Morphology and cytoskeletal reorganization of RBE cells adhering on extracellular matrix proteins. Serum-starved wild-type (wt), mock-transfected (mock), or CEACAM1-L–expressing (CEACAM1-L) cells (3 × 10⁴) were plated on fibronectin (FN), laminin-1 (LN), or Matrigel coatings (Matrigel, diluted 1:100) under serum-free conditions for 75 minutes. (Ai) Nonadherent cells were removed by washing, and adherent cells were fixed and visualized with crystal violet. Original magnification, × 100. (ii) For discrimination of differential cell morphologies, cells with or without at least one protrusion longer than the intersection of the cell body were counted using the AxioVision software. Relative numbers of cells with extended protrusions are presented as means ± SD of 2 independent experiments done in triplicate. ^*P < .001. (Bi) Visualization of the actin cytoskeleton by phalloidin-TRITC in wt (wt), mock-transfected (mock), tyrosine double-mutant (CEACAM1-L/Y448F/Y513F), or CEACAM1-L–expressing (CEACAM1-L) RBE cells during spreading on fibronectin (FN) and laminin-1 (LN). CEACAM1-L + α-CC1 pAb indicates treatment with polyclonal rabbit anti-CEACAM1 antiserum (50 μg/mL). Note the occurrence of small protrusions with lamellae after α-CC1 pAb treatment of CEACAM1-expressing cells on fibronectin (arrows). Original magnification, × 400. (ii) Protrusion length of adhering cells (distance in μm from cell body to the tip) was measured with AxioVision software and means ± SD of 2 experiments done in duplicate are shown (n = 20). Antibodies (mAb Be9.2 IgG and rabbit polyclonal anti-CEACAM1 IgG [α-CC1 pAb]) as well as control rabbit IgG were added at 50 μg/mL. ^*Increase in protrusion length of CEACAM1-L–expressing cells by α-CC1 treatment represents significant difference, P < .001.