures to optimize this pump-like action.^[22] Positioned in the mid-prostatic urethra between the internal urethral sphincter at the bladder neck and the external urethral sphincter in the membranous urethra, the seminal colliculus plays a passive role during micturition. As both sphincters relax to permit voiding, the fixed structural mound does not actively contract or influence sphincter coordination but simply allows unimpeded passage of urine through the relaxed urethral segment.^[5] In younger individuals, the seminal colliculus exerts minimal impact on urinary function due to the absence of significant prostatic changes. However, age-related prostatic enlargement, such as benign prostatic hyperplasia, can secondarily compress the surrounding urethral tissue, altering flow dynamics around the colliculus and contributing to reduced urine stream velocity or incomplete emptying.

Clinical significance

Surgical applications

The seminal colliculus, also known as the verumontanum, serves as a critical anatomical landmark in transurethral resection of the prostate (TURP), guiding the depth of resection to prevent damage to the ejaculatory ducts and preserve antegrade ejaculation and urinary continence.^[23] During TURP, surgeons identify the verumontanum as a midline elevation on the posterior urethral wall and halt resection just proximal to it, avoiding injury to the external urethral sphincter located distal to this structure.^[24] This preservation is essential, as inadvertent resection beyond the verumontanum can lead to retrograde ejaculation or incontinence.^[25] In cystoscopy, the seminal colliculus aids navigation by orienting endoscopes within the prostatic urethra, facilitating accurate assessment during diagnostic evaluations or therapeutic interventions.^[26] Urologists use its position as a reference to establish the 6 o'clock orientation in the posterior urethra and to delineate the prostatic apex, enabling precise visualization of surrounding structures without disorientation.^[27] Its identifiable rounded eminence on the urethral crest allows for reliable endoscopic maneuvering in procedures such as bladder neck evaluations or biopsy guidance. Specific procedures target the seminal colliculus region, including transurethral ablation of prostatic utricle cysts, where holmium laser is employed to incise or vaporize the cyst wall while using the colliculus as a proximal landmark to avoid deeper urethral damage.^[28] Similarly, in the management of posterior urethral valves (PUV), endoscopic valve ablation relies on the verumontanum to localize the obstructive membranes in the distal prostatic urethra, with incision performed under cystoscopic guidance to relieve obstruction. These interventions carry risks of iatrogenic injury to the colliculus or adjacent urethra, potentially resulting in strictures that require subsequent dilation or urethroplasty.^[29]

Associated conditions

The seminal colliculus, also known as the verumontanum, is associated with several pathological conditions that can impact urinary and reproductive function. Posterior urethral valves (PUV) represent a key congenital anomaly involving this structure, consisting of obstructive membranes located distal to the seminal colliculus in the posterior urethra. These valves cause varying degrees of urinary tract obstruction, with an incidence of approximately 1 in 8,000 male live births, and frequently lead to bilateral hydronephrosis due to upstream pressure effects on the kidneys and ureters.^[30]^[31]^[32] Benign hypertrophy of the seminal colliculus can occur in both pediatric and adult populations, resulting in enlargement of the verumontanum that protrudes into the urethral lumen. This condition often manifests with lower urinary tract symptoms, including dysuria and intermittent hematuria, particularly when associated with cystic changes or inflammation.^[33]^[34] Obstruction at the ejaculatory duct openings on the summit of the seminal colliculus is a recognized cause of male infertility, leading to obstructive azoospermia by preventing sperm transport into the ejaculate. This form of ejaculatory duct obstruction accounts for 1-5% of infertility cases in men with azoospermia and low semen volume, often requiring diagnostic evaluation via transrectal ultrasound or seminal vesiculography.^[35]^[36] The seminal colliculus's anatomical proximity to the central zone of the prostate implicates it in certain prostate malignancies, particularly central zone tumors, where neoplastic involvement may extend to the ejaculatory ducts and verumontanum. Although rarely the primary site of prostatic adenocarcinoma, such tumors exhibit higher rates of ejaculatory duct invasion compared to peripheral zone cancers, with diagnosis typically involving targeted biopsy near the colliculus during transurethral or transrectal procedures.^[37]^[38]

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