Three-dimensional cultures of normal human osteoblasts: proliferation and differentiation potential in vitro and upon ectopic implantation in nude mice

Abstract

We report the establishment in vitro of three-dimensional (3D) cultures of human osteoblasts (hOB) derived from normal adults and supported uniquely by the extracellular matrix (ECM) they deposit. Osteoblasts were cultured in 3D cultures in vitro for up to 120 days. The 3D cultures, examined at 25, 31, and 48 days, expressed protein markers of osteoblastic cells, namely osteonectin, collagen type I, fibronectin, osteopontin, bone sialoprotein, biglycan, and decorin. Sequentially, alkaline phosphatase (AP) and then Ca incorporation, mineralization of matrix (monitored by histochemistry and transmission electron microscopy), and finally osteocalcin expression, were detected in the 3D cultures. Ultrastructurally, morphology progressed from early to mature osteoblast and to osteocyte-like. Cells were embedded in a matrix with organized collagen type I fibers containing, increasingly with time of culture, needle-shaped crystals, often associated with matrix vesicles, characteristic of those in bone. During the culture (up to 120 days) there was an outgrowth of proliferating osteogenic cells from the 3D structure. Subcutaneous implantation in nude mice for 20 days of osteoblasts cultured in 3D culture for different lengths of time in vitro, showed progression of mineralization from the inner region of the implant outward, with peripheral cells being embedded in nonmineralized, collagen-rich matrix. The 3D implants were invaded by vessels derived from the host.