We have reported the isolation of a unique subpopulation of human stromal cells from bone marrow termed marrow-isolated adult multilineage inducible (MIAMI) cells. The expression of embryonic stem cell markers SSEA-4, Oct-4, Rex-1, and felomerase reverse transcriptase indicates the developmentally immature status of these cells. They resemble primitive stem cells in their capacity to differentiate, at least in vitro, into mature-like cells from all three germ layers. MIAMI cells are characterized by a unique molecular profile that distinguishes them from other marrow stromal cell populations. Although the frequency of MIAMI cells, among all marrow nucleated cells, decreases from 0.01% at age 3 to 0.0018% at age 45, their numbers remain unchanged after age 45. The level of expression of the markers characteristic of MIAMI cells remains constant independent of age and gender. In long-term in vitro expansion experiments aging increased the population doubling time by about 30%, whereas specific in vitro differentiation of MIAMI cells toward osteoblastic cells was unaffected. Because the oxygen tension in bone marrow ranges front 1% to 7%, we examined the role of oxygen tension in regulating the capacity of MIAMI cells to self-renew and maintain their pluripotentiality during long-term culture. Low oxygen tension upregulated mRNAs for primitive embryonic stem cell markers. Our results suggest that maintaining developmentally primitive human cells in vitro at low oxygen tension is more physiologic and favors stemness. For osteoblastic differentiation, gap-junctional communication mediated by connexin43 is required. Its inhibition not only blocked osteoblastic differentiation but stimulated the adipocytic differentiation. © Mary Ann Liebert, Inc.