Marrow stromal cells are adult stem cells from bone marrow that can differentiate into multiple nonhematopoietic cell lineages. Previous reports demonstrated that single-cell-derived colonies of marrow stromal cells contained two morphologically distinct cell types: spindle-shaped cells and large flat cells. Here we found that early colonies also contain a third kind of cell: very small round cells that rapidly self-renew. Samples enriched for the small cells had a greater potential for multipotential differentiation than samples enriched for the large cells. Also, the small cells expressed a series of surface epitopes and other proteins that potentially can be used to distinguish the small cells from the large cells. The results suggested it will be important to distinguish the major subpopulations of marrow stromal cells in defining their biology and their potential for cell and gene therapy.B one marrow contains at least two kinds of stem cells, hematopoietic stem cells and stem cells for nonhematopoietic tissues (1-26), variously referred to as mesenchymal stem cells or marrow stromal cells (MSCs). MSCs are of interest because they are easily isolated from a small aspirate of bone marrow and readily generate single-cell-derived colonies (1-5, 25, 27); the single-cell-derived colonies can be expanded through as many as 50 population doublings in about 10 weeks (25), and they can differentiate into osteoblasts, adipocytes, chondrocytes (1-13), myocytes (9), astrocytes, oligodendrocytes, and neurons (17,23,26,27). For these reasons, the cells are currently being tested for their potential use in cell and gene therapy for a number of human diseases (22,24). Previous reports (3,11,12) demonstrated that single-cell-derived colonies of human MSCs are heterogeneous in that they contain at least two morphologically distinct kinds of cells: spindle-shaped cells and large cuboidal or flattened cells. Here we have extended our previous observations (25) to demonstrate that the colonies also contain extremely small cells that are rapidly self-renewing (RS cells). The RS cells appear to be the earliest progenitors in the cultures and have the greatest potential for multilineage differentiation. They differ from more mature cells in the same cultures by a series of surface epitopes and expressed proteins. MethodsIsolation and Growth of MSCs. To isolate human MSCs, bone marrow aspirates of 10-20 ml were taken from the iliac crest of normal donors ranging in age from 19 to 49 years old under an Institutional Review Board approved protocol. Nucleated cells were isolated with a density gradient [Ficoll͞Paque (Pharmacia)] and resuspended in complete culture medium [␣ MEM, GIBCO͞BRL; 20% FBS, lot-selected for rapid growth of MSCs (Atlanta Biologicals); 100 units/ml of penicillin͞100 g/ml of streptomycin͞2 mM L-glutamine (GIBCO͞BRL)]. All of the cells were plated in 25 ml of medium in a 175-cm 2 culture dish (Falcon) and incubated at 37°C with 5% humidified CO 2 . After 24 h, nonadherent cells were discarded, and adherent cells ...

Summary. Marrow stromal cells (MSCs) were isolated from bone marrow obtained by aspirates of the iliac crest of normal volunteers. The cells were isolated by their adherence to plastic and then passed in culture. Some of the samples expanded through over 15 cell doublings from the time frozen stocks were prepared. Others ceased replicating after about four cell doublings. The replicative potential of the cells in culture was best predicted by a simple colonyforming assay in which samples from early passages were plated at low densities of about 10 cells per cm 2 . Samples with high colony-forming ef®ciency exhibited the greatest replicative potential. The colonies obtained by plating early passage cells at low density varied in size and morphology. The large colonies readily differentiated into osteoblasts and adipocytes when incubated in the appropriate medium. As samples were expanded in culture and approached senescence, they retained their ability to differentiate into osteoblasts. However, the cells failed to differentiate into adipocytes. The loss of multipotentiality following serial passage in culture may have important implications for the use of expanded MSCs for cell and gene therapy.

Cultures of plastic-adherent cells from bone marrow have attracted interest because of their ability to support growth of hematopoietic stem cells, their multipotentiality for differentiation, and their possible use for cell and gene therapy. Here we found that the cells grew most rapidly when they were initially plated at low densities (1.5 or 3.0 cells/cm 2 ) to generate single-cell derived colonies. The cultures displayed a lag phase of about 5 days, a log phase of rapid growth of about 5 days, and then a stationary phase. FACS analysis demonstrated that stationary cultures contained a major population of large and moderately granular cells and a minor population of small and agranular cells here referred to as recycling stem cells or RS-1 cells. During the lag phase, the RS-1 cells gave rise to a new population of small and densely granular cells (RS-2 cells). During the late log phase, the RS-2 cells decreased in number and regenerated the pool of RS-1 cells found in stationary cultures. In repeated passages in which the cells were plated at low density, they were amplified about 10 9 -fold in 6 wk. The cells retained their ability to generate single-cell derived colonies and therefore apparently retained their multipotentiality for differentiation.

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