# Bulbourethral (Cowper's) Glands: Anatomy and Function The bulbourethral glands, also called Cowper's glands, are a pair of small accessory sex glands that sit at the bulb of the urethra in many male mammals. They produce a clear, alkaline mucus that lubricates the urethra and pre-neutralizes its acidic environment before sperm pass through. They are not the prostate and they are not the seminal vesicles. They are separate organs with their own ducts, their own blood supply, and their own species-specific size and output. In humans each gland is about the size of a pea. In a boar, the same gland is enormous and contributes a gel-like fraction that can make up a large share of the ejaculate. In dogs, the glands are absent altogether. Those differences matter for anatomy exams, for imaging interpretation, and for anyone comparing reproductive biology across mammals. This guide covers the gross and microscopic anatomy of the bulbourethral glands, the mechanics of how they empty, what their secretion does, how they differ across species, and the rare conditions that bring them to clinical attention. A labeled male reproductive diagram accompanies this article as a separate figure. ## What the bulbourethral glands are, in one paragraph A bulbourethral gland is a paired, lobulated, tubuloalveolar gland positioned posterolateral to the membranous and bulbar urethra, embedded in the urogenital diaphragm and surrounded by skeletal and smooth muscle. Each gland drains through its own long duct into the bulbar urethra. The epithelium is columnar and secretes a mucoid, alkaline product rich in glycoproteins. The human gland is roughly pea sized, while in bulls and especially boars the gland is much larger and far more productive. In dogs and cats the gland is not part of the accessory gland set, and the dog relies on the prostate alone for accessory secretion. ## Location and Gross Anatomy ### Where the glands sit In humans, the two bulbourethral glands lie on either side of the membranous urethra, just above and behind the bulb of the penis, within the deep perineal pouch. Each is wrapped by the external urethral sphincter, the striated muscle ring commonly called the rhabdosphincter. Three-dimensional reconstruction of human cadavers shows the gland is surrounded laterally and posterosuperiorly by striated and smooth muscle respectively, and that the striated muscle bundle has a continuous connection running from the superficial transverse perineal muscle, the levator ani, and the external anal sphincter to the external urethral sphincter [1]. That anatomical detail is not trivia. It explains how the gland empties. ### The duct system Each gland has a single duct roughly 2.5 to 4 cm long in adult humans. The duct runs forward and downward, pierces the perineal membrane, and opens into the floor of the bulbar urethra. The two ducts usually open within a centimeter or so of each other, sometimes into a shared opening. The duct wall is thin and distensible, which is why it can dilate into a cystic structure called a syringocele under the right conditions [2]. ### Blood supply, lymphatics and nerves The glands receive branches of the internal pudendal artery, and the pudendal nerve supplies somatic innervation to the surrounding striated muscle. The smooth muscle component is supplied by autonomic fibers. In horses, researchers identified an unusually rich peptidergic innervation of the accessory glands, including neuropeptide Y, vasoactive intestinal peptide, and enkephalin-like immunoreactive endocrine cells inside the bulbourethral gland itself, which suggests the equine gland has paracrine signaling that has not been described in humans [3]. ### Microscopic structure The gland is a compound tubuloalveolar gland. A fibrous capsule sends trabeculae inward that divide the parenchyma into lobules packed with secretory acini. The lining is simple columnar epithelium containing principal secretory cells, columnar dense cells, and basal cells. In mice, the bulbourethral gland shows arborized acini lined by tall columnar epithelial cells, and immunofluorescence confirms expression of EphB4 and ephrin-B1 membrane proteins, with EphB2 detected only in the vesicular and bulbourethral glands [4]. Human Cowper's glands express glycoprotein 2 (GP2), a protein best known as a marker of intestinal M cells and pancreatic zymogen granules. Both the luminal cell membrane and the gland contents react strongly with an anti-GP2 antibody, and the stained regions are partly decorated with sialyl Lewis x glycans [5]. That glycan pattern hints at a role in cell recognition or mucosal defense, though the functional significance in the gland has not been settled. The rat