This presentation will review how myofibrils are formed in both types of vertebrate cross-striated myofibrils, i.e., cardiac and skeletal muscle.

Cardiomyocytes isolated from 7 to 9 day-old chick embryos and myoblasts isolated from 10-day old quail embryos were placed in tissue culture. The cardiac cells were allowed to spread in culture for 3 - 5 days. Similarly, the myoblasts were permitted to fuse to form myotubes which were then fixed at different points of elongation over 3 -5 days. Some cultures were fixed and stained with antibodies (e.g., sarcomeric isoform of alpha-actinin; non-muscle myosin IIB and muscle myosin II) to detect the disposition of the forming and mature myofibrils inside these cells. Sister cultures were transfected with plasmids expressing Green Fluorescent Protein (GFP) ligated sarcomeric proteins like alpha-actinin. The staining and transfection results revealed that three different types of fibrils form during myofibrillogenesis: premyofibrils, nascent myofibrils and mature myofibrils (1-2).