Abstract
Free full text
The Uterus Duplex Bicollis, Vagina Simplex of Female Chinchillas
Abstract
The available literature describing the morphology of the female chinchilla's uterine cervix varies and includes phrases such as ‘the cervical canal,’ ‘a single cervix,’ and ‘the cervix;’ alternatively, some publications describe 2 cervices. In this report, we provide an anatomically correct and definitive description of the uterine cervical morphology of the laboratory chinchilla. We further propose revised, anatomically precise nomenclature to characterize the female chinchilla reproductive tract as a whole.
Domesticated chinchillas (Chinchilla lanigera) are kept as pets, used in fur farming, and are a valuable laboratory species. Publications about chinchilla husbandry and biology commonly include descriptions of the female chinchilla reproductive tract. These publications consistently state that the female chinchilla has 2 uterine horns, but descriptions concerning the chinchilla uterine cervix are inconsistent. One reference mentions “the cervical canal” of chinchillas.2 Other references either state that chinchillas have “2 uterine horns, each with a separate opening into a single cervix”13 or have 2 uterine horns that “open separately into the cervix.”9-11,14,20 Additional references either state or imply (in diagrams) that chinchillas have 2 uterine horns and 2 cervices.1,5,6,12,17
The present report seeks to definitively classify the uterine cervical morphology of laboratory chinchillas (Chinchilla lanigera), order Rodentia, suborder Hystricomorpha. The intention is to establish a system for consistent morphologic description in other species of importance to laboratory animal research.
Materials and Methods
Animals.
We obtained 13 adult female chinchilla cadavers from a terminal study that did not require the reproductive tracts. The terminal study was approved by the University of Georgia IACUC, and the animals were euthanized in accordance with the AVMA guidelines. The animals originally were obtained from a breeding colony (Rauscher Chinchilla Ranch, La Rue, OH) and were mature females that were past their breeding prime.
Preliminary radiography.
The female reproductive organs, extending from the ovaries to the vulva, were removed by dissection from a fresh cadaver. The uteri were opened at their most cranial extremities to expose the uterine lumens. A 16-gauge ethylene tetrafluoroethylene intravenous catheter (Terumo, Somerset, NJ) was threaded into each lumen and stabilized by placing ligatures of 4-0 silk (Ethicon, Blue Ash, OH) around the outside of the uterus. Iohexol (240 mg I/mL; GE Healthcare, Pittsburgh, PA) diluted 50% with PBS was injected through the catheter into the lumen of the uterus until the lumen was full and contrast material flowed into the vagina. Contrast material was injected in this manner bilaterally, because it did not flow freely between the 2 uteri. A ligature was placed around the vagina near the vulva to retain the contrast medium within the lumen of the vagina and the cervical canals. The radiographic exposure factors were 5 mA, 35 kVp, and 30-s exposure time, used with a Torrex 120 Cabinet X-Ray unit (Torr X-Ray, Van Nuys, CA) and a Kodak RP X-Omatic processor (model M6B, SourceOne Healthcare Technologies, Mentor, OH).
Gross anatomy.
The female reproductive organs were removed from each cadaver. The suspensory ligaments and accompanying vasculature of the ovaries were transected, and the ovaries and uteri were reflected toward the vagina, with transection of the broad ligaments as the uteri were reflected. The urinary bladder and urethra were intimately associated with the ventral side of the reproductive tract; therefore the ureters were transected, and the urinary bladder and urethra were included with the reproductive tract. The ossa coxarum were separated by using bone cutters to cut through the pelvic symphysis, and the vagina was dissected to include the vulva and anus, transecting the rectum from the anus in the process. The tracts were rinsed briefly with tap water, placed in PBS (pH 7.4; Sigma, St Louis, MO), and refrigerated at 4 °C overnight. The following day, the tracts were carefully positioned in a dorsoventral plane between layers of gauze prior to placement in fixative (Carson modified Millonig 10% buffered formalin). The desired positioning was a flat presentation of the uterus through the cervix and into the vagina. The pull of the urinary bladder on the ventral surface of the vagina as the bladder was drawn laterally to achieve the desired positioning of the reproductive tract tended to create some twisting at the uterine cervix, resulting in varying degrees of undesirable superimposition of the cervical canals when sectioned for histology. The urinary bladder could have been excised and removed, but the organ was retained because it was a useful and needed marker for the ventral surface of the reproductive tract after the reproductive organs were removed from the cadaver.
