Stem Cell Potency

Totipotency

The word toti comes from the Latin totus, meaning entirely. Totipotency is the ability of a single cell to differentiate into all types of differentiated cells in an organism. Totipotent cells have two basic characteristics:

• 1.
  They have the ability to become any cell type in a fully developed human.
• 2.
  They can divide in unlimited numbers without losing their totipotency.

Totipotent stem cells can differentiate into any type of specialized cell, both embryonic (embryonic stem cells are derived from embryos that develop from eggs that have been fertilized) and extra embryonic (cells that support the embryo) and so have the highest differentiation potential. An example of totipotent cells is the zygote and the cells produced from its first few divisions. Each totipotent stem cell can give rise to a complete organism. A totipotent zygotic cell is formed by the union of an egg and a sperm. This cell now has the potential to give rise to virtually all human cells. The zygote divides and gives rise to more and more totipotent cells. After approximately four days, these cells begin to specialize into pluripotent cells. These pluripotent cells have decreased differentiation potential.

Pluripotency

Pluripotent stem cells are descendants of totipotent cells and have the ability to differentiate into nearly all cell types. They can differentiate and form cells from all three germ layers (cell layers found in early embryo): ectoderm, mesoderm, and endoderm.

The difference between totipotent and pluripotent stem cells is slight, but significant. Pluripotent cells are similar to totipotent cells in that they can give rise to any and all human cells. The difference is that they are not capable of giving rise to an entire organism. On day four of development, the embryo forms two layers: one becomes the placenta (a temporary organ that joins the mother and fetus, permitting oxygen and nutrients to transfer from the mother to the fetus and the release of waste products from the fetus) and the other becomes the baby. The cells that will become the baby can give rise to any human tissue type, but alone these cannot give rise to the entire organism.

Pluripotent stem cells are true stem cells and are being studied for their enormous research and therapeutic potential. There are various types of pluripotent cells.

• 1.
  Induced pluripotent cells (iPS cells): Induced pluripotent stem cells are generated directly from adult cells. These are actually reprogrammed cells with pluripotent capacity. The technology was first pioneered in 2006 in Japan, where they introduced four specific genes in an adult cell to convert it into a pluripotent cell; these were called induced pluripotent cells or iPS cells. These iPS cells can be made patient specific and may be used in creating transplants without the risk of immune rejection.
• 2.

  True embryonic stem cells (ES cells): These are cells derived from the inner cell mass of the blastocyst (the blastocyst is an early developmental stage of the embryo). Embryonic stem cells give rise to derivatives of all three basic germ layers (endoderm, ectoderm, and mesoderm). When given appropriate stimulation, they can develop into any specialized adult cell. ES cells therefore have immense potential to develop therapeutic cells and tissues.

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