Microscopic appearance of tumor cells and fibroblasts. (a) Numerous clumps of UACC-812 tumor cells resembling abortive mammary glands are evident. (b) UACC-812 breast cancer cells seeded at low density on a monolayer of serum-activated fibroblasts. Note the gland formation and the invasive infiltration by tumor cells into fibroblasts. Primary tumor cells derived from (c) human ductal carcinoma in situ and from (d) invasive ductal carcinoma co-cultured with fibroblasts yielded clonogenic growth next to myofibroblasts that expressed α-smooth muscle actin (pink immunostain). Breast cancer cells grown (e) on a monolayer of B16 cells or (f) without a monolayer of fibroblasts generally grew as individual cells identifiable by their red staining for epithelial membrane antigen. (g) Serum-activated fibroblasts also uniformly expressed abundant syndecan-1, as detected by immunocytochemical staining.