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Tumor invasion has been correlated with the ability of tumor cells to produce collagenolytic enzymes which are capable of degrading normal host tissues. However, the human small cell carcinoma implanted subcutaneously and growing progressively in athymic (nude) mice produced large quantities of collagenase but did not appear to significantly infiltrate adjacent host tissue. In comparison, subcutaneously implanted murine Lewis lung tumors produced similar quantities of collagenase and were locally invasive. The human tumors were surrounded by a compact layer of fibroblast cells in a fibrous matrix. This fibrous sheath exhibited anticollagenase activity and indicated a mechanism of host tissue resistance to invasion via the formation of inhibitors to degradative enzymes produced by tumor cells.

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