The derivation and characterization of stromal cell lines from the bone marrow of p53-/- mice: new insights into osteoblast and adipocyte differentiation.

We have derived a series of clonal cell lines from the bone marrow of p53-/- mice that represent different stages of osteoblast and adipocyte differentiation. All cell lines show indefinite growth potential (>300 population doublings) and have generation times of 12-20 h. These cell lines have been grouped into three categories. The least mature clones are heterogeneous and appear to contain a subpopulation of stem cells, which can spontaneously generate foci that contain either adipocytes or mineralizing osteoblasts. The second category of clones are homogeneous and clearly correspond to mature osteoblasts because they express high levels of the anticipated osteoblastic markers in a stable fashion and cannot differentiate into adipocytes even in the presence of inducers. The clones in the third category are the most unique. Initially they appeared to correspond to mature osteoblasts because they express alkaline phosphatase in a homogeneous manner, secrete type I collagen, show a significant cyclic adenosine monophosphate response to parathyroid hormone, secrete osteocalcin, and mineralize extensively after only 4-7 days. However, in contrast to the mature osteoblasts, these clones can be induced to undergo massive adipocyte differentiation, and this differentiation is accompanied by the complete loss of expression of all osteoblastic markers except alkaline phosphatase. These observations indicate that some cells that have acquired all of the characteristics of mature osteoblasts can be diverted to the adipocyte pathway. Further characterization of these clones may be particularly relevant to osteoporotic conditions where increased adipocyte formation appears to occur at the expense of osteoblast formation.