Abstract
The clinical, histological, and immunohistochemical features of 26 fibrous proliferative lesions in the vagina of 14 female dogs (13 intact and 2 neutered) were examined. These lesions presented as nodular or pedunculated masses. Histopathological examination revealed that all lesions consisted of spindle- and stellate-shaped cells with mild atypia, collagenous stroma, and blood vessels. Mitotic figures were rare. The number of blood vessels was significantly greater in pedunculated masses than in nodular masses. Immunohistochemically, spindle- and stellate-shaped cells were positive for progesterone receptor in all 26 lesions, estrogen receptor in 5 of 26 lesions, desmin in 20 of 26 lesions, and α-smooth muscle actin (αSMA) in 8 of 26 lesions. Lesions in neutered dogs exhibited lower progesterone receptor expression and higher estrogen receptor expression than those in intact females. No significant differences in desmin or αSMA expression were observed between the two groups. These findings suggest that the fibrous proliferative lesions in the canine vagina described in this study may represent progesterone-associated benign proliferative changes composed of myofibroblasts.
Keywords: dog, fibroma, hormone receptor, immunohistochemistry, vagina
INTRODUCTION
Fibrous proliferative lesions of the canine vagina are relatively common and typically present with two macroscopic appearances: nodular or pedunculated [14]. However, the detailed histopathological features and immunohistochemical profiles of proliferative cells in these lesions have not been well characterized. Vaginal lesions are not described in the WHO classification of tumors of domestic animals. There is some confusion regarding the nomenclature of mesenchymal tumors of the genital tract [3]. Lesions with similar histopathological features have been described as fibrous polyps [12], polyps [14], fibroepithelial polyps [1], and fibromas [11, 14].
In general, fibrous proliferative lesions of the genital tract do not occur in neutered dogs [3]. A previous study reported that surgical excision of vulvar or vaginal benign neoplasms combined with ovariohysterectomy was effective in preventing disease recurrence [14]. It has also been reported that 84.4% and 56.3% of canine smooth muscle tumors of the female genital tract express progesterone receptor (PR) and estrogen receptor (ER), respectively [9]. However, immunohistochemical studies on the expression of hormone receptors in fibrous proliferative lesions of the vagina have not been reported.
The aim of this study is to characterize pathological and immunohistochemical features of fibrous proliferative lesions in the canine vagina. These findings provide a basis for further investigation into the pathogenesis of these lesions.
MATERIALS AND METHODS
Lesion selection and clinical information
A total of 26 fibrous proliferative lesions of the vagina of 14 female dogs (13 intact and 2 neutered) were submitted to the Shikoku Cytopathological Laboratory between 2018 and 2021 for histopathological examinations. Data were collected on the canine breed, age, and neuter status. The size of each formalin-fixed tumor was recorded by measuring its longest diameter.
Histopathology and immunohistochemistry
The tissues were fixed in 10% formalin and embedded in paraffin. Sections were cut at a thickness of 3 μm for histopathological and immunohistochemical examinations. The sections were stained with hematoxylin and eosin (HE) and Masson’s trichrome. The diagnoses of fibrous proliferative lesions were based on histological and immunohistochemical findings. The mitotic index was defined as the number of mitotic figures per ten high-power fields (HPFs; ×400, 0.237 mm2) under a light microscope.
The primary antibodies used for immunohistochemistry are listed in Table 1. Immunohistochemical processing was performed using an automated immunostainer (Leica Bond-III, Leica Biosystems, Melbourne, Australia) with a polymer-based detection system with diaminobenzidine (DAB) as the chromogen and hematoxylin as the counterstain (Bond Polymer Refine Detection kit, Leica Biosystems, Newcastle Upon Tyne, UK). For antigen retrieval, all sections were treated with Bond Epitope Retrieval Solution 2 (Leica Biosystems) for 20 min at 100°C. Canine uterine tissue was used as a positive control for PR, ER, α smooth muscle actin (αSMA), and desmin. Canine peripheral nerve tissue was used as a positive control for S100.
