Immunohistochemical localization of trk receptor protein in pediatric small round blue cell tumors

Expression of Trk protein has been documented by Northern analysis in neuroblastomas with good prognosis. To localize the expression of this protein at the cellular level within individual tumors, we adapted a recently characterized panTrk antibody for use in formalin fixed, paraffinembedded tissue. We have examined a group of small round blue cell tumors occurring in children, including both high and low stage neuroblastomas, to assess the presence or absence of Trk expression and its cellular localization. Positive staining for Trk protein was observed in four of four low stage (good prognosis) neuroblastomas, five of five primitive neuroectodermal tumors/Ewing's sarcoma, five of five rhabdomyosarcomas, and no lymphomas. Within the neuroblastomas, expression of Trk protein was most striking in ganglion cells, in which positive cytoplasmic staining was demonstrated regardless of tumor stage. The latter observation may lend further insight into the pathobiology of this malig-nant childhood tumor.