A 13-day-old male Holstein calf presented with a horn-like mass in the left and rostral maxillary gingiva (Fig. 1). The calf had good weight gain, and showed normal eating and drinking behavior, although a small amount of hemorrhage was found near the top of the mass during eating. Respiratory signs including cough, and dyspnea were not evident. The horn-like mass was 1.5 cm in width and 3.5 cm in height, and was colored similar to the gingival mucosa. On palpation, the mass was firmly fixed with the deeper structures. Palpation did not reveal whether the mass was derived from maxillary bones, although a hard structure could be felt inside of the mass. Under sedation with intravenous xylazine hydrochloride (0.01 mg/kg), the calf was examined by computed tomography (CT) scanning using a helical CT device (Pronto SE, Hitachi Co. Ltd, Tokyo, Japan) for preoperative observation of the positional relationship of the mass with deeper structures (the nose and maxillary bones). The mass was demonstrated to be a homogenous and slightly bright gray structure than the neighboring soft tissues within the root of the mass, and included no radiopaque structure (i.e., bone or dent) (Fig. 2a). Neither osteolysis nor ossification was evident in the maxillary bone near the mass. Three-dimensional (3D) CT revealed that the mass arose cranial than the rostal edge of the maxillary bone and possessed anatomical continuity with the nasal structures (Fig. 2b). The mass was removed from the basal region by cutting along the gingival mucosa with an electric scalpel soon after the CT examination. The mass had a dual structure comprised of white cartilaginous tissues and soft tissues surrounding a cartilaginous core. The root of the mass extended into the deeper site of the maxillary gingiva toward the nasal cavity, and was removed as deeply as possible. The operative opening was covered by suturing the gingival mucosa over the opening with a monofilament absorbable suture. Follow-up one year after surgery found no recurrence.

The mass was composed of white cartilaginous tissues and soft tissues reaching from the basal part to the tip of the horn-like mass on the cut surface. The excised tissue was fixed in 10 % neutral buffered formalin, and embedded in paraffin wax. Sections were cut at a thickness of 5 micrometer, and stained with haematoxylin and eosin (H&E). The surface structures of the mass were lined with stratified squamous mucosal cells with gland-like nasal structures. Two types of luminal structures composed of cystic and tubular structures were seen in deeper areas of the surface structures (Fig. 3). The cystic structures were surrounded by epithelial cells, and possibly originated from tubular structures. Tubular structures lined with stratified ciliated mucosal columnar cells were surrounded by arterial and venous vessels, and gland-like structures that contained basophilic mucinoid materials (Fig. 4). Within the tubular structures, some goblet cells were occasionally aggregated in the mucosal cell layer, and lymphocytes and plasma cells frequently infiltrated beneath the mucosa. The cartilage-like structures, which were randomly observed near cystic and tubular structures, were compsed of an island and a meandering massive focus of mature cartilaginous tissues, and were surrounded by collagenous tissues (Fig. 5). Mitotic figures and atypic cells were not found among the overall structures of the mass.

Gingival hamartomas have been occasionally reported in calves ranging in age from 1 day to 2.5-months [1–4], and were diagnosed as vascular hamartomas based on pathological findings of large numbers of disorganized capillaries lined by mature endothelial cells [1]. The pathological findings of the present case can be summarized as follows: 1) the mass was comprised of cartilage-like structures, and tubular structures consisting of stratified ciliated mucosal columnar cells and gland-like structures; 2) the cellular composition in the cartilage-like and tubular structures of the mass were identical to those commonly found in nasal cavities; and 3) all structures were formed by proliferation of normal mature cells without mitotic abnormalities. Based on these histological findings, the mass was diagnosed as a nasal tissue-derived hamartoma.

There are large differences in the gross appearance of gingival vascular hamartomas versus nasal tissue-derived hamartoma. Gingival vascular hamartomas are characterized as reddish-colored flat masses (2–7 cm in widths and 1.5 cm in heights), whereas nasal tissue-derived hamartoma are a horn-like mass (1.5 cm in width and 3.5 cm in height) colored similar to the gingival mucosa [1–4]. Moreover, nasal tissue-derived hamartomas in the maxillary gingiva have different locations than gingival vascular hamartomas, which have been found in the rostral levels of the mandibular gingiva [1–4]. The cause of this difference may be that the presently described lesion originated from nasal structures and protruded to the oral cavity.

Hamartomas derived from nasal structures have been described in humans and cats [7–11]. Most human and feline patients with various types of nasal hamartomas have presented with upper respiratory signs, and the lesions were observed within the nasal cavity [7–11]. However, there are reported cases in which the lesions were first discovered during the course of a dental examination [7], which resembles the present case showing a clinical pattern of a nasal lesion that invaded the oral cavity. In human patients, nasal hamartomas have been reported to be predominantly composed of mesenchymal tissues (called as nasal chondromesenchymal hamartomas) or epithelial tissues (called as respiratory epithelial adenomatoid hamartomas) [8]. Nasal chondromesenchymal hamartomas are characterized by a mixture of mesenchymal elements (spindle cells, and collagen fibers) and irregular islands of osseous and chondroid tissues [8]. Respiratory epithelial adenomatoid hamartomas are characterized by formation of large and small adenomatoid structures lined by ciliated respiratory epithelium [7].

The present case was likely to have pathological findings identical to a mixture of these two types of lesions. Some human patients have been reported to have a respiratory epithelial adenomatoid hamartoma component together with an additional chondro-osseous component [9]. In veterinary medicine, nasal hamartomas have been reported in six feline cases: one case was diagnosed as a nasal vascular hamartoma and five cases were diagnosed as nasal mesenchymal hamartomas [10, 11]. Feline nasal mesenchymal hamartoma is diagnosed based on the pathological finding of the tissues mainly comprised of various mesenchymal components including cartilage, woven bone, and spindle cells [10]. Pathological findings of these lesions would be similar to those of nasal chondromesenchymal hamartomas in humans [8]. Nasal epithelial lesions identical to respiratory epithelial adenomatoid hamartomas in humans have not previously been reported in veterinary medicine. The present case may be the first reported animal case presenting with a mixed-type nasal hamartoma comprised of both epithelial and mesenchymal tissues.

All diagnostic and therapeutic procedures were performed by the approved veterinarians in the course of routine veterinary health management.