Gross imaging.
The reproductive organs were removed from formalin, rinsed with tap water, and lightly blotted to remove excess water. The uteri were opened at their most cranial extremities to expose the uterine lumens. An 11.4-cm, open-end catheter (Covidien, Mansfield, MA) was inserted into the lumen of one of the uteri and threaded toward the cervix, stopping when resistance was met just cranial to the internal uterine ostium. The opposite uterus was then slit along its lateral edge to the internal uterine ostium; the incision was continued through the cervical canal and the external uterine ostium. The catheter was then threaded through the cervical canal of the opposite uterine cervix until it emerged through the external uterine ostium. Images were made by using a stereomicroscope (model SZX9, Olympus, Tokyo, Japan) and a digital camera (model DP25, Olympus).
Histology.
The reproductive organs were removed from cadavers as previously described and placed in PBS (pH 7.4; Sigma). The organs were kept at 4 °C overnight and then transferred into Carson modified Millonig 10% buffered formalin. The organs were trimmed , loaded into embedding cassettes (Simport, Beloeil, Quebec, Canada), and processed in a Tissue-Tek VIP vacuum infiltration processor (Sakura Finetek USA, Torrance, CA). The tissue was embedded in a Tissue-Tek embedding console system (Miles, Elkhart, IN). Sections were cut by using a rotary microtome (model 1512, Wild Leitz, Wetzlar, Germany), and mounted on Superfrost Plus microscope slides (Fisher Scientific, Pittsburgh, PA). Routine staining with hematoxylin and eosin was performed robotically by using an Autostainer XL (Leica Microsystems, Buffalo Grove, IL). Special stains were performed manually. Mayer mucicarmine stain (Poly Scientific R and D, Bay Shore, NY) for epithelial mucin was selected as a special stain to enable differentiation between the endometrial and cervical epithelia. The periodic acid–Schiff reaction (Poly Scientific R and D) was used to demonstrate basement membranes and cervical mucin. A hematoxylin counterstain (Poly Scientific R and D) was used with both special stains.
Results
Radiography.
Contrast medium flowed freely through the cervices of the dissected chinchilla reproductive tracts. The medium revealed 2 distinct and parallel cervical canals (Figure 1).
Chinchilla uteri, cervices, and vagina. Iohexol contrast medium fills the lumen of both uteri, the cervical canals, and the vaginal lumen. Air bubbles that were introduced during injection of the contrast medium are present in the uterine and vaginal lumens. The arrows indicate the 2 separate cervical canals.
Gross imaging.
The portio vaginalis of the uterine cervices presented grossly as a single main horizontal slit and several smaller vertical slits radiating outwards. Each uterus had a separate cervix, with a shared septum between the 2 cervical canals. The external uterine ostia of the 2 cervical canals were not discernible at the portio vaginalis (Figures 2 and and33).
Chinchilla portio vaginalis and cranial vagina. The ventral vaginal wall is excised and reflected to expose the portio vaginalis of the uterine cervices. The portio vaginalis of this multiparous female chinchilla has a single main horizontal slit (arrow), with smaller vertical slits radiating outward. PV, portio vaginalis; DVW, dorsal vaginal wall; VVW, ventral vaginal wall (reflected).
Chinchilla uterine cervix and cranial vagina. This is the same specimen as seen in Figure 2, oriented in the opposite direction. A catheter (CA) was inserted into the lumen of the farthest uterus and threaded through the cervix, emerging through the external uterine ostium. The closest uterus was opened with a lateral incision that was made from the uterine lumen through the cervical canal and external uterine ostium. CS, cut surface; DVW, dorsal vaginal wall; arrow, external uterine ostium of closest (incised) uterus; rectangle, primary cervical mucosal folds (plicae); star, septum between the cervical canals.
Histology.
