Primate embryonic stem cells

Human embryonic stem cell and embryonic germ cell lines

Undifferentiated human embryonic stem (ES) cells and embryonic germ (EG) cells can be cultured indefinitely and yet maintain the potential to form many or all of the differentiated cells in the body. Human ES and EG cells provide an exciting new model for understanding the differentiation and function of human tissue, offer new strategies for drug discovery and testing, and promise new therapies based on the transplantation of ES and EG cell-derived tissues.

Human embryonic stem cells

Embryonic stem (ES) cells are cells derived from the early embryo that can be propagated indefinitely in the primitive undifferentiated state while remaining pluripotent; they share these properties with embryonic germ (EG) cells. Candidate ES and EG cell lines from the human blastocyst and embryonic gonad can differentiate into multiple types of somatic cell. The phenotype of the blastocyst-derived cell lines is very similar to that of monkey ES cells and pluripotent human embryonal carcinoma cells, but differs from that of mouse ES cells or the human germ-cell-derived stem cells. Although our understanding of the control of growth and differentiation of human ES cells is quite limited, it is clear that the development of these cell lines will have a widespread impact on biomedical research.

Lineage-restricted neural precursors can be isolated from both the mouse neural tube and cultured ES cells

We have previously identified multipotent neuroepithelial (NEP) stem cells and lineage-restricted, self-renewing precursor cells termed NRPs (neuron-restricted precursors) and GRPs (glial-restricted precursors) present in the developing rat spinal cord (A. Kalyani, K. Hobson, and M. S. Rao, 1997, Dev. Biol. 186, 202-223; M. S. Rao and M. Mayer-Proschel, 1997, Dev. Biol. 188, 48-63; M. Mayer-Proschel, A. J. Kalyani, T. Mujtaba, and M. S. Rao, 1997, Neuron 19, 773-785). We now show that cells identical to rat NEPs, NRPs, and GRPs are present in mouse neural tubes and that immunoselection against cell surface markers E-NCAM and A2B5 can be used to isolate NRPs and GRPs, respectively. Restricted precursors similar to NRPs and GRPs can also be isolated from mouse embryonic stem cells (ES cells). ES cell-derived NRPs are E-NCAM immunoreactive, undergo self-renewal in defined medium, and differentiate into multiple neuronal phenotypes in mass culture. ES cells also generate A2B5-immunoreactive cells that are similar to E9 NEP-cell-derived GRPs and can differentiate into oligodendrocytes and astrocytes. Thus, lineage restricted precursors can be generated in vitro from cultured ES cells and these restricted precursors resemble those derived from mouse neural tubes. These results demonstrate the utility of using ES cells as a source of late embryonic precursor cells.

Multipotent and restricted precursors in the central nervous system

Acquisition of cell type-specific properties in the nervous system is likely a process of sequential restriction in developmental potential. At least two classes of pluripotent stem cells, neuroepithelial (NEP) stem cells and EGF-dependent neurosphere stem cells, have been identified in distinct spatial and temporal domains. Pluripotent stem cells likely generate central nervous system (CNS) and peripheral nervous system (PNS) derivatives via the generation of intermediate lineage-restricted precursors that differ from each other and from multipotent stem cells. Neuronal precursors termed neuronal-restricted precursors (NRPs), multiple classes of glial precursors termed glial-restricted precursors (GRPs), oligodendrocyte-type 2 astrocytes (O2As), astrocyte precursor cells (APCs), and PNS precursors termed neural crest stem cells (NCSCs) have been identified. Multipotent stem cells and restricted precursor cells can be isolated from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. Analysis in multiple species illustrates similarities between rat, mouse, and human cell differentiation raising the possibility that similar factors and markers may be used to isolate precursor cells from human tissue or ES cells. Anat Rec (New Anat): 257:137-143, 1999.

Embryonic stem cell lines derived from human blastocysts

Human blastocyst-derived, pluripotent cell lines are described that have normal karyotypes, express high levels of telomerase activity, and express cell surface markers that characterize primate embryonic stem cells but do not characterize other early lineages. After undifferentiated proliferation in vitro for 4 to 5 months, these cells still maintained the developmental potential to form trophoblast and derivatives of all three embryonic germ layers, including gut epithelium (endoderm); cartilage, bone, smooth muscle, and striated muscle (mesoderm); and neural epithelium, embryonic ganglia, and stratified squamous epithelium (ectoderm). These cell lines should be useful in human developmental biology, drug discovery, and transplantation medicine.

Cell therapy: in search of pluripotency

Cell replacement as a route to treat cellular disease and injury is currently limited by the availability of suitable donor cell populations, but recent results with mouse embryonic stem cells suggest that isolated human pluripotent cells could provide a source of cells for transplantation and gene therapy.