Properties and origin of osteoblasts.

Osteoblastic and chondroblastic (i.e., osteogenic) cells belong to the stromal cell system, which is associated with bone marrow, and bone and is separate from the hematopoietic stem-cell system. Stromal stem cells are capable of producing reticular, fibroblastic, osteogenic, and adipose stromal lines. Marrow-derived osteogenic cells are a component of marrow stroma, which in vitro form fibroblastic-type colonies. These colonies are a heterogeneous population with varying enzymatic expressions and potencies that differentiate into fibroblastic, reticular, adipocytic, and osteogenic populations. It is postulated that these colonies are a component of the stem- and progenitor cell populations. Progenitors of osteogenic cells are widely distributed in the extraskeletal organs. On contact with an adequate inductor, they differentiate into chondro- and/or osteoblasts, thus producing ectopic (i.e., induced) cartilage and/or bone. Such osteoprogenitor cells were termed inducible osteoprogenitor cells, in contrast to the determined osteoprogenitor cells, which are present in the bone marrow stroma and produce bone spontaneously. To the class of determined osteoprogenitors also belong endosteal cells, periosteal cells, and osteoblastic established cell lines. There is no evidence of the presence of osteogenic cells in the blood and peritoneal fluid. The concept of mesenchymal cells as an osteoblastic precursor in adult organisms is open to question.