Pathology: ectomesenchymoma


  • Young adult; paratesticular

  • Nests and fascicles with heterogeneous composition

  • Fascicles of spindle-rhabdoid cells with abundant eosinophilic cytoplasm; conspicuous cross striations; round-ovoid nuclei with mild atypia and occasional multi-nucleation; inconspicuous mitotic activity; occasional myotube-like structures

  • Sheets of small round cells with scant cytoplasm; round nuclei with mild atypia; conspicuous mitotic activity

  • Interspersed ganglion-type cells; abundant cytoplasm with occasional Nissl substance; central nuclei with macronucleoli

  • IHC: myogenin, synaptophysin

  • Frequent HRAS mutations (PMID: 26872011)