Anatomical and histological characterization of solitary lobules of the monostomatic sublingual gland in dogs

Abstract

Sialocele recurrence in dogs following surgical excision of the monostomatic sublingual gland is often attributed to incomplete removal of sublingual gland tissue posterior to the site of duct rupture, suggested as solitary lobules of the monostomatic sublingual gland. Here we examined these solitary lobules in six healthy beagle cadavers. Gross anatomical observation revealed solitary lobules distributed along the major sublingual duct and separated from the rostral lobe of the monostomatic sublingual gland. Histologically, the solitary lobules were connected to the major sublingual duct and showed a mixed glandular structure similar to the monostomatic sublingual gland but distinct from the polystomatic sublingual gland. These findings elucidate the anatomical basis of sialocele recurrence and emphasize the importance of completely excising solitary lobules.

Salivary mucocele and sialocele are both common lesions in dogs. Salivary mucocele is characterized by the accumulation of saliva within an obstructed salivary duct, whereas sialocele involves the leakage of saliva into subcutaneous tissue due to injury to the salivary gland or duct [2]. Although these conditions arise from different pathological mechanisms, advanced salivary mucocele may lead to ductal rupture, resulting in a clinical presentation similar to sialocele. Therefore, in this study, we use the term sialocele to describe the lesion. Sialoceles are classified as sublingual (swelling under the tongue at the base of the lingual frenulum), cervical (swelling in the intermandibular region), pharyngeal (swelling within the submucosa of the pharyngeal wall), or zygomatic (periorbital swelling), with the cervical type being the most prevalent in dogs [4, 9]. They are primarily associated with the monostomatic sublingual gland or the mandibular gland, except in the zygomatic type. Leakage of saliva from the monostomatic sublingual gland–duct system has been reported in most cases [4]. Consequently, the preferred treatment of choice is surgical excision of the monostomatic sublingual gland. However, because of its close anatomical association with the mandibular gland, both glands and their ducts are typically removed together [1].

Recurrence of sialocele following surgical excision has been reported in 5–14% of cases. Because the mandibular gland, a single oval mass, is completely removed during the initial surgery, recurrence is mainly attributed to incomplete excision of the sublingual gland tissue posterior to the site of duct rupture site [1, 4, 9]. This rupture often occurs near the sublingual caruncle, where salivary leakage most frequently arises [1, 4, 7]. This suggests the presence of salivary gland lobules in the rostral portion near the sublingual caruncle that are connected to the major sublingual duct and may contribute to recurrence if not fully excised [4, 7].

According to the anatomy textbook The Viscera of the Domestic Mammals by Nickel et al. [8], the monostomatic sublingual gland is divided into a caudal lobe, a narrow rostral lobe, and solitary lobules located adjacent to the polystomatic sublingual gland. It is assumed that recurrence is caused by solitary lobules of the monostomatic sublingual gland remaining in the rostral portion.

However, definitive information regarding these lobules is lacking in anatomy textbooks [2, 3, 6, 10], aside from schematic representations found in the above-mentioned textbook [8]. Against this background, we examined the monostomatic sublingual gland in healthy beagles and subjected the solitary lobules to both anatomical and histological analyses.

The cadavers of six beagles (three males and three females) were obtained following their use in anatomy instruction for students and were handled according to the Guide for Care and Use of Animal Experimentation at Gifu University, Japan (Permission No. AG-P-N-20240171). The animals were euthanized for educational purposes by intravenous overdose injection of pentobarbital sodium (50 mg/kg) under sedation with xylazine (3 mg/kg). After euthanasia, the cadavers were perfusion-fixed via the carotid artery using ALTFiX fixative (Falma Co., Ltd., Tokyo, Japan) and a starch solution containing red dye, then frozen for 3 months before use. During instructional use, cadavers were kept at room temperature and stored under refrigeration outside of class hours, for approximately 2.5 months.

