Human Embryonic Stem Cells:
a Possible Source for Replacement Tissues

    Stem cells generally are
self–renewing embryonic cells that can develop into functional, differentiated cells. Human embryonic stem cells are unique because they are totipotent: that is, they can develop into all cells and tissues in the body.
    There are two types of human embryonic stem cells, also called
 1. human embryonic stem (hES) cells, which
     were derived from donated
in vitro fertilized
or very early-stage embryos;
 2. and human embryonic germ (hEG) cells,
     which were derived from
donated fetal
hES and hEG cells are capable of developing into all three cellular layers, including the gut epithelium (endoderm); cartilage, bone, and smooth and striated muscle (mesoderm); and neural epithelium, embryonic ganglia and stratified squamous epithelium (ectoderm).

    In addition to their
totipotent characteristics, hES cells express telomerase and can therefore multiply or replicate indefinitely in an undifferentiated state. This capability for self-renewal is a unique characteristic that distinguishes hES cells from all other stem cells discovered to date in humans and allows them to be a potential source for the manufacture of all cells and tissues of the body. Other stem cells such as blood or gut stem cells express telomerase at very low levels or only periodically; they therefore age, limiting their use in research or therapeutic applications.

    Human embryonic stem cells also maintain a structurally normal set of chromosomes even after prolonged growth in culture. They do not, for example, have any abnormal additions, deletions or rearrangements in their chromosomal structure as is characteristic of cell lines derived from tumors or immortalized by viruses. They are therefore expected to be a continuous source of normal cells for replacement tissues in the future.

Researchers hoped to use
hESC technology to:
1. identify and assign function to the genes that control human development;
  2. enable the development of transplantation therapies by providing standard starting material for the manufacture of cells and tissues; and
  3. facilitate pharmaceutical research and development practices by providing cells for screening, and assigning function to newly discovered genes.

     Human embryonic stem cells from days-old human embryos can multiply without limit and also differentiate into the 200 or so types of cells that make up the body. But because extracting them typically destroys the embryo, experiments have been attacked by those who believe even the earliest stages of human life have moral standing.