Stem cell research

Stem cells have the remarkable potential to develop into many different cell types in the body. Serving as a sort of repair system for the body, they can theoretically divide without limit to replenish other cells as long as the person or animal is still alive. When a stem cell divides, each new cell has the potential to either remain stem cells or become another type of cell with a more specialized function, such as muscle cells, red blood cells, or skin cells.

Promise of Stem Cells

There are three major types of stem cells: embryonic, fetal and adult, and induced pluripotent stem cells. Each comes from different sources and has somewhat different properties (Figure 1).

Embryonic Stem Cells

When a sperm fertilizes an egg, it becomes what is known as a “zygote.” Many scientists view the zygote as the ultimate stem cell because it can develop into any cell  not only of the embryo, but also of the surrounding tissues, such as the placenta. Because the zygote has the highest degree of plasticity, it is referred to as a “totipotent” stem cell.

Thirty hours after fertilization, the zygote begins to divide, and by the fifth or sixth day, the cells form a kind of a bubble or “blastocyst.” These stem cells are somewhat less plastic and more specialized than totipotent zygote stem cells: those on the outer surface of the blastocyst develop into the placenta and other tissues that surround the fetus, while those inside, referred to as “embryonic stem cells,” become the cells of all the fetal organs and tissues. Such stem cells that can become any of the more than 200 types of cells in the body are called “pluripotent.”

Figure 1. Stem cells derived from fertilized egg and their differentiation

Fetal stem cells

As the embryo grows, it accumulates additional embryonic stem cells in the yolk sac; as the fetus grows from 8 to 12 weeks, it accumulates “fetal stem cells” in the liver. Both embryonic and fetal stem cells generate the developing tissues and organs. At this stage, the stem cells are more tissue-specific rather than generating all of the body’s 200 different cell types. For example, fetal stem cells in the liver tend to generate liver and blood cell families. Such stem cells are generally designated as “multipotent.” However, some research suggests that at least some multipotent stem cells may be more plastic than first thought and may, under the right circumstances, become pluripotent.

Adult stem cells

Currently, the chief sources of adult stem cells are bone marrow, the bloodstream, and cord blood. The stem cells derived are blood (hematopoietic) precursor cells, i.e., they generate the major blood cell types: red blood cells, white blood cells, and platelets. Some research suggests that these blood stem cells may, under the right circumstances, be “pluripotent” and able to generate other cell families. Other types of multipotent stem cells exist in different tissues, but scientists have not been able yet to extract them in sufficient quantities for therapeutic use.

Induced pluripotent stem (iPS) cells

iPS cells can be generated by somatic cell reprogramming following the exogenous expression of specific transcription factors (Oct-3/4, KLF4, SOX2, and c-Myc). Stem cell pluripotency can be defined and experimentally evaluated by assessing the ability of starting cell populations to differentiate into somatic cell types that arise from the three vertebrate germ layers: ectoderm, mesoderm, and endoderm. In addition, embryonic and induced pluripotent stem cells are characterized by the ability to undergo long-term self-renewal in vitro when cultured with growth factors provided by feeder cells or defined culture media. Pluripotent stem cell phenotypes can be modulated by chemical compounds and small bioactive molecules that enhance the proliferation or direct the differentiation of pluripotent stem cells.