Marrow mesenchymal stem cells are well known for their differentiation into bone-forming osteoblasts and in vitro mineralized tissue formation. However, process details, including tissue structure and cellular environments, remain unclear. The present study demonstrates three-dimensional visualization of tissue fabricated by culturing MSCs in the presence of calcein, a fluorescent marker for bone mineralization. The 3D visualization was performed by computer-assisted confocal laser scanning microscopy and revealed that the in vitro tissue consisted of layers of a mineralized matrix with round cells in the matrix lacunae, an unmineralized matrix (osteoid), and osteoblastic cells on the osteoid surface. The findings show that the mineralization by cultured MSCs is an in vitro counterpart of in vivo bone formation and indicate that the novel technique of visualization without tissue fixation could be useful for continuous monitoring of tissue organization in an ongoing culture. (C) 2004 Elsevier Inc. All rights reserved.