Antibody ligation of CD9 in cocultures of EML-C1 and MS-5 cells results in increased cobblestone formation. EML-C1 cells were cocultured with MS-5 cells for 5 days and typical cultures were photographed at the magnification of 400×. (A) Hematopoietic cells were observed growing under MS-5 cells as phase-dark cobblestones in the presence of 1 μg/mL KMC8.8 (right panel) or isotype matched control antibodies (left panel). (B) Numbers of cobblestone areas per 3.5-cm dishes were also counted with a phase-contrast inverted microscope. Results represent the mean ± SD of triplicate cultures and statistically significant differences from control values are indicated by two (P < .01) asterisks. The results are typical of three similar experiments.