Ultrastructural evidence for divergent and alternating differentiations in spindle cell sarcoma xenografts

Seven spindle cell sarcomas, 5 poorly differentiated ones and 2 moderately well differentiated ones, were established on nude mice and long term passaging was done. Sarcoma strains were analysed electron microscopically in an attempt to get further insight in spindle cell sarcoma differentiation pathways. Ultrastructurally, the tumours were classified as malignant peripheral nerve sheath tumour (3/7), leiomyosarcoma (2/7), rhabdomyosarcoma (1/7), and spindle cell sarcoma not otherwise classifiable (1/7). Undifferentiated tumour cells including fibroblastoid ones predominated in most xenografts, whereas cells harbouring cytoplasmic specificities tended to be few in number. Nevertheless, divergent differentiations exhibiting unusual double or triple patterns could be documented ultrastructurally in 12/30 xenografts with juxtaposed myomatous as well as nerve sheath-like cells and, in addition, histiocytoid (MFH-like) elements in 3 of the xenografts. Moreover, sarcoma strains alternated fine structural constellations in the course of passaging, whereby different phenotypes, myomatous, nerve sheath-like, unspecific, or mixed ones, succeeded one another. These findings pursue recent immunohistochemical data on multidirectional sarcoma differentiation by means of electron microscopy. They, furthermore, fit well into the concept of multipotential stem cells as progenitors in mesenchymal differentiation and suggest microenvironment to play a modifying role in the expression of cell differentiation.