Table 1. List of primary antibody used for immunohistochemistry.
| Antibody | Host | Source | Clone | Dilution |
|---|---|---|---|---|
| PR | Mouse | Roche, Basel, Switzerland | 1E2 | RTU |
| ER | Mouse | Leica Biosystems, Newcastle Upon Tyne, UK | 6F11 | 1:120 |
| αSMA | Mouse | Agilent technologies, Santa Clara, USA | 1A4 | 1:50 |
| Desmin | Mouse | Leica Biosystems | DE-R-11 | RTU |
| S-100 | Rabbit | Agilent technologies | Polyclonal | 1:1,600 |
PR, progesterone receptor; ER, estrogen receptor; αSMA, α smooth muscle actin; RTU, ready to use.
Evaluation of histochemical and immunohistochemical results
The number of spindle- and stellate-shaped cells comprising the fibrous proliferative lesions was counted in each photomicrograph (0.09358 mm2), and these values were used to calculate the cell density (cells per mm2). Photomicrographs were obtained from areas with the highest cellularity. The number of blood vessels was counted using αSMA immunostained sections in 3 HPFs. Masson’s trichrome stain was performed in all samples to assess the amount of collagen. The blue-stained area within the photographed region (0.37432 mm2) was measured using the WinROOF 2018 software (Mitani Corp., Tokyo, Japan).
Nuclear staining was considered immunopositive for PR and ER. Cytoplasmic staining was considered immunopositive for desmin and αSMA. Cytoplasmic and nuclear staining patterns were considered immunopositive for S100. The number of immunopositive cells was counted in each photomicrograph (0.09358 mm2) and calculated as a percentage of the total. Photomicrographs were obtained from hotspot areas.
Statistical analysis
Statistical analysis was performed using the non-parametric Mann-Whitney U-test. Differences between nodular and pedunculated masses, as well as between intact and neutered dogs, were considered statistically significant at P<0.05.
RESULTS
Clinical and gross findings
Of the 26 mass lesions examined, 20 occurred in 13 intact dogs and 6 occurred in 2 neutered dogs (Table 2). Multiple masses were found in 5 dogs (3 intact and 2 neutered).
Table 2. Summary of clinical characteristics and gross morphology of fibrous proliferative lesions of the vagina in dogs.
| Case No. | Canine breed | Age (Y) | Lesion No. | Neuter status | Shape | Size (longest diameter, cm) |
|---|---|---|---|---|---|---|
| 1 | Shetland Sheepdog | 11 | 1 | Intact | Pedunculated | 7.5 |
| 2 | Papillon | 15 | 1 | Intact | Pedunculated | 1.6 |
| 3 | Mixed | 12 | 1 | Intact | Nodular | 3.4 |
| 4 | Shetland Sheepdog | 10 | 1 | Intact | Nodular | 5.6 |
| 2 | Neutered | Nodular | 1.5 | |||
| 3 | Neutered | Nodular | 1.2 | |||
| 4 | Neutered | Nodular | 1.0 | |||
| 5 | Neutered | Nodular | 0.7 | |||
| 5 | Papillon | 12 | 1 | Intact | Nodular | 1.7 |
| 6 | French Bulldog | 10 | 1 | Neutered | Nodular | 6.6 |
| 2 | Neutered | Nodular | 0.3 | |||
| 7 | Shiba Inu | 11 | 1 | Intact | Nodular | 3.0 |
| 2 | Intact | Nodular | 1.5 | |||
| 3 | Intact | Nodular | 1.5 | |||
| 4 | Intact | Nodular | 0.6 | |||
| 5 | Intact | Pedunculated | 0.6 | |||
| 6 | Intact | Nodular | 0.5 | |||
| 8 | Pomeranian | 8 | 1 | Intact | Pedunculated | 2.8 |
| 9 | Pomeranian | 12 | 1 | Intact | Pedunculated | 4.0 |
| 10 | Mixed | 11 | 1 | Intact | Pedunculated | 3.3 |
| 11 | Samoyed | 10 | 1 | Intact | Nodular | 6.6 |
| 12 | Pomeranian | 12 | 1 | Intact | Pedunculated | 3.3 |
| 2 | Intact | Nodular | 0.6 | |||
| 13 | Yorkshire Terrier | 13 | 1 | Intact | Pedunculated | 3.0 |
| 14 | Toy Poodle | 13 | 1 | Intact | Pedunculated | 2.4 |
| 2 | Intact | Pedunculated | 4.4 |
The ages of all dogs ranged from 8 to 15 years, with a median age of 11.5 years at the time of histopathological diagnosis. The dog breeds included 3 Pomeranians, 2 mixed-breed dogs, 2 Shetland Sheepdogs, 2 Papillons, and one each of Toy Poodle, French Bulldog, Shiba Inu, Samoyed, and Yorkshire Terrier.
