Isolation and perivascular localization of mesenchymal stem cells from mouse brain.

Bone marrow is a complex tissue and is a source of several stem/progenitor cell populations. In addition to hematopoietic stem cells and endothelial progenitors,1–4 the bone marrow is also a source of a less well-characterized stem cell/progenitor population referred to as mesenchymal stem cells (MSCs). This cell population was originally identified in the 1970s by Friedenstein et al,1,5 who placed whole bone marrow in plastic culture dishes and, after removing the nonadherent (hematopoietic) cells, was left with an adherent cell population that had the ability to differentiate into colonies resembling bone or cartilage. Castro-Malaspina et al6 used similar methods to isolate adherent bone marrow cells, which others showed could be induced to differentiate into a variety of mesenchymal elements, including fat, bone, cartilage, and bone marrow stroma.7–10 Most recently, Pittenger and Marshak4 and Pittenger et al9 isolated a bone marrow population, referred to as MSCs, that grew as adherent cells on plastic, could be expanded ex vivo to high passage numbers, and could be differentiated into multiple mature connective tissue cell types, particularly adipocytes, chondrocytes, and osteocytes. Although there is no specific surface marker that defines MSCs, human MSCs are negative for CD45 and CD31 and express CD105, CD73, CD90, CD44, and CD166.11 In the mouse, there is less consensus regarding the defining surface markers,12–14 but several studies indicate that mouse MSCs are negative for CD45, CD11b, and CD31 and positive for stem cell antigen-1 (Sca-1), and CD9.14,15 Physiologically, these cells are thought to give rise to the bone marrow stromal elements that provide the microenvironment and supporting structure necessary for marrow homeostasis.1,4,16–19 For the neurosurgeon, MSCs are of interest because of their potential use in the therapy of a variety of neurological disorders.12 In the field of neuro-oncology, we and others have reported that ex vivo expanded human bone marrow–derived MSCs (BM-MSCs) selectively localize to human gliomas after systemic administration and can be engineered to deliver antiglioma therapies to experimental human glioblastomas such as interferon-β and tumor necrosis factor-α–related apoptosis-inducing ligand.20,21 Exogenous MSCs also localize to areas of stroke,22 traumatic brain injury,23 and intracerebral hemorrhage,24 suggesting a potential role in these pathological processes. MSCs are also of interest because of the potential involvement of endogenous MSCs in normal physiological responses to injury and inflammation and because they may comprise the stroma of solid tumors, the milieu of which is much like a nonhealing wound.25 Recent evidence suggests that endogenous MSCs may be an important component of glioblastoma multiforme, the most common adult malignant brain tumor. Specifically, cells resembling MSCs can be isolated from fresh glioma specimens obtained at surgery.26,27 Like BM-MSCs, these tumor derived-MSCs grow as adherent cells in culture; differentiate into adipocytes, chondrocytes, and osteocytes; and express the requisite MSC surface markers.26,27 These tumor-derived MSCs are thought to contribute to the stroma of human gliomas. Although it is postulated that MSCs may be recruited into areas of inflammation, injury, and tumors from a pool of circulating BM-MSCs originating from the bone marrow, to date there is conflicting evidence supporting the notion that BM-MSCs actually circulate in the bloodstream. However, an alternative source of MSCs may be the local tissues within peripheral organs. Cells similar to MSCs have been reported to be found in tissues other than the bone marrow, including fat, dermis, muscle, liver, pancreas, lung, endometrium, kidney, vessels, and brain.28–36 It is suggested that MSCs may reside throughout the body in most postnatal organs.28,32 Although Rutka et al37 reported in the 1980s that nonglial, mesenchymal cells can be cultured from normal brain tissue, the isolation of cells with the specific characteristics of MSCs from the brain has only begun to be explored, and the existence of brain-derived MSCs (Br-MSCs) and their location within the brain remains unclear. Identifying MSCs in normal brain tissue would provide new insight into the capacity of central nervous system tissues to respond to inflammation and tumor formation. In this context, we sought to determine the extent to which cells with the features of MSCs exist in normal brain tissue and to determine the location of these cells in the brain. Consequently, we isolated and cultured cells from normal mouse brain using the same methods used to isolate BM-MSCs and thereby identified cells that were similar to BM-MSCs in terms of morphology, surface markers, and differentiation pattern, which we called brain-derived MSCs (Br-MSCs). Histological studies suggest that these Br-MSCs reside within the perivascular niche. We propose that these Br-MSCs may represent a pool of local stem cells that maintain normal homeostasis of the perivascular niche and are capable of responding to tissue stresses within the brain.