Osteogenesis and chondrogenesis of bone marrow stromal cells

Bone marrow stromal cells (MSC1)

The existence of non-hematopoietic cells in bone marrow was first suggested by Cohnheim about 130 years ago 16). Bone marrow-derived stromal cells (MSC1) can differentiate into most somatic cells, including osteoblasts, chondrocytes, myoblasts, cardiomyocytes 17-21), and adipocytes, when placed in appropriate in vitro 20) and in vivo environments 22), and thus are a useful cell source for regenerative medicine 23). Recent studies suggest that MSC1 can also differentiate into a neuronal lineage 24), and murine bone marrow-derived adult progenitor cells can differentiate into dopaminergic neuronal cells 25, 26). Since the use of MSC1 entails no ethical or immunological problems, and bone marrow aspiration is an established routine procedure, these cells provide a useful and almost routine source of material for transplantation and tissue repair or regeneration (Fig. 1: http://blogs.yahoo.co.jp/akihiroumezawa/19883767.html).

1) Osteogenesis
KUSA-A1 cells, a murine marrow stromal cell line, are capable of generating mature bone in vivo 27). They are a unique, mature osteoblast cell line and serve as a very suitable model for in vivo osteogenesis. Bone forms in subcutaneous tissue after subcutaneous injection of the cells into mice. The osteogenesis by KUSA-A1 is not mediated by chondrogenesis and thus is considered to be membranous ossification. Follow-up study on the fate of bone by immortalized osteoblasts shows that the ectopically-generated bone keeps its size and shape for 12 months 21). Furthermore, the implanted cells do not metastasize like tumor cells. These unique characteristics of KUSA-A1 cells provide an opportunity to analyze the process of membranous ossification in detail.

2) Chondrogenesis
Chondrocytes differentiate from mesenchymal cells during embryonic development 28) and the phenotype of the differentiated chondrocyte is characterized by the synthesis, deposition, and maintenance of cartilage-specific extracellular matrix molecules, including type II collagen and aggrecan 29-31). The phenotype of differentiated chondrocytes is rapidly lost since it is unstable in culture 32-35). This process is referred to as 'dedifferentiation' and is a major impediment to use of mass cell populations for therapy or tissue engineering of damaged cartilage. When isolated chondrocytes are cultured in a monolayer at low density, the typical round chondrocytes morphologically transform into flattened fibroblast-like cells, with profound changes in biochemical and genetic characteristics, including reduced synthesis of type II collagen and cartilage proteins 36). When cultured three-dimensionally in a scaffold such as agarose, collagen, and alginate, redifferentiated chondrocytes re-express the chondrocytic differentiation phenotype.

KUM5 mesenchymal cells, a MSC1 line, generate hyaline cartilage in vivo and exhibit endochondral ossification at a later stage after implantation 37). OP9 cells, another MSC1 line, derived from macrophage colony-stimulating factor-deficient osteopetrotic mice, and also known to be niche-constituting cells for hematopoietic stem cells, express chondrocyte-specific or -associated genes, such as type II collagen 1, Sox9, and cartilage oligomeric matrix protein at an extremely high level, as do KUM5 cells. OP9 micromasses exposed to TGF- 3 and BMP2 form type II collagen-positive hyaline cartilage within two weeks in vivo. The unique characteristics of KUM5 and OP9 cells provide an opportunity to analyze the process of endochondral ossification.