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The urethra is the main place of entry for sexually transmitted pathogens. However, there is little literature on the morphology of the urogenital system, principally the urethra and ducts of the sex accessory glands. The Mongolian gerbil is an insectivorous, herbivorous and monogamous rodent with nocturnal habits; it has been used successfully as a laboratory animal since the 1960s. Therefore, the objective of the present paper was to describe the structure and ultrastructure of the urethra and its relations to the ducts of the accessory sex glands of the Mongolian gerbil (Meriones unguiculatus), contributing to the understanding of the reproductive biology of the rodent and aiming to provide data for future experimental studies. Conventional techniques of light and scanning electron microscopy were utilized. The urethra and ducts of the accessory sex glands are similar to those of the albino rat and the mouse. However, there is variation in drainage type among accessory sex glands for the inner urethra. The ducts of the seminal vesicle, the ductus deferens, drain their contents independently into the ampullary duct that opens in the urethra. The ducts of the prostate, coagulating and bulbourethral glands drain their contents independently into the urethra.
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The origin of the distal part of the male urethra has been disputed by researchers, although there is general agreement on the development of the proximal part (Kanagasuntheram & Anandarajara, 1960). Embryologists have stated that the urethra within the navicular fossa was formed by an ectodermal surface, while the remainder of the urethra was formed by an endodermal surface. Despite the embryological and histological interest described above, the urethra has been given little attention (Nakano & Muto, 1989), in contrast to the abundant studies on the urinary bladder, ureter and kidney (Fulker et al. 1971; Kumagai, 1975; Kjaer et al. 1976; Newman & Hicks, 1977; Tannenbaum et al. 1978; Hayakawa, 1979; Nelson et al. 1979).
To date, the study of the structure of the sex ducts in mammals (including man and laboratory rodents), the accessory glands, has been reviewed only partially. The Mongolian gerbil (Meriones unguiculatus) is a rodent known for its suitability for laboratory use. By nature, it is a curious and friendly rodent, almost without odour, with monogamous behaviour and physiological mechanisms of corporal-water conservation, presenting spontaneous attacks of epilepsy and relative absence of natural illnesses. The purpose of the present paper was to describe the structure of the pelvic and penile urethra and their relationship to the ducts of the accessory sex glands, and to provide data for future experimental studies.
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Dellmann (1993) considered three distinct portions in the urethra: pelvic, from the neck of the urinary bladder to the penile bulb; bulbar contained in the penile bulb; and penile, from the bulbous extremity to the external ostium of the urethra. Getty (1986) and Dyce et al. (1997) described two portions of the male urethra in domestic animals: pelvic, from the internal ostium of the urethra to the seminal colliculus; and penile, from the seminal colliculus to the external ostium of the urethra. According to Nakano & Muto (1989), the urethra of various mammals including man is divided into three parts: prostatic, membranous and spongy (penile). The prostatic urethra is lined with the transitional epithelium, while the membranous and spongy parts are lined with stratified (or pseudostratified) columnar epithelium. According to Carrol & Dixon (1992), anatomically the human male urethra can be divided into posterior and anterior. The posterior urethra is composed of the membranous and prostatic segments. The anterior urethra is composed of the penile urethra with its bulbar and pendulous segments. Williams et al. (1989) decsribe four regional parts in the human male urethra: preprostatic, prostatic, membranous and spongiose. In the rat and mouse, according to Hayes (1965), the urethra presents two portions: pelvic and penile. The pelvic urethra initiates in the internal ostium of the urethra and finishes in the penile bulb. The penile bulb contains a large urethral diverticle, an expansion of the penile urethra. The penile urethra initiates in the penile bulb and extends to the external ostium of the urethra. In the gerbil, the urethra was divided into pelvic and penile, similar to that seen in the rat, mouse and domestic animals.
The penile bulb of the gerbil shelters the urethral diverticle that receives the duct of the bulbourethral gland. The diverticle was situated between the pelvic and penile urethra, matching the point of urethral transition, the isthmus. The topographical anatomical position of the urethral diverticle in the gerbil is comparable to that of the rat and the mouse (Hayes, 1965; Çiner et al. 1996) and with the suburethral diverticle in the sow and the cow. In these animals the urethra forms a small pouch immediately before entering the vagina (Çiner et al. 1996). Among domestic mammals, the male urethra of small ruminants extends beyond the tip of the penis, forming the urethral process (processus urethrae) (Ghoshal & Bal, 1976), and is not a homologue of the urethral diverticle in the gerbil, rat or mouse. Goats, sheep, cattle and swine present a urethral recess projecting dorsocaudally in the region of the junction between the pelvic and penile urethra. The urethral recess constitutes the urethral diverticle that, like that in the Muridae, receives the duct of the bulbourethral gland (Garrett, 1987).
