Growth and composition of a transplantable murine Leydig cell tumor

C57BL/6J mice inoculated sc with 50 mg of a transplantable Leydig cell tumor (M5480) demonstrated a reproducible pattern of slow tumor growth up to day 10 followed by rapid growth at the rate of approximately 0.5 g per day through day 27. The mean survival time of tumor-bearing mice was 33 days, when tumor weight accounted for more than 25% of total body weight. The rapid increase in weight occurring around day 10 resulted largely from a 20-fold increase in the quantity of extravasated blood inside the tumor, which in turn promoted a 50% reduction in host hematocrit. Sustained enlargement of M5480 during the third and fourth weeks of growth was supported by proliferation of tumor cells. Apart from blood-filled cavities, over 90% of the tumor consisted of neoplastic Leydig cells exhibiting generalized cytoplasmic features usually associated with mitotic activity. An activated macrophage was the next most abundant cell type, accounting for 2-3% of the nucleated cell mass. The remaining 3% was occupied by vascular elements, leukocytes, and giant cells. Depending on the age of the tumor, varying proportions of the cell population showed signs of anoxic degeneration. Degenerate cells were minimal at day 14, accounting for less than 4% of the total population, and maximal beyond day 21, when they occupied more than 50% of the cell mass.