One side of the head was examined in each cadaver. The monostomatic sublingual glands of the six cadavers were studied from various anatomical perspectives: four cadavers (one male and three females) from the left lateral side, one male from the right lateral side, and another male from the right medial side, allowing observation from multiple angles. To visualize the gland laterally, the unilateral mandible was removed after partial or complete dissection of the masseter, temporal, medial and lateral pterygoid, digastric, and mylohyoid muscles. To observe the gland medially, the mucous membrane of the oral cavity between the right mandible and the tongue was partially removed following resection of the left cheek region, including the left mandible and associated muscles.

After gross dissection, the polystomatic and monostomatic sublingual glands, along with the major sublingual and mandibular ducts, were excised from each cadaver and fixed in 10% formalin for several days. Tissue blocks containing the rostral lobe and solitary lobules, as well as those containing the polystomatic sublingual gland and solitary lobules, were dissected from these fixed samples and embedded in paraffin. Semi-serial sections, 5 μm thick, were obtained at 30–40 μm intervals and stained with hematoxylin and eosin.

The monostomatic sublingual gland was located beneath the mucosa of lateral sublingual recess, between the mandibular ramus and the body of the tongue (Fig. 1). The main lobe was the caudal lobe, situated on the dorso-rostral portion of the mandibular gland. The rostral lobe, covered by the mylohyoid muscle, extended along the major sublingual duct from the caudal lobe to a point corresponding to the level of the second upper molar and the root of the tongue, which is densely covered by conical papillae (Fig. 1, white line). In the rostral region, solitary lobules were present along the major sublingual duct, separate from the rostral lobe of the monostomatic sublingual gland (Fig. 1, white dotted line).

Fig. 1.

Gross anatomy of the monostomatic sublingual gland (MoSLG). (A): A representative image of the MoSLG on the left lateral side. The white line outlines the rostral lobe of the MoSLG (rMoSLG), while the white dotted line outlines its solitary lobules (sMoSLG). (B): Schematic representation of the MoSLG shown in panel A. Li, lingual nerve; M1, first upper molar; M2, second upper molar; MD, mandibular duct; MG, mandibular gland; MaSLD, major sublingual duct; cMoSLG, caudal lobe of the MoSLG; P4, fourth upper premolar; PoSLG, polystomatic sublingual gland.

Solitary lobules of the monostomatic sublingual gland were observed in all cadavers, located along the major sublingual duct in the rostral portion, anterior to the second upper molar and the root of the tongue (Fig. 2, white dotted lines). No sex-related differences were identified, but individual variation was noted in terms of the number, size, and degree of diffusion of the lobules. One cadaver exhibited multiple small lobules scattered along the duct (Fig. 2B), others showed clusters of lobules gathered together (Fig. 2C, and 2D). Some cadavers had fewer but more distinct lobules (Fig. 2A, and 2E). In one cadaver (Fig. 2E), a connecting duct between a solitary lobule and the major sublingual duct was observed (Fig. 2F, white arrow).

Fig. 2.

Anatomical variation in the solitary lobules of the monostomatic sublingual gland (MoSLG). (A)–(E): Photographs and schematic illustrations of the MoSLG from observed cadavers. White dotted lines indicate solitary lobules of the MoSLG (sMoSLG). In panels A, C, and E, the sMoSLGs are observed on the left lateral side. In panel B, the sMoSLG is observed on the right medial side. In panel D, the sMoSLG is observed on the right lateral side. (F): Higher magnification view of the boxed area in panel E. The white dotted line indicates a sMoSLG, and the white arrow indicates a connecting duct between the sMoSLG and the major sublingual duct (MaSLD). MD, mandibular duct; cMoSLG, caudal lobe of the MoSLG; rMoSLG, rostral lobe of the MoSLG; PoSLG, polystomatic sublingual gland.

Histological analysis of solitary lobules is shown in Figs. 3 and 4. In semi-serial sections of the rostral lobe, the major sublingual duct, located between the thick mandibular duct and the sublingual glands, was generally narrow but showed localized dilation near the solitary lobules (Fig. 3A–C). The solitary lobules were also histologically separated from the rostral lobe, with one directly connected to the major sublingual duct via an interlobular duct (Fig. 3B, black arrows). Terminal lobules exhibited a mixed gland structure with serous acini and mucous acini containing serous demilunes, consistent with the rostral lobe (Fig. 3D, and 3E). In the semi-serial sections of the polystomatic sublingual gland, solitary lobules were also histologically separated and clearly distinguishable (Fig. 4A). The polystomatic sublingual gland displayed a mixed gland structure that was characterized by few serous acini and mucous acini lacking serous demilunes in their terminal portions (Fig. 4B, and 4C). Taken together, these findings indicate that the solitary lobules observed in this study belong to the monostomatic sublingual gland and are structurally distinct from the polystomatic sublingual gland.