Grossly, the lesions appeared as either nodular masses (16 lesions, Fig. 1) or pedunculated masses (10 lesions, Fig. 2). The nodular masses were whitish, whereas the pedunculated masses were reddish or black. The pedunculated masses occurred only in intact female dogs. The mean diameters of the nodular and pedunculated masses were 2.3 cm (range: 0.3–6.6 cm) and 3.3 cm (range: 0.6–7.5 cm), respectively.
Fig. 1.
Gross findings of a fibrous proliferative lesion of the vagina in Case 6. A white nodular mass in the vagina.
Fig. 2.
Gross findings of a fibrous proliferative lesion of the vagina in Case 9. A dark red pedunculated mass in the vagina.
Eight intact female dogs underwent vaginal mass resection concurrently with either ovariectomy or ovariohysterectomy. In one dog (Case 4), a single vaginal mass was excised at the time of ovariohysterectomy, and 4 additional vaginal masses developed 69 days after the surgery. Another neutered dog (Case 6) developed 2 vaginal masses 11 days after ovariohysterectomy. No recurrence was reported in any of the other dogs during the study.
Histopathological findings
Histopathological features of fibrous proliferative lesions in the canine vagina are shown in Figs. 3–6. All lesions were composed of spindle- and stellate-shaped mesenchymal cells with collagenous stroma and various numbers of blood vessels. The spindle-shaped cells were arranged in streams, interlacing bundles, herringbone patterns, or storiform patterns, admixed with varying amounts of collagen bundles (Figs. 3 and 4). Collagen stained with blue color was confirmed in Masson’s trichrome-stained sections (Fig. 5). The spindle- and stellate-shaped cells had oval to spindle-shaped nuclei with finely dispersed chromatin and a small nucleolus. Cellular and nuclear atypia were minimal (Fig. 6). The mean number of mitotic figures was 0.067 per 10 HPFs (0.067/2.37 mm2), ranging from 0 to 1. In pedunculated masses, small arteries and veins extended through the narrow stalk toward the apex, where numerous capillaries and small vessels were observed. Ulceration was observed in 20 lesions from all intact female dogs, whereas no ulceration was observed in lesions from neutered females. Lymphocytic infiltration was observed in 13 of the 26 lesions. Necrosis was not observed in any of the lesions.
Fig. 3.
Histopathological findings of a fibrous proliferative lesion of the vagina in Case 7. Spindle-shaped cells are arranged in interlacing bundles with collagen. Hematoxylin and eosin stain, ×100.
Fig. 6.
Histopathological findings of a fibrous proliferative lesion of the vagina in Case 7. The cells have oval to spindle-shaped nuclei with dispersed chromatin and small nucleoli. Mitotic figure is absent. Hematoxylin and eosin stain, ×400.
Fig. 4.
Histopathological findings of a fibrous proliferative lesion of the vagina in Case 13. Spindle-shaped cells are arranged in a herringbone or fascicular pattern. Hematoxylin and eosin stain, ×100.
Fig. 5.
Histopathological findings of a fibrous proliferative lesion of the vagina in Case 7. Abundant collagen is stained blue. Masson’s trichrome stain, ×100.
