Figure 1.

The presence of fibrillar collagen blocks the cell cycle of M24met cells in the G₁ phase without stimulating apoptosis. (A) M24met human melanoma cells were cultured for 20 h on top of a fibrillar collagen, embedded into the collagen gel, or plated in tissue culture dishes precoated with nonfibrillar collagen. Cells collected by collagenase treatment were permeabilized and stained with PI for nuclear DNA content analysis by flow cytometry. The mean fluorescence intensity of cells with a relative DNA content of 1 (G₀/G₁ phases) was 400 and that of cells with a relative DNA content of 2 (G₂/M phases) was 790. (B) Distribution of cells in different phases of the cell cycle (average + SD), based on 10 independent experiments. (C) In some experiments, we compared cells cultured in the presence or absence of serum. The histograms present the percentage of cells in S phase, as the average + SD in one representative experiment performed in triplicate. NFC, nonfibrillar collagen; FC, fibrillar collagen. (D) Unpermeabilized cells were incubated in the presence of PI and FITC-conjugated-Annexin V and analyzed by bivariate flow cytometry for apoptosis. Note in both conditions a small (8–9%) percentage of PI-positive cells but an absence of cells binding Annexin-V at their surface.