bulbourethral gland is unusual among accessory glands because it carries a distinct striated muscular layer, and its basal cells are genuine myoepithelial cells, unlike the prostate where basal cells are not myoepithelial [6]. This is a useful contrast for anyone learning accessory gland histology, because it shows that "basal cell" does not mean the same thing in every gland. ## How the Glands Secrete: The Emission Mechanism The secretion is not a passive drip. It is actively expelled through coordinated muscle action. In humans, the current understanding from three-dimensional cadaveric dissection is that emission is carried out through cooperation of striated and smooth muscle. The striated muscle bundle connected to the external urethral sphincter compresses the gland during contraction, and the smooth muscle derived from the deep transverse perineal muscle enters between the two glands and their lobules to help squeeze the secretion out [1]. In rats, researchers characterized the striated muscular layer of the bulbourethral gland and showed that its fibers discharge in response to mechanical stimulation of the prepuce, glans, and penile urethra. The motor branch of the sacral plexus supplies this musculature, the same nerve that supplies the striated penile muscles. That finding demonstrates a somatic reflex arc driving glandular release rather than purely autonomic control [7]. The practical takeaway is that bulbourethral secretion is a reflex event tied to genital stimulation, not a continuous background process. That is why the secretion appears at the start of arousal and is largely finished before the main ejaculatory emission. ## What the Secretion Does The product is a clear, colorless to opalescent, viscous alkaline mucus. Its documented roles are lubrication and neutralization of residual acidity in the urethra. Because urine is acidic and leaves the urethra at low pH, sperm passing through would encounter an unfavorable environment without pre-treatment. The alkaline mucus buffers that residual acidity, and the mucus coat lubricates the passage. There is a common claim that pre-ejaculatory bulbourethral fluid carries enough sperm to cause pregnancy. That claim belongs to the pre-ejaculate as a whole and is a separate question from gland physiology. It is not a function of the gland itself. The gland's job is lubrication and pH, not sperm transport. A second common claim is that Cowper's glands produce most of the semen volume. They do not. The bulk of semen volume comes from the seminal vesicles and the prostate. Bulbourethral output is small in humans, measured in fractions of a milliliter. In species where the gland is large, its role is correspondingly larger. Boars produce a copious gelatinous fraction from the bulbourethral glands that forms a plug in the female tract. That plug is a reproductive strategy, not just lubrication. ## Comparative Anatomy Across Species Size and presence of the bulbourethral glands vary dramatically across mammals. The table below summarizes the standard textbook picture for five familiar species. | Species | Presence | Approximate size (adult) | Secretion and role | |--|--|--|--| | Human | Present, paired | Pea sized, roughly 0.5 to 1 cm each | Small volume of clear alkaline mucus. Lubricates and buffers the urethra. | | Bull | Present, paired | Walnut to egg sized | Thick, ropy secretion contributes a small but consistent pre-ejaculatory fraction. | | Boar | Present, very large | Among the largest of any mammal, up to several hundred grams | Copious gel-forming secretion that makes up a large fraction of the ejaculate volume and forms a cervical plug. | | Stallion | Present, paired | Smaller than the boar but well developed | Thin, watery to mucoid secretion, minor volume fraction. | | Dog | Absent | Not applicable | No bulbourethral contribution. The prostate provides all accessory secretion. | The boar is the classic exam example of a huge bulbourethral gland. A boar's ejaculate is delivered in fractions, and the bulbourethral component is the gelatinous, sperm-poor tail end that coagulates in the cervix. Because it is so voluminous, the boar's gland is used as a model in andrology and in studies of gel fraction biology. Not every species has the same gland complement, and the differences are deeper than size. In many bats, the accessory gland set consists of an intra-abdominal prostatic complex, urethral glands, and a pair of inguinal bulbourethral glands, with no ampullary glands and no seminal vesicles [8]. In *Noctilio albiventris* and *Rhynchonycteris naso*, the bulbourethral glands sit extra-abdominally in the inguinal region and have a simple columnar epithelium with PAS-positive secretion [9]. The aardvark has seminal vesicles, a prostate gland, and bulbourethral glands with histology similar to other species [10]. The sugar glider has a prostate in the pelvic cavity but its accessory gland set differs from the standard eutherian arrangement [11]. The African straw-colored fruit bat shows seasonal changes in the gland, with larger acini and greater epithelial heights during the rainy season than the dry season [12]. In dogs, the bulbourethral glands are absent. This is why the canine prostate is the sole accessory gland and why canine prostate disease has no parallel bulbourethral pathology to confuse it with. Cats also lack bulbourethral glands. For practitioners and students, knowing the canine accessory gland set is prostate only is a common exam point. ## Development and Innervation Details That Show Up on Imaging ### Development The bulbourethral glands develop from endodermal outgrowths of the urogenital sinus, similar in origin to the prostate. This shared origin explains why both glands can express overlapping markers, and why Cowper's gland tissue can occasionally be mistaken for prostate tissue on biopsy or imaging. ### The sphincter relationship over time In the fetus, the striated urethral sphincter consists of scantily dispersed muscle fibers. In young infants, it becomes well defined with a tail-like structure running posteriorly toward the perineal body. At 3 to 4 months of life, at the level of the bulbourethral glands, that tail disappears and the sphincter becomes horseshoe-shaped, with two branches bifurcating posteriorly to envelop the glands [13]. This maturational change is one reason pediatric urethral anatomy differs from adult anatomy, and it influences how syringoceles and sphincter abnormalities present at different ages. ### Normal imaging appearance Normal Cowper glands and ducts are imperceptible at imaging [2]. That is a critical baseline. If you can see the gland on routine cross-sectional imaging, something is abnormal or the gland is hyperplastic. Cowper's gland hyperplasia can present as an encapsulated mass of 1.3 to 3.0 cm with heterogeneous low or intermediate T2 signal intensity in the genitourinary diaphragm, and targeted biopsy of such a lesion shows benign Cowper's gland tissue. It is a recognized pitfall at MRI of the prostate because it can mimic an exophytic benign prostatic hyperplasia nodule or local spread of prostate cancer [14]. ## Conditions That Affect the Glands ### Cowperitis Cowperitis is inflammation of the gland or its duct. Recurrent cowperitis is usually associated with an underlying syringocele, a urethral fistulous connection to the gland, or a perianal fistula. MRI detects the underlying structural abnormality and the superimposed inflammation [2]. ### Syringocele A Cowper duct syringocele is a congenital or acquired dilatation of the Cowper duct in the bulb of the corpus spongiosum. Retrograde urethrography is the standard test to diagnose a communicating (open) syringocele, and MRI is used for further morphologic assessment [2]. Syringoceles are uncommon but not vanishingly rare. In a series of 100 consecutive urethrograms from patients with unreconstructed strictures, 33 percent showed filling of Cowper ducts or glands, occurring predominantly in patients with bulbar strictures. Among 26 symptomatic patients, 15 presented with poor flow, 8 with perineal pain, and 4 developed perineoscrotal abscesses. In 4 patients a grossly dilated Cowper duct obstructed the urethra [15]. The same study found that only 1 of 26 patients with non-bulbar strictures had a visible duct or gland, so the association is specific to bulbar disease. In children, a retrospective series of 122 patients found that half also had posterior urethral valves or a single flap valve. The overall incidence of syringoceles seen at urethrocystoscopy in that series was 3.0 percent [16]. A 2025 case report described a 6-month-old boy with a Cowper's gland syringocele and a simultaneous left ureteropelvic junction obstruction, treated with transurethral incision and laparoscopic pyeloplasty in a single anesthetic [17]. The combination is exceedingly rare, with only two prior reported cases. ### Cowper stones Cowper stones are typically radiopaque and are found in both the Cowper gland and the duct [2]. ### Hemorrhagic cyst A Cowper gland hemorrhagic cyst may be found incidentally at MRI of the prostate. Follow-up MRI may be required to confirm resolution of hemorrhage and exclude an underlying neoplasm [2]. ### Carcinoma Cowper gland carcinoma is rare. Reported types include adenocarcinoma and adenoid cystic carcinoma [2]. ### Perineal swelling and abscess When the gland or duct obstructs, infection can track into the perineum. In the adult stricture series, four patients developed perineoscrotal abscesses from complex multicystic lesions within the bulbourethral glands [15]. Perineal pain and swelling in a male with a history of urethral stricture should raise the question of gland involvement. ## Common Mistakes and Limitations **Confusing the bulbourethral glands with the prostate.