Quantitative analysis of histological findings
The density of spindle- and stellate-shaped cells ranged from 790.77 to 7,159.65 per mm2, with a median of 2,313.52. The number of blood vessels in the lesions ranged from 4 to 43 per 3 HPFs each (4–43 / 0.711 mm2), with a median of 18. The number of blood vessels was significantly greater in pedunculated masses than in nodular masses (Fig. 7). Masson’s trichrome staining confirmed collagen areas stained blue. The percentage of collagen area ranged from 35.12% to 71.40%, with a median of 50.09%.
Fig. 7.
Number of blood vessels in nodular and pedunculated fibrous proliferative lesions of the vagina of dogs. The box plots show the median and quartiles, whiskers show the minimum and maximum values, dots represent individual data points, and crosses indicate the mean values.
Immunohistochemistry
Immunohistochemical findings are shown in Figs. 8–11 and Table 3. Immunohistochemical analysis confirmed PR expression in all 26 lesions (Fig. 8). The mean PR positivity rate in spindle- and stellate-shaped cells was 80.14% (median 86.87%). The expression of ER was confirmed in 5 of 26 lesions, including 4 from neutered female dogs (Fig. 9). The mean ER positivity rate in spindle- and stellate-shaped cells was 6.33% (median 0%). The expression of desmin was confirmed in 20 of 26 lesions (Fig. 10). The mean desmin positivity rate in spindle- and stellate-shaped cells was 27.62% (median 20.26%). The expression of αSMA was confirmed in 8 of 26 lesions (Fig. 11). The mean αSMA positivity rate in spindle- and stellate-shaped cells was 3.40% (median 0%). Hormone receptors, desmin, and αSMA were expressed in some stromal cells of the vaginal lamina propria. The expression of S100 was not detected in any of the lesions.
Fig. 8.
Representative immunohistochemical findings of a fibrous proliferative lesion of the vagina in Case 2. The nuclei of most spindle-shaped cells are positive for progesterone receptor. ×400.
Fig. 11.
Representative immunohistochemical findings of a fibrous proliferative lesion of the vagina in Case 6. The cytoplasm of some spindle-shaped cells is positive for α smooth muscle actin. ×400.
Table 3. Results of immunohistochemistry for fibrous proliferative lesions of the vagina in dogs.
| Case No. | Lesion No. | PR (%) | ER (%) | Desmin (%) | αSMA (%) |
|---|---|---|---|---|---|
| 1 | 1 | 95.90 | 0.00 | 48.77 | 2.46 |
| 2 | 1 | 98.96 | 0.00 | 38.02 | 0.00 |
| 3 | 1 | 86.21 | 0.00 | 0.00 | 0.00 |
| 4 | 1 | 87.54 | 0.00 | 1.13 | 0.85 |
| 2 | 100.00 | 0.00 | 1.21 | 0.00 | |
| 3 | 54.51 | 0.82 | 0.00 | 0.00 | |
| 4 | 64.20 | 2.47 | 55.55 | 0.00 | |
| 5 | 83.57 | 0.00 | 0.95 | 0.00 | |
| 5 | 1 | 60.37 | 52.53 | 27.65 | 0.00 |
| 6 | 1 | 50.94 | 39.62 | 21.07 | 9.43 |
| 2 | 19.31 | 69.16 | 0.06 | 9.66 | |
| 7 | 1 | 81.48 | 0.00 | 76.85 | 6.48 |
| 2 | 61.32 | 0.00 | 0.00 | 0.00 | |
| 3 | 100.00 | 0.00 | 20.79 | 0.00 | |
| 4 | 100.00 | 0.00 | 0.00 | 0.00 | |
| 5 | 100.00 | 0.00 | 0.00 | 22.81 | |
| 6 | 71.82 | 0.00 | 0.00 | 0.00 | |
| 8 | 1 | 53.03 | 0.00 | 66.67 | 3.64 |
| 9 | 1 | 77.03 | 0.00 | 98.65 | 0.00 |
| 10 | 1 | 94.88 | 0.00 | 4.19 | 0.00 |
| 11 | 1 | 96.79 | 0.00 | 19.72 | 0.00 |
| 12 | 1 | 98.44 | 0.00 | 75.00 | 33.00 |
| 2 | 88.81 | 0.00 | 67.91 | 0.00 | |
| 13 | 1 | 100.00 | 0.00 | 33.97 | 0.00 |
| 14 | 1 | 87.83 | 0.00 | 0.00 | 0.00 |
| 2 | 70.66 | 0.00 | 59.88 | 0.00 |
PR, progesterone receptor; ER, estrogen receptor; αSMA, α smooth muscle actin.
