Bioactive factors affect proliferation and phenotypic expression in progenitor and pluripotent stem cells

Adult-derived stem cells and their potential for use in tissue repair and molecular medicine

This report reviews three categories of precursor cells present within adults. The first category of precursor cell, the epiblast-like stem cell, has the potential of forming cells from all three embryonic germ layer lineages, e.g., ectoderm, mesoderm, and endoderm. The second category of precursor cell, the germ layer lineage stem cell, consists of three separate cells. Each of the three cells is committed to form cells limited to a specific embryonic germ layer lineage. Thus the second category consists of germ layer lineage ectodermal stem cells, germ layer lineage mesodermal stem cells, and germ layer lineage endodermal stem cells. The third category of precursor cells, progenitor cells, contains a multitude of cells. These cells are committed to form specific cell and tissue types and are the immediate precursors to the differentiated cells and tissues of the adult. The three categories of precursor cells can be readily isolated from adult tissues. They can be distinguished from each other based on their size, growth in cell culture, expressed genes, cell surface markers, and potential for differentiation. This report also discusses new findings. These findings include the karyotypic analysis of germ layer lineage stem cells; the appearance of dopaminergic neurons after implantation of naive adult pluripotent stem cells into a 6-hydroxydopamine-lesioned Parkinson's model; and the use of adult stem cells as transport mechanisms for exogenous genetic material. We conclude by discussing the potential roles of adult-derived precursor cells as building blocks for tissue repair and as delivery vehicles for molecular medicine.

Adult stem cells

Development of a multicellular organism is accomplished through a series of events that are preprogrammed in the genome. These events encompass cellular proliferation, lineage commitment, lineage progression, lineage expression, cellular inhibition, and regulated apoptosis. The sequential progression of cells through these events results in the formation of the differentiated cells, tissues, and organs that constitute an individual. Although most cells progress through this sequence during development, a few cells leave the developmental continuum to become reserve precursor cells. The reserve precursor cells are involved in the continual maintenance and repair of the tissues and organs throughout the life span of the individual. Until recently it was generally assumed that the precursor cells in postnatal individuals were limited to lineage-committed progenitor cells specific for various tissues. However, studies by Young, his colleagues, and others have demonstrated the presence of two categories of precursor cells that reside within the organs and tissues of postnatal animals. These two categories of precursor cells are lineage-committed (multipotent, tripotent, bipotent, and unipotent) progenitor cells and lineage-uncommitted pluripotent (epiblastic-like, ectodermal, mesodermal, and endodermal) stem cells. These reserve precursor cells provide for the continual maintenance and repair of the organism after birth.

Existence of reserve quiescent stem cells in adults, from amphibians to humans

Several theories have been proposed to explain the phenomenon of tissue restoration in amphibians and higher order animals. These theories include dedifferentiation of damaged tissues, transdifferentiation of lineage-committed stem cells, and activation of quiescent stem cells. Young and colleagues demonstrated that connective tissues throughout the body contain multiple populations of quiescent lineage-committed progenitor stem cells and lineage-uncommitted pluripotent stem cells. Subsequent cloning and cell sorting studies identified quiescent lineage-uncommitted pluripotent mesenchymal stem cells, capable of forming any mesodermal cell type, and pluripotent epiblastic-like stem cells, capable of forming any somatic cell type. Based on their studies, they propose at least 11 categories of quiescent reserve stem cells resident within postnatal animals, including humans. These categories are pluripotent epiblastic-like stem cells, pluripotent ectodermal stem cells, pluripotent epidermal stem cells, pluripotent neuronal stem cells, pluripotent neural crest stem cells, pluripotent mesenchymal (mesodermal) stem cells, pluripotent endodermal stem cells, multipotent progenitor stem cells, tripotent progenitor stem cells, bipotent progenitor stem cells, and unipotent progenitor stem cells. Thus, activation of quiescent reserve stem cells, i.e., lineage-committed progenitor stem cells and lineage-uncommitted pluripotent stem cells, resident within the connective tissues could provide for the continual maintenance and repair of the postnatal organism after birth.

Clonogenic analysis reveals reserve stem cells in postnatal mammals. II. Pluripotent epiblastic-like stem cells

Undifferentiated cells have been identified in the prenatal blastocyst, inner cell mass, and gonadal ridges of rodents and primates, including humans. After isolation these cells express molecular and immunological markers for embryonic cells, capabilities for extended self-renewal, and telomerase activity. When allowed to differentiate, embryonic stem cells express phenotypic markers for tissues of ectodermal, mesodermal, and endodermal origin. When implanted in vivo, undifferentiated noninduced embryonic stem cells formed teratomas. In this report we describe a cell clone isolated from postnatal rat skeletal muscle and derived by repetitive single-cell clonogenic analysis. In the undifferentiated state it consists of very small cells having a high ratio of nucleus to cytoplasm. The clone expresses molecular and immunological markers for embryonic stem cells. It exhibits telomerase activity, which is consistent with its extended capability for self-renewal. When induced to differentiate, it expressed phenotypic markers for tissues of ectodermal, mesodermal, and endodermal origin. The clone was designated as a postnatal pluripotent epiblastic-like stem cell (PPELSC). The undifferentiated clone was transfected with a genomic marker and assayed for alterations in stem cell characteristics. No alterations were noted. The labeled clone, when implanted into heart after injury, incorporated into myocardial tissues undergoing repair. The labeled clone was subjected to directed lineage induction in vitro, resulting in the formation of islet-like structures (ILSs) that secreted insulin in response to a glucose challenge. This study suggests that embryonic-like stem cells are retained within postnatal mammals and have the potential for use in gene therapy and tissue engineering.

