Fig. 5. Penetration of premyofibrils and myofibrils into the nuclear grooves and invaginations of myotubes. (A) Penetration of both muscle- and non-muscle-type actin-containing premyofibrils. (B) Penetration of muscle-type actin containing premyofibrils. Non-muscle-type actin is not present in the premyofibrils. (C) Penetration of actin- and myosin-containing premyofibrils. (D) Penetration of actin- and myosin-containing myofibrils with crossstriations. PC, phase-contrast images; H33258, nuclear staining with H33258; actin, total actin staining with rhodamine-phalloidin; NM actin, non-muscle actin staining with absorbed antigizzard actin pAb; MyHC, sarcomeric MyHC staining with the mAb A4.1025; H33258 + actin, H33258 + NM actin, H33258 + MyHC, doubly merged images; Merge, triply merged images. Bar, 10 µm.