Two cloned cell lines were isolated from cultures of mouse bone-marrow
cells. One of the lines, D1, exhibited osteogenic properties and
synthesized type-I collagen (alpha 1)2 alpha 2. The second cell line, D2,
was not osteogenic and produced a collagen homotrimer (alpha 1)3. Whereas
the extracellular matrix of the D1 cell cultures contained striated
collagen fibrils, presumably composed of type-I collagen, the
homotrimer-producing D2 cells did not demonstrate striated collagen
fibrils. Instead, they had thin filaments without detectable striations.
Sodium ascorbate stimulated collagen synthesis at the transcriptional level
in both the D1 and the D2 cells. The bone-producing characteristics of D1
in vitro included high levels of alkaline phosphatase, increased cyclic
adenosine monophosphate on treatment with parathyroid hormone, and
expression of osteocalcin mRNA. The D1 cells, unlike the D2 cells, produced
a mineralized matrix in vitro. Mineralization in the cultures of the D1
cells occurred in nodules of increased cell density, which also contained
the cells with the highest concentrations of collagen mRNA, as shown by in
situ hybridization. When the D1 cells were implanted in a diffusion chamber
in vivo, a mixture of both osteogenic and adipogenic tissues was formed.
This indicates that the D1 cell line is derived from an early marrow
stromal precursor that is multipotential.