Fig. 4. Pioneer myofibers fuse with younger cells during development of multinucleated intercostal muscles. Frontal sections through a E8 quail embryo pulse-chased with thymidine and counterstained for desmin (brown reaction product) as described in the Materials and Methods. (A) Low magnification to show localization of intercostal muscles (M) between successive ribs (R). Arrows indicate clearly distinguishable fibers containing nuclei with thymidine grains. (B-D) Higher magnifications illustrating several parallel fibers each containing multiple nuclei stained with Hematoxylin (light blue). Arrowheads indicate thymidine-positive pioneer nuclei which are an integral part of the multinucleated fiber. Scale bar: 100 µm in A; 22 µm in B-D.
