Fig. 4. Cardiomyocyte and fibroblast spreading. Cardiomyocytes and pericardial fibroblasts were plated from 10-day old embryonic myocardium without purification and spread areas were measured (± s.d. for >15 cells in triplicate studies) after 4 or 24 hours. Fibroblasts were detected as -actinin-negative cells and were the larger fraction of cells, consistent with in vivo proliferation (Fig. 2B). Half maximal `set points' for projected cell area correspond closely to the peaks in passive tissue elasticity in Fig. 2B.