JPC SYSTEMIC PATHOLOGY

MUSCULOSKELETAL SYSTEM

April 2025

M-N08

 

SIGNALMENT (Accession#3144240): Three-week-old, male, Mox-2 Cre transgenic mouse (Mus musculus).

HISTORY: A mass was noted on left front leg. 

GROSS PATHOLOGY: A firm mass encompassed the left front leg from the shoulder to

the paw (~2 cm in diameter).

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle, left forelimb: Effacing skeletal muscle, elevating the overlying dermis and epidermis, and multifocally infiltrating the panniculus carnosus, is a well-demarcated, infiltrative, unencapsulated, densely cellular, nodular neoplasm composed of spindle cells arranged in long streams and bundles on a fine, fibrovascular stroma. Neoplastic cells have indistinct cell borders, moderate amounts of finely fibrillar basophilic cytoplasm, an oval to elongate nucleus with finely stippled chromatin and 1-2 distinct magenta nucleoli. There is moderate to marked anisocytosis and anisokaryosis with occasional nuclear pleomorphism and karyomegaly. There are more than 50 mitotic figures per 2.37mm², and they are frequently atypical. Neoplastic cells are often multinucleated with nuclear rowing (strap cells or myotube-like) or are plump and eosinophilic (myoblast-like). Rarely neoplastic cells contain faint, linear, cytoplasmic cross-striations. There are multifocal areas of lytic and coagulative necrosis as well as hemorrhage, fibrin and edema. Remaining entrapped skeletal muscle fibers are shrunken (atrophy). There are scattered lymphocytes, plasma cells and macrophages in the surrounding subcutis and dermis. Overlying the mass, there is a focal area of epidermal ulceration with large numbers of neutrophils infiltrating the underlying dermis and subcutis. The adjacent epidermis is mildly hyperplastic.      

MORPHOLOGIC DIAGNOSIS: Skeletal muscle, left forelimb: Rhabdomyosarcoma, Mox-2 Cre transgenic mouse, murine.

GENERAL DISCUSSION:

• Rhabdomyosarcoma (RMS) is a rare type of sarcoma arising from pluripotent progenitor cells that differentiate towards skeletal muscle
• They can arise in skeletal muscle, but more commonly occur in locations that normally lack skeletal muscle
• Typically aggressive with rapid growth, local invasion, and metastasis to unusual locations (i.e. other muscles, heart, spinal cord, as well as lung, liver, and spleen)
• Embryonal RMS and its botryoid variant are the most common form in animals
• Embryonal RMS
  • Round/myoblast-like (most common) or elongated/myotube-like (strap cells)
  • Mononuclear to multinucleated
  • Most common in young animals involving the head or neck; an important differential for non-histiocytic, non-lymphoid round cell neoplasms of the head and neck
• Botryoid variant of embryonal RMS (U-N03)
  • Most often in the trigone area of urinary bladder in young large breed dogs; occurs in females more than males
  • Spindle and stellate cells within myxoid stroma
• Alveolar RMS
  • Sheets of undifferentiated small round cells, forming alveolar-like structure
• Pleomorphic RMS
  • Least common; must completely lack features of embryonal and alveolar rhabodomyosarcomas
• Cardiac rhabdomyosarcoma
  • Rare, only reported in dogs; poorly differentiated and invasive

PATHOGENESIS:

• Animal models indicate that suppression of p53 is required for tumor development
• Cytogenetic abnormalities play a role in human RMS, unknown in animals
• Possible genetic component as related litters of pigs developed RMS

TYPICAL CLINICAL FINDINGS:

• Fleshy growth in a variety of locations
• Botryoid RMS can cause urinary obstruction (dysuria, stranguria, and hematuria) and be associated with hypertrophic osteopathy

TYPICAL GROSS FINDINGS:

• Pink, fleshy mass most often affecting the head, neck, urinary bladder, tongue, larynx, heart, or orbit, but can occur anywhere
• Botryoid RMS most often occur in the trigone of bladder as polypoid, grape-like (i.e. botryoid) masses protruding into the lumen

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Highly variable presentation- Round cells, spindle cells, or mixed morphologies
  • Primitive embryonal round cells with either prominent eosinophilic cytoplasm (rhabdomyoblasts), or indistinct cytoplasm (lymphoid-like appearance), and rare cross striations
  • More differentiated spindle cells with myotube formation, multinucleation, frequent cross striations, and strap/racquet cells
• Often considerable pleomorphism with multinucleated cells
• Cross striations when present aide the diagnosis, but are often not seen; polarized light microscopy may assist in visualization
• Elongated ‘strap’ cells or ovoid ‘racquet’ cells may be present

ADDITIONAL DIAGNOSTIC TESTS:

• Electron microscopy –Primitive myofilaments and Z-band structures
• Phosphotungstic acid hematoxylin (PTAH): Highlights cross striations, which are variably present depending on differentiation
• Immunohistochemistry:
  • Positive for: Vimentin, desmin, muscle specific actin, sarcomeric actin, myoglobin, myogenin, and MyoD1
    • Sarcomeric actin, myoglobin, myogenin and MyoD1 are specific for skeletal muscle
    • Myogenin and MyoD1 are skeletal muscle specific nuclear transcription factors expressed early in skeletal muscle development (Tuohy, Vet Pathol. 2021)
      • MyoD1 expression is associated with immature rhabdomyoblasts that have a higher proliferative capacity
      • Myogenin is expressed during myoblast fusion and is associated with exit from the cell cycle
      • RMSs that are composed of relatively undifferentiated cells are expected to express less desmin, actin, myosin, and myoglobin and more MyoD1 and myogenin
    • Negative for: cytokeratin, smooth muscle actin

DIFFERENTIAL DIAGNOSIS:

• Poorly differentiated and multinucleated sarcomas (that do not express muscle markers) are often misdiagnosed as rhabdomyosarcoma
• Rhabdomyoma – Most often occurs as a round cell tumor in the larynx of adult dogs, non-invasive, low mitoses, no metastasis
• Leiomyosarcoma - Smooth muscle actin positive
• Tumors metastatic to muscle
• Soft tissue sarcoma
• Lymphoma
• Oncocytoma
• Chondroma/sarcoma
• Osteochondroma (larynx)

COMPARATIVE PATHOLOGY:

• Reported in numerous species
• Horse: Case report of disseminated pleomorphic rhabdomyosarcoma (Carpenter, J Vet Diagn Invest. 2025)
• Cattle: Case report of rhabdomyosarcoma of the tongue in a neonatal calf (Hishikawa, J Comp Pathol. 2024)
• Cat: Associated with feline injection sites (Ipek, J Comp Pathol. 2024)
• Dog: Case report of invasive spindle-cell rhabdomyosarcoma with osteolysis (Shi, J Comp Pathol. 2023)
• Mice: A strain and BALBc overrepresented as well as GEMs and SCID
• Pig: Cardiac rhabdomyoma (some consider it a hamartoma); possible Purkinje cell orgin (PGP 9.5 expression)
• Guinea pig: Cardiac rhabdomyomatosis (C-M02)
• Avian: Infrequently reported in pet birds, wild birds, and poultry within the heart and skeletal muscle; strap cells and large multinucleated cells common features
• In chickens, caused by avian leukosis

REFERENCES:

1. Barnes HJ, Abdul-Aziz T, Fletcher OJ. Chapter 4: Muscular System. In: Abdul-Aziz T, Fletcher OJ, Barns HJ, eds. Avian Histopathology. 4th ed. Madison, WI: Omnipress; 2016: 129.
2. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. West Sussex, UK. John Wiley & Sons, Inc.; 2016: 115-116, 241-242.
3. Carpenter AL, Townsend KS, Johnson PJ, Kim DY. Disseminated pleomorphic rhabdomyosarcoma in a horse. J Vet Diagn Invest. 2025;37(1):203-207.
4. Caserto BG. Comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol. 2013; 50:806-826.
5. Cooper BJ, Valentine BA. Muscle and Tendon. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St Louis, MO: Elsevier; 2016: 241-243.
6. Dal Pont TP, Santos IR, Pereira VC, Ferrari FE, Schmidt VRQ, Bandinelli MB, Alievi MM, Pavarini SP. Spindle cell rhabdomyosarcoma in a cockatiel (Nymphicus hollandicus). J Comp Pathol. 2025;217:34-38.
7. Hishikawa S, Sunden Y, Imamura A, Nishikawa T, Morita T. Rhabdomyosarcoma of the tongue in a neonatal calf. J Comp Pathol. 2024;209:36-39.
8. Ipek V, Karagul I, Gulbenli Turkoglu B. Unlocking immunotherapy targets: programmed death 1 and its ligand and their correlation with tumour grade in feline injection site sarcoma. J Comp Pathol. 2024;213:10-19. 
9. Schmidt R, Reavill DR, Phalen DN. Pathology of Pet and Aviary Birds. 2nd ed. Ames, IA: John Wiley & Sons, Inc.; 2015: 205.
10. Schwarz S, Mathes K, Wohlsein P. Rhabdomyosarcoma on the Forelimb of a Common Musk Turtle (Sternotherus odoratus). J Comp Pathol. 2021;186:73-76.
11. Shi J, Gao R, Zhang J, Xu R, Jia Q, Ma Y, Lu H, Zhao K, Gao F, He W. Invasive spindle-cell rhabdomyosarcoma with osteolysis in a dog: case report and literature review. J Vet Diagn Invest. 2023;35(2):168-172. 
12. Tuohy JL, Byer BJ, Royer S, et. al. Evaluation of Myogenin and MyoD1 as Immunohistochemical Markers of Canine Rhabdomyosarcoma. Vet Pathol. 2021;58(3):516-526.
13. Valentine BA. Skeletal Muscle. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022: 1015-1016.

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