The role of stem cells in skeletal and cardiac muscle repair

In postnatal muscle, skeletal muscle precursors (myoblasts) can be derived from satellite cells (reserve cells located on the surface of mature myofibers) or from cells lying beyond the myofiber, e.g., interstitial connective tissue or bone marrow. Both of these classes of cells may have stem cell properties. In addition, the heretical idea that post-mitotic myonuclei lying within mature myofibers might be able to re-form myoblasts or stem cells is examined and related to recent observations for similar post-mitotic cardiomyocytes. In adult hearts (which previously were not considered capable of repair), the role of replicating endogenous cardiomyocytes and the recruitment of other (stem) cells into cardiomyocytes for new cardiac muscle formation has recently attracted much attention. The relative contribution of these various sources of precursor cells in postnatal muscles and the factors that may enhance stem cell participation in the formation of new skeletal and cardiac muscle in vivo are the focus of this review. We concluded that, although many endogenous cell types can be converted to skeletal muscle, the contribution of non-myogenic cells to the formation of new postnatal skeletal muscle in vivo appears to be negligible. Whether the recruitment of such cells to the myogenic lineage can be significantly enhanced by specific inducers and the appropriate microenvironment is a current topic of intense interest. However, dermal fibroblasts appear promising as a realistic alternative source of exogenous myoblasts for transplantation purposes. For heart muscle, experiments showing the participation of bone marrow-derived stem cells and endothelial cells in the repair of damaged cardiac muscle are encouraging.

Human reserve pluripotent mesenchymal stem cells are present in the connective tissues of skeletal muscle and dermis derived from fetal, adult, and geriatric donors

This study details the profile of 13 cell surface cluster differentiation markers on human reserve stem cells derived from connective tissues. Stem cells were isolated from the connective tissues of dermis and skeletal muscle derived from fetal, mature, and geriatric humans. An insulin/dexamethasone phenotypic bioassay was used to determine the identity of the stem cells from each population. All populations contained lineage-committed myogenic, adipogenic, chondrogenic, and osteogenic progenitor stem cells as well as lineage-uncommitted pluripotent stem cells capable of forming muscle, adipocytes, cartilage, bone, fibroblasts, and endothelial cells. Flow cytometric analysis of adult stem cell populations revealed positive staining for CD34 and CD90 and negative staining for CD3, CD4, CD8, CD11c, CD33, CD36, CD38, CD45, CD117, Glycophorin-A, and HLA DR-II.

Clonogenic analysis reveals reserve stem cells in postnatal mammals: I. Pluripotent mesenchymal stem cells

Clonal populations of lineage-uncommitted pluripotent mesenchymal stem cells have been identified in prenatal avians and rodents. These cells reside in the connective tissue matrices of many organs and tissues. They demonstrate extended capabilities for self-renewal and the ability to differentiate into multiple separate tissues within the mesodermal germ line. This study was designed to determine whether such cells are present in the connective tissues of postnatal mammals. This report describes a cell clone derived by isolation from postnatal rat connective tissues, cryopreservation, extended propagation, and serial dilution clonogenic analysis. In the undifferentiated state, this clone demonstrates a high nuclear-to-cytoplasmic ratio and extended capacity for self-renewal. Subsequent morphological, histochemical, and immunochemical analysis after the induction of differentiation revealed phenotypic markers characteristic of multiple cell types of mesodermal origin, such as skeletal muscle, smooth muscle, fat cells, cartilage, and bone. These results indicate that this clone consists of pluripotent mesenchymal stem cells. This report demonstrates that clonal populations of reserve stem cells are present in mammals after birth. Potential roles for such cells in the maintenance, repair, and regeneration of mesodermal tissues are discussed.

Immunobiology and the future of myoblast transfer therapy

Myoblast transfer therapy (MTT) is a cell-mediated gene transfer method aimed at the restoration of normal dystrophin expression in Duchenne muscular dystrophy (DMD). Initial clinical MTT trials were conducted amid much controversy, as they were based on very few animal studies. Unfortunately, the trials were of little therapeutic benefit. As a result, there has been a renaissance of interest in experimental studies in animal models. In MTT, myoblasts are obtained by muscle biopsy from normal, i.e., dystrophin-positive, donors, expanded in culture, and injected directly into the muscles of dystrophic recipients. The major requirement for successful MTT is the survival of injected donor myoblasts in the host environment. However, a vast majority of donor cells fail to survive for more than 1 h after injection, and very few last beyond the first week. This review on the immunological aspects of MTT focuses in particular on the roles of specific components of the host immune response, the effects of tissue culture on donor cells, and strategies under development to circumvent the problem of donor myoblast death after injection in vivo.

Muscle regeneration: molecular aspects and therapeutic implications

With respect to diverse clinical applications for muscle regeneration, this paper discusses the latest markers for identifying skeletal muscle precursor cells in regenerating muscle, the implications of alternative sources of myogenic precursor cells and putative stem cells, and the current status of administration of exogenous factors to enhance muscle repair.