This study identifies a subpopulation of rapidly self-renewing (RS) and multipotential adult stem cells within colonies of human marrow stromal cells (MSCs). MSCs are adult stem cells from bone marrow that can differentiate into multiple non-hematopoietic cell lineages. Previous studies showed that single-cell-derived colonies of MSCs contain two morphologically distinct cell types: spindle-shaped cells and large flat cells. The researchers found that early colonies also contain a third type of cell: very small round cells that rapidly self-renew. These RS cells have a greater potential for multipotential differentiation than the large cells. The RS cells express unique surface epitopes and proteins that distinguish them from the large cells. Bone marrow contains at least two types of stem cells: hematopoietic stem cells and non-hematopoietic stem cells (MSCs). MSCs are of interest because they can be easily isolated from bone marrow and generate single-cell-derived colonies. These colonies can be expanded for up to 50 population doublings in about 10 weeks and can differentiate into various cell types, including osteoblasts, adipocytes, chondrocytes, myocytes, astrocytes, oligodendrocytes, and neurons. MSCs are currently being tested for their potential use in cell and gene therapy for various human diseases. The study used methods such as isolation and growth of MSCs, electron microscopy, and assays for differentiation to identify the RS cells. The results showed that RS cells are 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. The study also found that RS cells can be distinguished from more mature cells (mMSCs) by surface epitopes and expressed proteins. The study concludes that RS cells may have the greatest potential for long-term engraftment and differentiation in vivo. However, even the RS cell population was heterogeneous, as they did not stain uniformly for several surface epitopes. Therefore, further subfractionation of the RS cell population is needed to determine the potentials of the subpopulations for multilineage differentiation and engraftment to specific tissues. The study was supported by grants from the National Institutes of Health, the Oberkotter Foundation, the HCA–Healthcare Company, and the Louisiana Gene Therapy Research Consortium.This study identifies a subpopulation of rapidly self-renewing (RS) and multipotential adult stem cells within colonies of human marrow stromal cells (MSCs). MSCs are adult stem cells from bone marrow that can differentiate into multiple non-hematopoietic cell lineages. Previous studies showed that single-cell-derived colonies of MSCs contain two morphologically distinct cell types: spindle-shaped cells and large flat cells. The researchers found that early colonies also contain a third type of cell: very small round cells that rapidly self-renew. These RS cells have a greater potential for multipotential differentiation than the large cells. The RS cells express unique surface epitopes and proteins that distinguish them from the large cells. Bone marrow contains at least two types of stem cells: hematopoietic stem cells and non-hematopoietic stem cells (MSCs). MSCs are of interest because they can be easily isolated from bone marrow and generate single-cell-derived colonies. These colonies can be expanded for up to 50 population doublings in about 10 weeks and can differentiate into various cell types, including osteoblasts, adipocytes, chondrocytes, myocytes, astrocytes, oligodendrocytes, and neurons. MSCs are currently being tested for their potential use in cell and gene therapy for various human diseases. The study used methods such as isolation and growth of MSCs, electron microscopy, and assays for differentiation to identify the RS cells. The results showed that RS cells are 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. The study also found that RS cells can be distinguished from more mature cells (mMSCs) by surface epitopes and expressed proteins. The study concludes that RS cells may have the greatest potential for long-term engraftment and differentiation in vivo. However, even the RS cell population was heterogeneous, as they did not stain uniformly for several surface epitopes. Therefore, further subfractionation of the RS cell population is needed to determine the potentials of the subpopulations for multilineage differentiation and engraftment to specific tissues. The study was supported by grants from the National Institutes of Health, the Oberkotter Foundation, the HCA–Healthcare Company, and the Louisiana Gene Therapy Research Consortium.