Earliest steps in primary tumor formation and micrometastasis resolved with histochemical markers of gene-tagged tumor cells

To facilitate detection of tumor cells at the highest resolution in any organ in athymic nude mouse model systems, a histochemical marker gene [bacterial lacZ or human placental alkaline phosphatase (ALP)] was transfected into specified transformed/tumor cells (fibrosarcoma or neuroblastoma). The fates of tumor cells were followed qualitatively and quantitatively by histochemical staining of whole organs or organ sections. Primary tumors developed initially via formation of 'curly-haired' complexes of cells in the subcutis or dermis, followed by division of a large fraction of cells. When two tumor classes were mixed before injection, outgrowth occurred in regional concentrations of the primary tumor. Blood microvessels were detectable within 72 hr of injection, growing into tumor regions. iv injection routinely yielded multicellular foci in the lungs within minutes as precursors of experimental metastases. Micrometastasis was further resolved with cells 'inactivated' by different treatments and by co-injection of two different tagged cell types. These approaches using different histochemical marker genes to 'tag' different tumor cell classes, along with more advanced molecular biological approaches, permit us to characterize gene expression and its reversibility during the earliest stages of primary tumor formation and micrometastasis to virtually any organ in the recipient animal.