Fig. 3.

Histology of the solitary lobules of the monostomatic sublingual gland (MoSLG). (A): Low-magnification view of a section including the solitary lobules (sMoSLG) and the rostral lobe (rMoSLG). The boxed area in the schematic indicates the location of the tissue block. The boxed areas correspond to regions shown at higher magnification in panels B and C. Bar=1 mm. (B): Higher magnification view of the boxed area in panel A, showing the sMoSLG and the major sublingual duct (MaSLD). Arrows indicate an interlobular duct between the sMoSLG and the MaSLD. Bar=200 μm. (C): Higher magnification view of the boxed area in panel A, showing the rMoSLG and the MaSLD. Bar=200 μm. (D): Higher magnification view of the boxed area in panel B, showing the glandular structure of the sMoSLG. Bar=100 μm. (E): Higher magnification view of the boxed area in panel C, showing the glandular structure of the rMoSLG. Bar=100 μm. M, mucous acini; MD, mandibular duct; cMoSLG, caudal lobe of the MoSLG; PoSLG, polystomatic sublingual gland; S, serous acini.

Fig. 4.

Comparison of the histology between the solitary lobules of the monostomatic sublingual gland (MoSLG) and the polystomatic sublingual gland (PoSLG). (A): Low-magnification view of a section including the solitary lobules (sMoSLG) and the PoSLG. The boxed area in the schematic indicates the location of the tissue block. The boxed areas correspond to regions shown at higher magnification in panels B and C. Bar=1 mm. (B): Higher magnification view of the boxed area in panel A, showing the glandular structure of the sMoSLG. Bar=100 μm. (C): Higher magnification view of the boxed area in panel A, showing the glandular structure of the PoSLG. Bar=100 μm. M, mucous acini; MD, mandibular duct; cMoSLG, caudal lobe of the MoSLG; rMoSLG, rostral lobe of the MoSLG; PoSLG, polystomatic sublingual gland; S, serous acini.

This study clarified the anatomical and histological characteristics of solitary lobules of the monostomatic sublingual gland in dogs. These lobules were consistently located along the major sublingual duct in the rostral portion, anatomically distinct from the rostral lobe, and varied in number, size, and distribution. Their position near the sublingual caruncle, where salivary leakage frequently occurs, and their direct ductal connection to the major sublingual duct suggest clinical relevance, particularly in the context of sialocele recurrence [4, 7]. Histologically, the features of the monostomatic and polystomatic sublingual glands observed in this study align with previous reports indicating that the polystomatic sublingual gland in dogs consists predominantly of mucous acini relative to the monostomatic sublingual gland [5]. In our observations, the solitary lobules were similar to the rostral lobe of the monostomatic sublingual gland but distinct from the polystomatic sublingual gland observed in the same cadavers. Overall, these findings provide new insights into the detailed characterization of solitary lobules, which have been poorly described in several earlier studies. This distinction is particularly important because prior studies have misclassified these rostrally located structures, associated with sialocele recurrence, as part of the “polystomatic sublingual gland portion” [1, 4, 7, 9]. The present findings demonstrate that these structures are, in fact, solitary lobules of the monostomatic sublingual gland. Their consistent location, direct ductal connection to the major sublingual duct, and histological features support this classification.

A limitation of this study is the small sample size, which may not fully represent the anatomical variation across different dog breeds or sexes. Nevertheless, this clarification is significant for understanding the anatomical basis of sialocele recurrence after surgical excision, particularly when complete removal of the solitary lobules is not achieved due to their diffuse and variable distribution. Future studies with larger and more diverse samples are needed to validate these findings and to explore their implications for surgical strategies aimed at preventing recurrence.

CONFLICT OF INTEREST

We declare no conflicts of interest associated with this manuscript.