Fig. 3.

Sarcomere assembly and titin integration in differentiating ES cells. Alternating fluorescent titin bands in newly assembled sarcomeres are delineated with red and green dots, respectively (merged panels). The white and gray arrows indicate more and less mature integration sites, respectively. (A) α-Actinin staining identifies the punctate Z-bodies of the early developing premyofibrils before titin is integrated and Z-disks. (B) Staining for sarcomeric actin (thin filament) that partially overlaps with Z-disk titin. Mature sarcomeres with titin and actin integrated into the sarcomere lattice (white arrow) coexist with less mature regions where titin is integrated but actin is more distributed (gray arrow).