Fig. 9.
Representative immunohistochemical findings of a fibrous proliferative lesion of the vagina in Case 5. The nuclei of some spindle-shaped cells are positive for estrogen receptor. ×400.
Fig. 10.
Representative immunohistochemical findings of a fibrous proliferative lesion of the vagina in Case 2. The cytoplasm of some spindle-shaped cells is positive for desmin. ×400.
The positivity rate of PR was significantly higher, and that of ER was significantly lower, in lesions from intact female dogs compared to lesions from neutered dogs (P<0.05) (Figs. 12 and 13). No significant difference was observed in the positivity rates of αSMA and desmin between lesions from intact and neutered dogs.
Fig. 12.
Median positive rates of progesterone receptor (PR) in fibrous proliferative lesions of the vagina in intact and neutered female dogs. The box plots show the median and quartiles, whiskers show the minimum and maximum values, dots represent individual data points, and crosses indicate the mean values.
Fig. 13.
Median positive rates of estrogen receptor (ER) in fibrous proliferative lesions of the vagina in intact and neutered female dogs. The box plots show the median and quartiles, whiskers show the minimum and maximum values, dots represent individual data points, and crosses indicate the mean values.
DISCUSSION
The present study demonstrated the clinical presentation, gross findings, histopathological features, and immunohistochemical results of fibrous proliferative lesions of the vagina in female dogs. These lesions occurred in 13 intact female dogs and 2 neutered female dogs. The lesions were grossly classified into two types: nodular and pedunculated masses. Histopathological features were similar between the two types, composed of spindle- and stellate-shaped cells with collagenous stroma and various numbers of vessels. Small arteries and veins extended through the narrow stalks of the pedunculated masses. The spindle- and stellate-shaped cells within the lesions were positive for PR in all lesions, whereas ER, desmin, and αSMA positivity varied.
During this study, none of the dogs developed recurrent vaginal lesions after surgery except for one dog (Case 4). In this case, a recurrent lesion was recognized 69 days after the ovariohysterectomy, but there was no local recurrence after the second surgery. No distant metastatic lesions were reported in any of the dogs. Consistent with previous reports [10, 13, 14], the clinical presentation and outcome of the dogs in the present study suggest that fibrous proliferative lesions in the vagina are benign, and surgical resection of the lesion together with ovariectomy provides a good prognosis.
Two types of lesions that were grossly distinguished, nodular and pedunculated masses, are presented in a schematic diagram (Fig. 14). These lesions were histopathologically comparable and characterized by spindle- or stellate-shaped cells arranged in multiple patterns within a highly collagenous stroma. The cellular components exhibited minimal atypia and mitotic figures, which aligns with the benign biological nature of this disease. Pedunculated masses had blood vessels and characteristic vascular runs in their stalks. Moreover, the number of blood vessels was significantly greater in pedunculated masses than in nodular masses. Nodular masses were whitish, whereas pedunculated masses were red or black. These differences in gross morphology and lesion color may be attributed to differences in vascularization.
Fig. 14.
Schematic diagram of nodular mass and pedunculated mass of fibrous proliferative lesions of the vagina in dogs. In the pedunculated mass, blood vessels extend through the narrow stalk toward the apex, where numerous blood vessels are observed.