Transverse- and dorsal-plane sections were made of the 2 cervices. The sections began at the caudal portions of the uteri near the internal ostia and ended at the portio vaginalis, including the initial portion of the vagina. The layers of the uterine walls were typical until the uteri came into close proximity, at which point layers were either lost or fused. Unless otherwise noted, our later descriptions of the uterine wall focus on the region of contact and fusion between the 2 uteri.
Transverse-plane sections.
The perimetria and the outer longitudinal layers of the myometria were not visible in the region of apposition. The inner layers of the myometria had both longitudinally and circularly arranged smooth-muscle fascicles. The combined strata vasculare provided an approximate line of demarcation between the inner layers of the myometria, but blending of the smooth muscle fascicles was present between the 2 layers. On the outer perimeter of the 2 apposed uteri, the outer longitudinal layers of the myometria were continuous; the perimetria were continuous as well.
The lamina epithelialis of the uterine mucosa ranged from simple cuboidal to simple columnar cells; the same types of epithelia also lined the endometrial glands, which were simple coiled tubular glands with some branching. The endometrial epithelium and that lining the endometrial glands were both negative for mucins.
The lamina epithelialis of the uterine cervical mucosa contained a mixture of epithelial types; simple and stratified cuboidal and columnar epithelia were present, as was a pseudostratified columnar epithelium. The cervical epithelium was an epithelium in transition near the external uterine ostium, where the tissue acquired characteristics of the stratified squamous epithelium of the vagina (note that an epithelium in transition is not the same as transitional epithelium, which is a distinct classification of epithelium). Intraepithelial lymphocytes were frequently noted. Mucin staining was abundant within the uterine cervical mucosa, providing a means to differentiate between the uterus and the uterine cervix, in addition to the demarcation provided by the presence or absence of the endometrial glands. The cervical propria–submucosa exhibited primary, secondary, and tertiary folds. The cervical folds frequently had dilated lumens filled with mucin, especially toward the base of the folds. In some sections, particularly those in which the cervical folds were cut obliquely, there was a false impression that there were simple branching tubular glands present within the cervical mucosa.
Dorsal-plane sections.
The perimetria and myometria of the 2 uteri remained separate until they were in close proximity near the uterine cervices. In the region of close proximity, the perimetrial layers fused and then ended, as did the outer longitudinal layers of smooth muscle of the myometria. The strata vasculare persisted but appeared to anastamose; these helicine vessels became smaller and less coiled as they continued between the cervical canals. The inner layers of the myometria had both longitudinally and circularly arranged smooth-muscle fascicles. The smooth-muscle fibers (cells) within the fascicles were hyperplastic. At the level of the uterine cervices, as evidenced by the cessation of the endometrial glands and the beginning of the mucin-positive cervical mucosa, the smooth-muscle fibers became hypertrophied and extended into the cervical folds to the level of the propria–submucosa. The tunica muscularis of each cervix contained smooth-muscle fascicles in both longitudinal and circular arrangement; frequent and regularly arranged fascicles extended in a third plane from the muscularis into the propria–submucosa. Near the portio vaginalis of the uterine cervices, the fascicles of smooth muscle become very blended.
The simple, branching, coiled tubular glands of the endometrium ceased at the cervical mucosa. The lamina epithelialis of the cervical folds ranged from simple columnar epithelium (nearer the uterine mucosa), pseudostratified columnar epithelium (throughout the bulk of the cervical mucosa), and stratified columnar epithelium (near the vaginal mucosa). Intraepithelial lymphocytes were common. The lamina epithelialis became more atypical toward the external uterine ostium, exhibiting the characteristics of an epithelium in transition. Mucin staining was absent in the endometrial glands, although the cervical mucus that extruded into the uterine lumen stained positively for mucins. The lamina epithelialis of the cervical mucosa had abundant apical cytoplasmic granules that stained positively for mucins. In addition, all luminal mucus within the cervical canals stained positively for mucins. The mucin-positive epithelium began at each of the internal uterine ostia and continued through the cervical canals to the external uterine ostia. The epithelium in transition near the external uterine ostia stained positively for mucins for variable distances, but the positive signal eventually ceased near the portio vaginalis of the uterine cervices.
