Cell cringe in response to cAMP. Numbers in each panel indicate the approximate time (±2 s) before or after addition of cAMP to starved cells of the normal strain (top row) or MHC− cells (bottom row). Observe how the central cell in each panel is more elongate before addition of cAMP but becomes more rounded at, or just subsequent to, addition of cAMP. Specifically, the normal cell is less elongated at +12 s compared with −9 or −33 s, and the MHC− cell is nearly round at +15 s compared with −30 or −15 s. By +30 s, this particular MHC− cell begins to show signs of new pseudopodia (arrow) and by +85 s is once more elongate. This normal cell shows new pseudopodia (arrow) by the +30-s time point. cAMP concentrations ranging from 10−7–10−4 M were tested, and no significant differences were observed by this assay. For the images shown, 10−4 M cAMP was used in the top row, and 10−7 M cAMP was used in the bottom row. Bar, 5 μm.