Multiple etiological factors were considered to contribute to the pathogenesis of fibrous proliferative lesions. Progesterone may play an important role in the development of these lesions. Fibrous proliferative lesions of the genital tract are uncommon in neutered female dogs [3]. In the present study, although most affected dogs were intact females, two neutered dogs developed these lesions, a finding that has not been previously reported. In addition, PR positivity was confirmed in all cases. In female dogs, progesterone reaches high concentrations during diestrus and remains elevated for a prolonged period (about 60 days), whereas estrogen rises transiently around ovulation [2, 8]. These hormonal dynamics suggest that the lifetime cumulative exposure to progesterone in female dogs may be greater than that to estrogen. Accordingly, the fibrous proliferative lesions of the canine vagina may represent progesterone-dependent proliferative changes. Interestingly, vaginal lesions developed 69 days and 11 days after ovariohysterectomy in Case 4 and Case 6, respectively. In both cases, the lesions formed within a relatively short period after ovariohysterectomy. In addition, immunohistochemical analysis revealed that PR expression was significantly higher and ER expression was significantly lower in lesions from intact female dogs than in those from neutered dogs. Progesterone levels rapidly decrease within a short period after ovariectomy [5, 6]. Changes in hormonal dynamics after ovariohysterectomy, persistent stimulation of hormone receptors, and the effects of residual hormones may influence the development of these lesions. The cause remains unclear. The small number of neutered dogs and the lack of circulating hormone measurements are limitations of this study. An additional limitation of this study is that multiple lesions from the same individual were included in the analysis. Because vaginal fibrous proliferative lesions are often multifocal, the assumption of statistical independence among samples may not have been fully met. Additional cases are required to confirm the differences in hormone receptor expression between intact and neutered female dogs.
Another possible etiological factor is the involvement of inflammation or trauma. In addition to hormone receptors, the expression of desmin and αSMA, myogenic differentiation markers, was immunohistochemically examined. Spindle- and stellate-shaped cells were variably positive for desmin, whereas αSMA positivity was limited. These results indicate that these mesenchymal cells have myofibroblast-like and fibroblast-like features. Similar desmin-positive stromal cells have been reported in fibroepithelial polyps [1]. The transient activation of fibroblasts into myofibroblasts contributes to tissue repair, whereas persistent activation leads to pathological fibrosis [15, 16]. Since ulceration and lymphocytic infiltration were observed in many of the lesions examined, a reparative response to mucosal damage and chronic inflammation may be involved in the development of fibrous proliferative lesions of the vagina in dogs.
Based on the findings of the present study alone, it was difficult to determine whether these lesions should be classified as non-neoplastic lesions, such as reactive lesions or hyperplastic changes, or as true neoplasms.
Superficial myofibroblastoma of the lower genital tract in women is a benign neoplasm composed of spindle-shaped cells [4]. The tumor is thought to arise from subepithelial, hormonally sensitive, mesenchymal tissue and develops as a well-circumscribed nodular or polypoid mass. The neoplastic spindle-shaped cells are positive for desmin, CD34, ER, and PR, but negative for S100 [4, 7]. The clinical and pathological findings of fibrous proliferative lesions of the vagina in dogs in the present study shared several pathological features with superficial myofibroblastoma described in humans. However, we believe that demonstration of clonal proliferation is necessary to establish fibrous proliferative lesions of the canine vagina as true neoplasms.
In conclusion, the fibrous proliferative lesions of the vagina in dogs described in this study are benign proliferative changes composed of fibroblasts and myofibroblasts that may be influenced by progesterone.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
Acknowledgments
We gratefully acknowledge the contributions of the members of the Shikoku Cytopathological Laboratory. We also thank Dr. Koji Obayashi and Dr. Kazunori Yamasaki for providing the photographs.