Discussion
This report provides a detailed characterization of the chinchilla uteri. Presently, the literature contains limited and sometimes conflicting information in this regard. In our hands, when dorsal-plane sections demonstrated the desired epithelial transitions, they generally did not simultaneously demonstrate the dual nature of the cervical canals. Transverse-plane sections were especially difficult to interpret; documentation of the presence or absence of a Y-shaped cervical canal was unachievable with this orientation.
Normal chinchilla cervical canals grossly are lined by numerous primary mucosal folds (plicae; Figure 3). Microscopically, secondary and tertiary folds are present also; some of these folds resemble simple branching tubular glands (Figure 4). The common lumen of each cervical canal did not always appear to communicate with the lumen of each of the folds that radiated from the canal (Figure 5); this microscopic finding is not uncommon in a tubular organ with deep mucosal folds. For example, uterine tube mucosa typically has this appearance when sectioned transversely. The lumens of the mucosal folds were often dilated with accumulations of secretory material that stained positive for mucin (Figure 5). Microscopically, the cervical canals terminated separately and simultaneously into the grossly visible horizontal mucosal slit of the portio vaginalis (Figure 6 D).
Chinchilla uterine cervix; microscopic imaging, dorsal plane. PAS stain of cervical mucosal folds. Note the mucin-filled apical cytoplasm of the lamina epithelialis. PF, lumen of primary fold; SF, lumen of secondary fold.
Chinchilla uterine cervix; microscopic imaging, obliqued transverse plane. Mayer mucicarmine stain of cervical mucosal folds. EO, external uterine ostium; CC, cervical canal lumen; DL, dilated cervical fold lumen filled with mucus.
Chinchilla uteri and uterine cervices; microscopic imaging, dorsal plane. (A) PAS stain of uterus. The lamina epithelialis of the endometrium was negative for mucins. UL, uterine lumen; rectangle, simple coiled tubular endometrial gland. (B) Cranial cervical regions, hematoxylin and eosin stain. There are 2 separate cervical canals. CC, cervical canal lumen. (C) Middle cervical regions, hematoxylin and eosin stain. The 2 cervical canals remain separate. CC, cervical canal lumen. (D) Caudal cervical regions, hematoxylin and eosin stain. Each cervical canal terminates at an external uterine ostium. CC, cervical canal lumen; EO, external uterine ostium.
Because the transverse-plane orientation was less informative, we sectioned the chinchilla cervices in longitudinal orientation (dorsal plane). Six of these specimens were oriented well enough to document the dual cervical canals of chinchillas (Figures 6 and and7).7). By the strictest definition, chinchillas do not have a ‘true’ duplex uterus like those of monotremes and most marsupials, which have 2 completely separate uteri and 2 vaginae. However, a ‘uterus duplex’ may be the most accepted classification for chinchillas5,12. As stated previously, duplex uteri are “often joined externally at their cervical ends but always open independently into 2 cervical canals,” thus implying the presence of 2 internal uterine ostia.15 Chinchillas clearly have 2 internal uterine ostia and 2 cervical canals housed within what appears grossly to be a single anatomic cervix. The author15 also notes that for the bicornuate uterus 2 horns are joined for 5% to 50% of their lengths and always have a body (corpus) and a single cervical canal, indicating the presence of a single internal uterine ostium. By definition, chinchillas cannot have a uterine body; because the species has 2 internal uterine ostia, it does not have a bicornuate uterus (uterus bicornis). In addition, the author15 says that “in some species, these canals may join within the cervical region and then communicate with the vagina by a single [external] ostium,” implying some species have a Y-shaped cervical canal. In the present study, we found no evidence of a Y-shaped cervical canal in chinchillas. Two separate external uterine ostia were documented microscopically; however, grossly these ostia are hidden by the bulging or overhanging mucosa of the portio vaginalis (Figure 2).
