by Academician Vladimir YARYGIN (Russian Academy of Medical Sciences), Corresponding Member Gennady SUKHIKH (Russian Academy of Medical Sciences), Konstantin YARYGIN, Dr. Sc. (Med.), Russian State Medical University; Igor GRIVENNIKOV, Dr. Sc. (Biol.), Institute of Molecular Genetics, Russian Academy of Sciences
The human body is composed of a huge number (-10 13 ) of cells of different structural and functional types (about 250 in all) - myoblasts, epithelial, nerve (ganglion), immune cells, among others. All of them come from one single fertilized ovum, or egg cell. Dividing, it gives rise to a blastocyst (embryo vesicle) made up of spherical trophoblast and somatic (body) cells, the building bricks of a growing organism. All these body mass cells are identical, and they are also known as totipotent, or omnipotent, embryonic stem cells.
Now the stem cell is a progenitor of a tree of descendants - it is from this cell, which is at the foot of the stem, that the tree grows. Some daughter cells may be identical to the stem cell and go to build up the stem, while others may be specialized (myoblasts, epithelial, nerve cells and others) which go to form branches. Dividing, the stem cell produces at least one copy. This one, while dividing, may beget another stem cell (in what we call symmetric cell division). But it may also be a specialized cell (asymmetric division). This is how all specialized cells that form body tissues and organs come into being. However, stem cells, too, keep up in any body organ - as long as the organism is alive - to replace the spent, dead cells or else regenerate the tissue if it is impaired. Such physiological regeneration, whereby dead cells are substituted, proceeds at different rates. At one pole we have nerve cells which persist all through man's life. And at the other pole are intestinal epithelium cells, all of them replaced within hours.
The transformation of a stem cell into a specialized one occurs in a process known as different ... Read more