Stem cells are defined based on their behavior in vivo, or in the intact organism. For example, a zygote is a totipotent stem cell, meaning that it can differentiate into all the tissues that make up the embryo, as well as the extra-embryonic tissues, placenta, and umbilical cord. Pluripotent stem cells can give rise to cells that are derived from all three embryonic germ layers (ectoderm, mesoderm, and endoderm) throughout the course of natural embryonic development. In other words, they can differentiate into any cell that comprises the embryo. Unipotent stem cells are found in the adult organism and can only differentiate into one cell lineage in vivo. This allows for a steady state of renewal within the tissue that the unipotent cell reside

Because of the pluripotency that is expressed by embryonic stem cells, it may be possible to grow differentiated cells in vitro for in vivo transplantation. Dr. John Gearhart and colleagues at John’s Hopkins University have reported evidence that cells derived from embryonic stem cells can be used to restore movement in rats with amyotrophic lateral sclerosis (Lou Gehrig’s disease), which progressively destroys efferent neurons in the spinal cord. During the experiment, the rats were exposed to the Sindbis virus, which simulates the disease by infecting efferent neurons in the spinal cord. The rats that survived infection were left with myoplegia. The researchers implanted retinoic acid-treated embryoid bodies from mouse embryonic stem cells into an injured rat spinal cord and found that they soon differentiated into astrocytes, neurons, and oligodendrocytes and promoted motor recovery. Autopsy reports confirmed that the cells had migrated throughout the spinal cord. It is still unknown whether the new neurons replaced the old or if they released trophic (growth) factors that promoted regeneration.

Research involving stem cells is still preliminary, and it will take time before our understanding of stem cells will have any real clinical applications. The research is promising though, and will hopefully lead us to a better understanding of embryogenesis and factors that promote cell differentiation and migration within our species. This knowledge could help us develop therapies for currently incurable diseases.

Embryonic stem cells proliferate extensively in the embryo, and can be isolated and grown in vitro (in a laboratory setting), where t

Some topics in this essay:
, Lou Gehrig’s, Alpha TGF, stem cells, Hopkins University, embryonic stem, embryonic stem cells, stem cell, adult stem, parkinson’s disease, spinal cord, John Gearhart, unipotent stem cells, embryoid bodies, derived embryonic, cell types, differentiate cell, adult stem cells, embryonic stem cell, disease stem cells,