Musculoskeletal System
April 2019

SIGNALMENT (NIEHS#15-0076):  11.5-month-old male B6.129-Trp53tm1Brd mouse

HISTORY:  This mouse was exposed to a chemical via inhalation for 5 days a week for up to 8 weeks.  At the end of the exposure period the mouse was held without further exposure until death.  The mouse died prior to the time of scheduled sacrifice.

GROSS PATHOLOGY: Subcutaneous mass located above and around right front shoulder.

HISTOPATHOLOGIC DESCRIPTION:  Striated muscle and fibrovascular tissue, right shoulder (per contributor):  Infiltrating and effacing the normal adjacent skeletal muscle and subcutaneous adipose tissue is an unencapsulated, well circumscribed, 2x2cm, densely cellular neoplasm composed of tightly packed moderately to highly pleomorphic spindle cells  arranged in disorganized interlacing streams bundles and solidly cellular sheets on a scant collagenous matrix surrounded by multifocal varisized areas of coagulative and lytic necrosis and hemorrhage.  Neoplastic cells have indistinct borders, moderate to abundant eosinophilic fibrillar to vacuolated cytoplasm and large eccentrically located pleomorphic nuclei with finely stippled chromatin and 1-4 nucleoli. Mitoses average 1-4 per 400x hpf and are occasionally bizarre.  Anisokaryosis and anisocytosis are marked. Multinucleated and karyomegalic cells are numerous and occasionally these cells contain nuclei that line up within elongated cytoplasm (strap cells), or rarely, are globoid with one end tapering into an elongate or wispy tail (racquet cell).  These cells contain large, round eosinophilic glycogen inclusions and rarely very faint cytoplasmic cross striations.  Multifocally scattered throughout are small numbers of neutrophils.  Adjacent pre-existing muscle fibers are surrounded and separated, shrunken with loss of cross striations (atrophy).

MORPHOLOGIC DIAGNOSIS:  Striated muscle and fibrovascular tissue, right shoulder (per contributor): Rhabdomyosarcoma, B6.129-Trp53tm1Brd mouse, murine.










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