Surface marker expression, SA-β-gal staining and telomerase activity in HBCEC. A. Determination of the percentage of cell surface marker expression in HBCEC at different ages. Expression of the surface marker proteins CD24, CD44, CD227 was maintained during long term culture of HBCEC. Whereas CD24 and CD44 were similarly expressed after 176d and 462d, CD227 increased from 52% to 88% in HBCEC 462d. The flow cytometry measurements varied by about 8%. B. SA-β-gal staining of primary HBCEC and HMEC cultures. Staining for SA-β-gal of a HBCEC population after 722d in culture revealed little if any positive cell. Normal HMEC in passage 16, however, displayed already predominantly enlarged senescent cells after 32d, demonstrated by the dark-green stain (bar = 200 μm). C. Telomerase (TRAP-)assay of primary cultures from breast cancer biopsies. Telomerase activity was analyzed according to the Telomeric Repeat Amplification Protocol (TRAP). HBCEC populations demonstrated telomerase activity independent of the age of the culture and the harvest method. The human embryonic kidney (HEK) 293T cell line was used as a positive control and 1× CHAPS buffer served as a negative control. Quantification was performed using densitometric analysis.