Changes in the macrophage content of lung metastases at different stages in tumor growth

The macrophage content of experimental B16 melanoma metastases at different stages of their growth has been quantified with the use of morphometry in conjunction with a recently developed histochemical method for selectively staining intratumoral macrophages. Data are presented from analyses of 954 sections of 155 individual lung metastases, showing that the macrophage content of individual B16 melanoma lung metastases not only varies significantly but also falls dramatically once metastases contain more than 700 tumor cells. In addition to providing new information on host response reactions of micrometastases, these experiments also indicate that conclusions on intratumoral macrophages derived from studies of large primary tumors and metastases in advanced stages of growth may have little or no relevance to events in micrometastases.