REFERENCES
- 1.Brown PJ, Evans HK, Deen S, Whitbread TJ. 2012. Fibroepithelial polyps of the vagina in bitches: a histological and immunohistochemical study. J Comp Pathol 147: 181–185. doi: 10.1016/j.jcpa.2012.01.012 [DOI] [PubMed] [Google Scholar]
- 2.Concannon PW, Hansel W, Visek WJ. 1975. The ovarian cycle of the bitch: plasma estrogen, LH and progesterone. Biol Reprod 13: 112–121. doi: 10.1095/biolreprod13.1.112 [DOI] [PubMed] [Google Scholar]
- 3.Dalen WA, MacLachlan NJ. 2017. Tumors of the genital systems. pp.689–722. In: Tumor in Domestic Animals, 5th ed. (Meuten DJ ed.), Wiley Blackwell, New Jersey. [Google Scholar]
- 4.Ganesan R, Howitt BE. 2020. Superficial myofibroblastoma. p. 490. In: WHO Classification of Tumours: Female Genital Tumours, 5th ed. (WHO Classification of Tumours Editorial Board ed.), World Health Organization, Geneva. [Google Scholar]
- 5.Gobello C, Baschar H, Castex G, de la Sota RL, Goya RG. 2001. Dioestrous ovariectomy: a model to study the role of progesterone in the onset of canine pseudopregnancy. J Reprod Fertil Suppl 57: 55–60. [PubMed] [Google Scholar]
- 6.Hydbring-Sandberg E, Larsson E, Madej A, Höglund OV. 2021. Short-term effect of ovariohysterectomy on urine serotonin, cortisol, testosterone and progesterone in bitches. BMC Res Notes 14: 265. doi: 10.1186/s13104-021-05680-y [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Laskin WB, Fetsch JF, Tavassoli FA. 2001. Superficial cervicovaginal myofibroblastoma: fourteen cases of a distinctive mesenchymal tumor arising from the specialized subepithelial stroma of the lower female genital tract. Hum Pathol 32: 715–725. doi: 10.1053/hupa.2001.25588 [DOI] [PubMed] [Google Scholar]
- 8.Maksimović A, Pehar B, Filipović S, Čengić B. 2025. The estrous cycle in bitches: mechanisms, influencing factors, and clinical relevance. Veterinaria 74: 131–142. [Google Scholar]
- 9.Millán Y, Gordon A, de los Monteros AE, Reymundo C, de las Mulas JM. 2007. Steroid receptors in canine and human female genital tract tumours with smooth muscle differentiation. J Comp Pathol 136: 197–201. doi: 10.1016/j.jcpa.2007.01.004 [DOI] [PubMed] [Google Scholar]
- 10.North SM, Banks TA. 2009. Tumors of the urogenital tract. pp. 151-171. In: Small Animal Oncology. An Introduction. Elsevier, Amsterdam. [Google Scholar]
- 11.Rollón E, Millán Y, de las Mulas JM. 2008. Effects of aglepristone, a progesterone receptor antagonist, in a dog with a vaginal fibroma. J Small Anim Pract 49: 41–43. doi: 10.1111/j.1748-5827.2007.00424.x [DOI] [PubMed] [Google Scholar]
- 12.Sato Y. 2008. A case of fibrous polyp on the vaginal mucosa in a female dog. Nihon Jui Masui Gekagaku Zasshi 39: 17–20. [Google Scholar]
- 13.Soderberg SF. 1986. Vaginal disorders. Vet Clin North Am Small Anim Pract 16: 543–559. doi: 10.1016/S0195-5616(86)50060-7 [DOI] [PubMed] [Google Scholar]
- 14.Thacher C, Bradley RL. 1983. Vulvar and vaginal tumors in the dog: a retrospective study. J Am Vet Med Assoc 183: 690–692. doi: 10.2460/javma.1983.183.06.690 [DOI] [PubMed] [Google Scholar]
- 15.Tomasek JJ, Gabbiani G, Hinz B, Chaponnier C, Brown RA. 2002. Myofibroblasts and mechano-regulation of connective tissue remodelling. Nat Rev Mol Cell Biol 3: 349–363. doi: 10.1038/nrm809 [DOI] [PubMed] [Google Scholar]
- 16.Younesi FS, Miller AE, Barker TH, Rossi FMV, Hinz B. 2024. Fibroblast and myofibroblast activation in normal tissue repair and fibrosis. Nat Rev Mol Cell Biol 25: 617–638. doi: 10.1038/s41580-024-00716-0 [DOI] [PubMed] [Google Scholar]