The urethral diverticle of the gerbil is heart-shaped, presenting bilateral symmetry. In accord with Çiner et al. (1996), the bifid organization of the diverticle, also heart-shaped, can be considered a remnant from an original paired system, which is fused incompletely in the Muridae.
Next, the pelvic urethra of the gerbil presents its transition epithelium origin, changing to stratified (or pseudostratified) cylindrical, being similar therefore to those of man (Leeson & Leeson, 1977) and mouse (Nakano & Muto, 1989). Already, the penile urethra presents a lesser diameter than the pelvic urethra. The stratified cylindrical epithelium and rich propria lamina in the elastic, collagenous and smooth muscle fibres determine the folds in the mucosa (Leeson & Leeson, 1977; Nakano & Muto, 1989; Dellmann, 1993).
Throughout the extension of the male gerbil urethra, in the mucosa, urethral glands were observed, related to those of female rodents (Shehata, 1974), but rarely identified in humans (Newman & Hicks, 1981). The appearance of the female urethral glands only differs from the male in the openings of the ducts.
The glandular portions of the seminal vesicle are constituted by a simple cylindrical epithelium. The duct is located dorsally to the ductus deferens and its opening occurs in the ampullary duct, as in the albino rat (Hebel & Stromberg, 1976). In the gerbil, there is no formation of the ejaculatory duct as in primates, and in this way the gerbil is similar to the chinchilla (Martinez et al. 1995). The duct of the coagulating gland opens independently in the roof of the urethra, as occurs in the albino rat (Jesik et al. 1992). In the gerbil, a larger number of ventral ducts drain the dorsolateral lobe of the prostate. The ducts drain independently in the pelvic urethra. According to Jesik et al. (1992), the ventral lobes are drained by four or five ducts that originate in the central portion of the lobe and follow a straightforward course to the ventrolateral muscle layers of the urethra wall. Small ducts are occasionally observed that drain the acini and enter the main channels, which in turn empty into the urethra. In the lateral lobe, each duct drains only three or four acini, and the acini and their ducts are wrapped in connective tissue. There are seven or eight ducts draining each lateral lobe. The portions of the lateral lobe closest to ventral lobe area are drained by two ducts that enter the urethral lumen ventrolaterally. Four ducts that enter the urethral wall medially drain the middle portion of the lateral lobes. The most anterior parts of the lateral lobes are drained by one or two ducts that enter the dorsal urethra. The dorsal lobes are each drained by 10–14 long and slender ducts that enter the dorsal roof of the urethra. In the gerbil, it was not possible to quantify the number of ducts. However, in the albino rat it was shown that the biggest prostatic lobe (dorsolateral) possesses the greatest number of ducts. The heights of the epithelium in the ventral and dorsolateral prostatic lobes and in the ductus deferens gland do not present differences among themselves.
The ducts of the ductus deferens glands form diverticles, surrounding the papilla of the ductus deferens, constituting the ampullary duct, similar to those described in the albino rat (Hebel & Stromberg, 1976). They are presented firstly as next to the muscular layer of the ductus deferens and, subsequently, constitute the ampullary duct.
The ducts of the seminal vesicle, the ductus deferens gland and ductus deferens, drain their contents independently through the ampullary duct that opens into the pelvic urethra. The prostatic ducts and both coagulating and bulbourethral glands drain their contents independently into the pelvic urethra. It has been concluded that the structures of the urethra and of the ducts are similar to those of the albino rat (Hebel & Stromberg, 1976), the mouse (Greene, 1966) and the golden hamster (Silva et al. 1995). However, there are variations in the disposition of the opening of the ducts of the sex accessory glands toward the interior of the urethra.
|CG||coagulating gland/duct of the coagulating gland|
|D||duct of the bulbourethral gland|
|DL||dorsal prostatic lobe/dorsal prostatic duct|
|F||floor of the urethra|
|GDD ||ductus deferens gland|
|L||lateral part of the bulboespongiosus muscle|
|M||medial part of the bulboespongiosus muscle|
|Pa||papilla of the ductus deferens|
|R||roof of the urethra|
|SV||seminal vesicle/duct of the seminal vesicle|
|VL||ventral prostatic lobe/ventral prostatic duct|