(A) Representative phase contrast photomicrograph of pseudoemperipolesis of CLL B cells after 2-hour culture on the heterologous murine stromal cell line, M2-10B4. Cells that had not migrated beneath the stromal cells washed off, and the stromal cell layer containing the migrated CLL cells was photographed (200x magnification). Pseudoemperipolesis is characterized by the dark appearance of lymphocytes that have migrated into the same focal plane as the stromal cells. (B) For comparison, this photomicrograph shows reduced pseudoemperipolesis after pretreatment of CLL cells with pertussis toxin.