Tissue-like behavior of human pulp sphere cells. Figure (a) shows merging pulp-spheres in a human root canal model after five days. After 28 days of cultivation the two spheres merged, cells grew out of the spheres and coated the root dentin walls (white arrows). (b) Outgrowing cells also covered the underlying polystyrene culture dish and built up a dense cell layer (black asterisk). SEM investigation of the merged spheres (c and d) represents the even surface of the new cell construct and the widespread coverage of the root canal walls (c) as well as the randomly distributed different sized particles (d).