The role of osteoblasts in the hematopoietic microenvironment.

Hematopoietic stem cell differentiation occurs in direct proximity to osteoblasts within the bone marrow cavity. Despite this striking affiliation, surprisingly little is known about the precise cellular and molecular impact of osteoblasts on the bone marrow microenvironment. Recently, it has been proposed that human osteoblasts support the growth of primitive human hematopoietic cells in vitro and possibly in vivo. Evidence to support this hypothesis is reviewed as follows: the influence of osteoblasts on osteoclast development; the participation of osteoblasts in long-term bone marrow cultures; the production of positive hematopoietic regulatory molecules by osteoblasts; the production of cell-cycle inhibitory factors by osteoblasts, and cell-cell interactions between early hematopoietic cells and osteoblasts.