** They are separate organs with separate ducts, separate blood supply, and separate pathology. On MRI, Cowper's gland hyperplasia is a documented mimic of prostate cancer extension, and the distinction is made by location and biopsy [14]. **Confusing them with seminal vesicles.** The seminal vesicles contribute the largest share of semen volume in humans. The bulbourethral glands contribute a small mucus volume. **Assuming all mammals have them.** Dogs and cats do not. A canine prostate exam has no bulbourethral component to consider. **Assuming the human size applies everywhere.** The human gland is pea sized. The boar gland is vastly larger and produces a gel fraction. Exam questions and comparative anatomy discussions hinge on this difference. **Expecting to see the glands on routine imaging.** Normal Cowper glands and ducts are imperceptible [2]. Visibility implies hyperplasia, cyst, stone, or inflammation. **Treating a syringocele as automatically clinically significant.** In the adult series, 11 of 26 symptomatic patients had symptoms caused by a coexisting bulbar stricture, and the syringocele was incidental [15]. Treating the stricture may resolve the picture without addressing the syringocele. **Individual cases need veterinary or medical assessment.** The pattern described here is general. A specific animal or person with perineal swelling, urinary obstruction, or a suspected gland abnormality needs an individual workup. ## Frequently Asked Questions ### What are the bulbourethral glands? The bulbourethral glands, or Cowper's glands, are a pair of small accessory sex glands at the bulb of the urethra that secrete clear alkaline mucus to lubricate and buffer the urethra. ### Are Cowper's glands the same as the prostate? No. They are separate organs with their own ducts and their own pathology. The prostate surrounds the urethra below the bladder, while Cowper's glands sit in the urogenital diaphragm. ### Do dogs have bulbourethral glands? No. Dogs lack bulbourethral glands. Their only accessory sex gland is the prostate. ### Why are the bulbourethral glands so large in boars? The boar's gland produces a copious gel-forming secretion that becomes a large fraction of the ejaculate and forms a plug in the female reproductive tract. ### What does bulbourethral secretion do? It lubricates the urethra and neutralizes residual urinary acidity so sperm pass through a more favorable environment. ### Can Cowper's glands become infected? Yes, though it is uncommon. Cowperitis is inflammation of the gland or duct and is often linked to an underlying syringocele or fistulous connection. ### What is a Cowper's syringocele? It is a cystic dilatation of the Cowper duct in the bulb of the corpus spongiosum, either congenital or acquired, and it can obstruct urine flow in severe cases. ### How are the glands controlled? Secretion and emission depend on cooperation between striated and smooth muscle around the gland, with a somatic reflex component triggered by genital stimulation. ## Related Articles - [Bovine Mammary Gland Anatomy and Lactation Physiology](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/bovine-mammary-gland-anatomy-lactation-physiology) - [Canine Endocrine System: Glands and Hormonal Regulation](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-endocrine-system-glands-hormonal-regulation) - [Equine Larynx Anatomy and Function in Respiration](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-larynx-anatomy-function-respiration) - [Canine Lymphatic System: Anatomy and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/canine-lymphatic-system-anatomy-function) - [Equine Hoof Anatomy: Structures and Function](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/equine-hoof-anatomy-structures-function) - [Fish Gills: Anatomy, Function, and Common Health Issues](/knowledge/animal-farming/aquaculture/fish-gills-anatomy-function-and-common-health-issues) - [Lacrimal Gland: Anatomy and Tear Production](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/lacrimal-gland-anatomy-and-tear-production) - [Submandibular Gland: Anatomy, Ducts, and Comparative Notes](/knowledge/veterinary-medicine/veterinary-anatomy-physiology/submandibular-gland-anatomy-ducts-and-comparative-notes) ## Sources 1. 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