To form muscle fibers, masses of cells merge to form long tubes with many cell nuclei (above). Recent research reveals two distinct stages in this process. First, proteins on the outside of the cells (shown as spikes in A) promote the fusion of the cell membranes. The blue squares in B and C represent proteins inside the cells that facilitate the development of a pore between them, so their contents can merge. This process of cell fusion is a basic part of the biology of muscle cell fusion and of several other types of cells as well.