Marrow stromal stem cells.

Abstract

Multipotential marrow stromal stem cells were known as early as 1968 (1) through the work of Friedenstein and his coworkers, who established that cells that are adherent, clonogenic, nonphagocytic, and fibroblastic in habit (defined as colony-forming units–fibroblastic; CFU-Fs) can be isolated from the bone marrow stroma of postnatal organisms. CFU-Fs, as these investigators found, can give rise under appropriate experimental conditions to a broad spectrum of fully differentiated connective tissues, including cartilage, bone, adipose tissue, fibrous tissue, and myelosupportive stroma (2, 3). Evidence for the physiological relevance of the stromal system and stem cells rests primarily on the in vivo transplantation of marrow stromal cell strains obtained from marrow cell suspensions and expanded in culture. Transplantation of such cells in open systems, such as the space under the kidney capsule, results in the generation of a chimeric ossicle, that is, a structure replicating the histology and architecture of a miniature bone and comprising tissues of both donor and host origin. In these systems, bone trabeculae and cortices, myelosupportive stroma, and adipocytes are of donor origin, whereas the hematopoietic cells that colonize the ossicle and reach full maturity within it are of host origin (4). This outcome can be viewed as the mirror image of bone marrow transplantation, in which host stromal cells provide the structures within which donor cells undergo hematopoiesis. In addition to transplantation in open systems, stromal cell strains can also be transplanted in diffusion chambers that exclude the immigration of host hematopoietic cells into the forming stromal tissues. Under these conditions, an array of differentiated connective tissues — cartilage, bone, fibrous tissue, and adipocytes — develops, all of donor origin (3). In the nonvascularized diffusion chambers, cartilage is more frequently observed than in open transplants and is regularly distributed at sites of predicted low oxygen tension. This principle is reflected in current micromass culturing techniques for obtaining cartilage formation from stromal cells in vitro (5). Cell strains derived from the ex vivo expansion of a single clone (i.e., the progeny of a single CFU-F) are endowed with the same multipotentiality under the same or similar experimental conditions. Thus, a single CFU-F can give rise to ossicles identical to those generated by transplanted nonclonal stromal cell strains, which may include cells of multiple differentiated phenotypes (6). Based on such observations, Friedenstein, Owen, and others developed the concept that cartilage, fat, bone, and other connective tissues derive from a common ancestor, the stromal stem cell. Their studies also established that the stromal stem cell persists within the bone marrow of postnatal and even adult organisms. However, remarkable differences are observed between individual CFU-Fs. Cell morphology and rates of proliferation vary dramatically, as does the ability to form multilayer or nodular structures. Expression of various markers of the osteoblastic, chondrogenic, and adipogenic phenotypes is variable not only between different cell strains, but also within a cell strain, as a function of time in culture. Furthermore, upon transplantation, some CFU-Fs form bone and support hematopoiesis and adipogenesis, some only form bone, while others form only connective tissue (6). To date, no clear-cut phenotypic characteristics have been identified that allow CFU-F subsets to be isolated with predictably broad or restricted potential. Recent attempts, employing monumental numbers of putative markers to purify the true marrow stromal stem cell (inappropriately termed the “mesenchymal stem cell”) from a heterogeneous population of adherent stromal cells, have identified cells that are neither indefinitely self-renewing nor homogeneously multipotential (7). These mesenchymal stem cells, although supposedly purified, reproduce all of the known virtues and vices of the marrow CFU-F population as a whole, as known from Friedenstein’s studies and others’, except that these cells are obtained with considerably lower efficiency than with the earlier protocols. Ironically, the rediscovery of the widely known properties of marrow stromal cells in 1999 was celebrated in the scientific and lay press as the happy product of an extraordinary and successful hunt.