Spontaneous Rhabdomyosarcoma in a Common Marmoset ( Callithrix jacchus )

Abstract

The common marmoset (Callithrix jacchus) is now widely used in various research fields, including toxicology. However, information about the background pathology of this species is scarce. Here, we report a case of rhabdomyosarcoma that spontaneously occurred in a common marmoset. A 44-month-old male common marmoset was euthanized due to bilateral hind limb paralysis. At necropsy, a 2×2×5-cm intramuscular mass was observed in the lower right back. Histologically, the mass was mainly composed of interlacing bundles of spindle-shaped tumor cells. Immunohistochemically, the tumor cells were positive for myogenin, desmin, vimentin and alpha-smooth muscle actin. Ultrastructurally, the tumor cells contained bundles of myofilaments with Z-band-like structures. Thus, the tumor was diagnosed as a rhabdomyosarcoma. To our knowledge, this is the first report of spontaneous rhabdomyosarcoma that was definitely diagnosed in the common marmoset.

Keywords: Callithrix jacchus; immunohistochemistry; pathology; rhabdomyosarcoma.

Fig. 1.

Macroscopic appearance of the mass. A 2×2×5-cm intramuscular mass can be observed in the lower right back.

Fig. 2.

Histological appearance of the mass. (A) The mass is surrounded by the fascia (arrowheads) with atrophic muscle fibers, and the center of the mass is necrotic. HE. Bar = 2 mm. (B) The mass is mainly composed of interlacing bundles of spindle-shaped tumor cells. HE. Bar = 400 µm. (C) Tumor cells have elongated, blunt-ended nuclei and weakly basophilic, vaguely outlined cytoplasm. Some tumor cells have abundant eosinophilic cytoplasm, resembling rhabdomyoblasts (arrow). HE. Bar = 100 µm. (D) Many of the tumor cells have vacuolated cytoplasm, and some show a spider web-like appearance. HE. Bar = 100 µm. Inset: Higher magnification. Bar = 25 µm. (E) At the periphery of the mass, a focus of clear cells can be seen. (F) Higher magnification of Fig. E. Nests of round, clear tumor cells separated by thin fibrous septa can be seen. HE. Bars = 2 mm (E) and 100 µm (F). (G) Tumor cells invade the spinal canal and compress the spinal cord. HE. Bar = 2 mm. (H) Metastasis to the lung can be seen. HE. Bar = 2 mm.

Fig. 3.

Immunohistochemical findings of the mass. Most tumor cells are positive for myogenin (A), desmin (B), and vimentin (C). Also, many of the tumor cells are positive for alpha-smooth muscle actin (D). Bars= 100 µm.

Fig. 4.

Ultrastructural findings of the mass. (A) Ultrastructurally, the tumor cells are characterized by distorted nuclei and distinct nucleoli. Bar = 25 µm. (B) In the cytoplasm, the tumor cells contain bundles of myofilaments with Z-band-like structures (arrow). Bar = 3.5 µm (C). Some tumor cells have vacuoles in the cytoplasm. These vacuoles are filled with electron-lucent, flocculent materials, and a limiting membrane cannot be seen. Bar = 2.5 µm.