Finally, using the periodic acid–Schiff reaction and Mayer mucicarmine methods for staining epithelial mucin, we found clear demarcation between the uterine mucosa, the cervical mucosa, and the vaginal mucosa. With these methods, the epithelium lining the cervical folds stained a bright magenta due to the abundant mucin granules in the apical cytoplasm of the cervical epithelium (Figure 4). This pattern does not occur in the endometrial epithelium because of its typically serous secretions (Figure 6 A), nor does it occur in the stratified squamous epithelium of the vagina. From the gross anatomic perspective, the uterine cervix of chinchillas appears to be a single structure. Microscopically, there is fusion of the perimetrium and part of the myometrium of the 2 uteri where the 2 structures come into close proximity, which begins very near the uterine cervices. Some references either state that chinchillas have 2 uterine horns that open separately into a “single cervix”13 or into “the cervix.”9-11,14,20 Other references either state or imply (through a drawing) the presence of 2 uterine horns and 2 cervices.5,6,12,17 These discrepancies in the literature demonstrate the current confusion regarding chinchilla uterine morphology. The source of the confusion appears to be due to the fact that chinchillas grossly appear to have a single cervix, thus providing a plausible explanation of why so many references refer to “the cervix,” indicating a single gross anatomic structure.
The rabbit grossly appears to have 2 anatomic cervices, and accordingly, there appears to be little confusion in the literature concerning rabbit cervices.4,7,8,19 Interestingly, diverging opinions concerning rabbit uteri do emerge. Authors range from describing rabbit uteri as “duplex uteri” (but then referring to the uterine horns), characterizing the uteri as “bicornuate” (which implies the presence of a uterine body), and depicting the presence of “2 separate uterine horns” (again, implying a uterine body).4,7,8,16,19 If the reason for the varied descriptions of the chinchilla cervix is ultimately related to whether the chinchilla has 1 or 2 gross anatomic cervices, then the discrepancy should be cleared by referring to the definition of the term ‘cervix.’ Unfortunately, this resolution is not straightforward because the uterine cervix is generally defined as the neck of the uterus and more specifically as the region starting at the isthmus and extending into the vagina.3,18 The definitions do not include any description of the cervical canal(s), assumedly because they were derived from descriptions of normal human anatomy, and humans typically have a single cervical canal.
The Nomina Histologica21 lists 2 subdivisions of the cervix: the portio prevaginalis and the portio vaginalis. Again the number of cervical canals is not addressed, presumably because domestic species normally have only a single uterine cervix and a single cervical canal. According to anatomic terminology for humans and domestic species, a uterine cervix must be assumed to contain a single cervical canal. If, by definition, a cervix can have only one cervical canal, then chinchillas have 2 cervices despite the gross appearance of having only one. We believe this description to be the most appropriate for the chinchilla uterine cervix. The chinchilla by definition thus exhibits a uterus duplex bicollis, vagina simplex—that is, 2 uteri, 2 cervices, and a single vagina. This definition is an anatomically precise description of chinchilla cervical morphology that may serve as a model for other situations that lack established terminology for the gross anatomic configuration of the cervix.
References
Articles from Journal of the American Association for Laboratory Animal Science : JAALAS are provided here courtesy of American Association for Laboratory Animal Science
Similar Articles
To arrive at the top five similar articles we use a word-weighted algorithm to compare words from the Title and Abstract of each citation.
Uterus duplex bicollis, Vagina simplex in laboratory Guinea pigs (Cavia porcellus), rats (Rattus norvegicus forma domestica) and mice (Mus musculus forma domestica).
Anat Histol Embryol, 48(4):388-393, 10 Apr 2019
Cited by: 0 articles | PMID: 30968434
Morphological changes in the porcine cervix: A comparison between nulliparous and multiparous sows with regard to post-cervical artificial insemination.
Theriogenology, 127:120-129, 10 Jan 2019
Cited by: 2 articles | PMID: 30685687
The persistence of active smooth muscle in the female rat cervix through pregnancy.
Am J Obstet Gynecol, 212(2):244.e1-8, 06 Aug 2014
Cited by: 7 articles | PMID: 25108144
Vaginal and cervical modifications during the estrus cycle in the domestic cat.
Theriogenology, 64(3):679-684, 01 Aug 2005
Cited by: 3 articles | PMID: 15953629
Review
A normal uterus communicating with a double cervix and the vagina: a müllerian anomaly without any present classification.
Fertil Steril, 91(3):935.e1-3, 11 Nov 2008
Cited by: 10 articles | PMID: 19006794
Review