An extraskeletal osteosarcoma in the auricle of a Wistar Hannover rat

Abstract

An extraskeletal osteosarcoma was detected in the auricle of a 110-week-old female Wistar Hannover rat. Grossly, the tumor, measuring 15 mm in size, was observed in the subcutis as a solid and hard mass. Histologically, the majority of the mass comprised mature, compact bone. It was surrounded by neoplastic cells showing a variety of histologies, such as sarcoma, not otherwise specified, and myxosarcoma away from the bone-forming region. However, these different histological regions were considered to be components of a single bone tumor, based on the common expression of osterix and a similar mixture of constituent cells in each region. The tumor was diagnosed as an extraskeletal osteosarcoma because of the presence of infiltrative growth and abnormal mitosis and its development in the auricle without attachment to the skeleton. The present case is a rare histological type of an extraskeletal osteosarcoma with independent and different histological elements in rats.

Keywords: auricle; extraskeletal; osteosarcoma; osterix; rat.

Fig. 1.

(a) A schematic drawing showing the three trimmed sites (I, II, and III) of the auricular mass. Red arrows: section surface. Asterisk: maximum cut surface (MCS). (b) A schematic drawing showing the positional relationship among regions A to E in MCS of the auricular mass, derived from the section surface of trimmed site I (Fig. 1a). EC, ear canal; AC, auricular cartilage; region A, bone-forming; region B, sarcoma, not otherwise specified; region C, myxosarcomatous; region D, myxomatous; and region E, regions other than regions A to D. (c) A schematic drawing of the positional relationship between regions A and F in cut surfaces other than the MCS of the auricular mass, derived from the section surface of trimmed site II (Fig. 1a). AC, auricular cartilage; region A, bone-forming; and region F, regions other than regions A.

Fig. 2.

Representative histological images of regions A to F. (a) Region A is composed of mature, compact bone trabeculae and contains many lacunae. Nuclear chromatin is abundant in constituent cells, and constituent cells show a high nucleus/cytoplasm ratio. (b) Region B is composed of atypical mesenchymal cells with osteoid formation (arrows) which have undergone fascicular proliferation. (c) Region C is composed of elongated fibroblast-like cells which exhibit nuclear pleomorphism. The stroma is edematous. (d) Region D is composed of small cells which exhibit nuclear pleomorphism. The stroma is abundant in the mucoid substance. (e) Region E is composed of pleomorphic large cells having bizarre nuclei (arrows). The stroma is edematous. (f) Region F is composed of large pleomorphic cells with abundant cytoplasm. H&E stain. Scale bars = 50 μm (a–f).

Fig. 3.

Histological images showing diagnostic characteristics. (a) Region A (A) expands into the surrounding connective tissue (*) (arrows). (b) Tumor cells are abundant around osteoids (arrows) in region F but scarce around osteoids in region A (A). (c) Pleomorphic tumor cells in region F surround region A. Cells within bone trabeculae (arrows) are smaller than those surrounding bone trabeculae. (d) The proximal portion of the auricular cartilage (arrows). The thickness of the cartilage is irregular, but the cartilage is not connected to region A (A). H&E stain (a–c) and toluidine blue stain (d). Scale bars = 400 μm (a, b), 50 μm (c), and 500 μm (d).

Fig. 4.

Immunostaining with anti-osterix (a–f), anti-S-100 (g), and anti-proliferating cell nuclear antigen (PCNA) (h) antibodies. (a) Strong nuclear positivity for osterix in region A cells. (b) Area showing nuclear positivity for osterix in region C cells, but the intensity of the staining is weak compared with that in region A. (c) Cells surrounding osteoids (*) in region F show strong nuclear positivity for osterix. (d) Cells in region F which surround region A (A) are osterix positive in either the nucleus or cytoplasm, but the intensity of the staining is weak. (e) Cells in region E show cytoplasmic positivity for osterix. (f) Hyperplastic chondrocytes, but not normal chondrocytes (arrows), in the auricular cartilage show cytoplasmic positivity for osterix. (g) S-100 protein-positive cells in region F. (h) The highest number of PCNA-positive cells was observed in region B (B), followed by region E, and the lowest number of PCNA-positive cells was observed in fibroblasts in the hyalinized dermis (*). Scale bars = 400 μm (a, b, g), 500 μm (c, h), and 50 μm (d–f).