Stem Cells

Stem cells are cells that divide to form

• one daughter that goes on to differentiate, and
• one daughter that retains its stem-cell properties.

1. Totipotent cells. In mammals, totipotent cells have the potential to become
   • any type in the adult body;
   • any cell of the extraembryonic membranes (e.g., placenta).

   The only totipotent cells are the fertilized egg and the first 4 or so cells produced by its cleavage (as shown by the ability of mammals to produce identical twins, triplets, etc.).

   In mammals, the expression totipotent stem cells is a misnomer: these cells fail to meet the second criterion — they cannot make more of themselves.

2. Pluripotent stem cells. These are true stem cells, with the potential to make any differentiated cell in the body, but cannot contribute to making the extraembryonic membranes (which are derived from the trophoblast).

   Three types of pluripotent stem cells have been found

   • Embryonic Stem (ES) Cells. These can be isolated from the inner cell mass (ICM) of the blastocyst — the stage of embryonic development when implantation occurs. For humans, excess embryos produced during in vitro fertilization (IVF) procedures are used. Harvesting ES cells from human blastocysts is controversial because it destroys the embryo, which could have been implanted to produce another baby (but often was simply going to be discarded).
   • Embryonic Germ (EG) Cells. These can be isolated from the precursor to the gonads in aborted fetuses.
   • Embryonic Carcinoma (EC) Cells. These can be isolated from teratocarcinomas, a tumor that occasionally occurs in a gonad of a fetus. Unlike the other two, they are usually aneuploid.

   All three of these types of pluripotent stem cells

   • can only be isolated from embryonic or fetal tissue;
   • can be grown in culture, but only with special methods to prevent them from differentiating.
   
   
3. Multipotent stem cells. These are true stem cells but can only differentiate into a limited number of types. For example, the bone marrow contains multipotent stem cells that give rise to all the cells of the blood but not to other types of cells. [Discussion]

   Multipotent stem cells are found in adult animals; perhaps most organs in the body (e.g., brain, liver) contain them where they can replace dead or damaged cells. These adult stem cells may also be the cells that — when one accumulates sufficient mutations — produce a clone of cancer cells.

Using Stem Cells for Human Therapy - The Dream

Examples:

• Insulin-dependent diabetes mellitus (IDDM) where the beta cells of the pancreas have been destroyed by an autoimmune attack;
• Parkinson's disease; where dopamine-secreting cells of the brain have been destroyed;
• spinal cord injuries leading to paralysis of the skeletal muscles; [View]
• ischemic stroke where a blood clot in the brain has caused neurons to die from oxygen starvation;
• multiple sclerosis with its loss of myelin sheaths around axons.
• blindness caused by damage to the cornea.

While some success has been achieved with laboratory animals, not much has yet been achieved with humans.

One exception: culturing human epithelial stem cells and using their differentiated progeny to replace a damaged cornea. This works best when the stem cells are from the patient (e.g. from the other eye). Corneal cells from another person (an allograft) are always at risk of rejection by the recipient's immune system.

Using Stem Cells for Human Therapy - The Problems

The Solution?

One way to avoid the problem of rejection is to use stem cells that are genetically identical to the host.

This is already possible in the rare situations when the patient has healthy stem cells in an undamaged part of the body (like the stem cells being used to replace damaged corneas).

In this technique,

• A human egg has its own nucleus removed and replaced by
• a nucleus taken from a somatic (e.g., skin) cell of the patient.
• The now-diploid egg is allowed to develop in culture to the blastocyst stage when
• embryonic stem cells can be harvested and grown up in culture.

  This much has now been achieved with humans - Link

• When they have acquired the desired properties, they can be implanted in the patient with no fear of rejection.

• Imprinted Genes.

  Sperm and eggs each contain certain genes that carry an "imprint" identifying them later in the fertilized egg as being derived from the father or mother respectively.

  Link to discussion of gene imprinting.

  Creating an egg with a nucleus taken from an adult cell may not allow a proper pattern of imprinting to be established.

  When the diploid adult nucleus is inserted into the enucleated egg (at least those of sheep and mice), the new nucleus becomes "reprogrammed". What reprogramming actually means still must be learned, but perhaps it involves the proper methylation and demethylation of imprinted genes. For example, the inactive X chromosome in adult female cells must be reactivated in the egg, and this actually seems to happen.

• Aneuploidy.

  In primates (in contrast to sheep, cattle, and mice), the process of removing the resident nucleus causes molecules associated with the centrosome to be lost as well. Although injecting a donor nucleus allows mitosis to begin, spindle formation may be disrupted, and the resulting cells fail to get the correct complement of chromosomes (aneuploidy).

• Somatic Mutations. This procedure also raises the spectre of amplifying the effect(s) of somatic mutations. [Link to discussion]

  In other words, mutations that might be well-tolerated in a single somatic cell of the adult (used to provide the nucleus) might well turn out to be quite harmful when they become replicated in a clone of cells injected later into the patient.

• Political Controversy.

  The goal of this procedure (which is often called "therapeutic cloning" even though no new individual is produced) is to culture a blastocyst that can serve as a source of ES cells.

  But that same blastocyst could theoretically be implanted in a human uterus and develop into a baby that was genetically identical to the donor of the nucleus. In this way, a human would be cloned.

  And in fact, Dolly and other animals are now routinely cloned this way. Link to a description.

  The spectre of this is so abhorrent to many that they would like to see the procedure banned despite its promise for helping humans.

  In fact, many are so strongly opposed to using human blastocysts — even when produced by nuclear transfer — that they would like to limit stem cell research to adult stem cells (even though these are only multipotent).

  Two possible solutions (both so far demonstrated only in mice):

  • ES cells can be derived from a single cell removed from an 8-cell morula. We know that, in humans, removing a single cell from the morula does not destroy it — the remaing cells can develop into a blastocyst, implant, and develop into a healthy baby. [link to evidence].
  • In altered nuclear transfer (ANT) — a modified version of SCNT (somatic-cell nuclear transfer) — a gene necessary for later implantation (Cdx2 — endocoding a homeobox transcription factor) is turned off (by RNA interference) in the donor nucleus before the nucleus is inserted into the egg. The blastocyst that develops
    • has a defective trophoblast that cannot implant in a uterus;
    • but the cells of the inner cell mass are still capable of developing into cultures of ES cells. (The gene encoding the interfering RNA can then be removed using the Cre/loxP technique.)
  If these procedures work in humans, neither would involve the destruction of a potential human life.