Saving Superman:
A Look into Stem Cell Research

What is a stem cell?  Simply put, it is a primitive, undifferentiated cell that gives rise to other types of cells.  Cells have a finite lifespan, and thus most cells in the body duplicate and replenish themselves.  Through the isolation and targeted manipulation of cells in culture, scientists are finding ways to identify the various types of stem cells in order to find ways to use them to replace diseased, damaged, or dead cells in the body that cannot repair themselves.  It's analogous to the concept of an organ transplant, except this time scientists are transplanting stem cells, not organs.

Stem cells share the three following general characteristics:

1. The ability to differentiate into specialized cells.
2. The ability to regenerate an infinite number of times.
3. The ability to relocate and differentiate where needed.

There are three main classes of stems cells:  totipotent, pluripotent, and multipotent.

Totipotent Cells:  After fertilization (union of sperm and egg), the zygote created is a totipotent cell, meaning it has the genetic potential to create every cell of the body and the nourishing placenta and extra-embryonic tissues, and thus can form a human being.  This one totipotent cell divides into multiple totipotent cells for up to five days (three to four cellular divisions) after fertilization.

Pluripotent Cells (aka Embryonic Stem Cells):  After about five days, these totipotent cells begin to differentiate, or specialize, and form a hollow ball of cells called a blastocyst.  The blastocyst has an outer layer of cells (which becomes the placenta and fetal-supporting tissues within the uterus) and a cluster of cells inside the hollow sphere called the inner cell mass (which becomes every cell of the body).  This inner cell mass constitutes pluripotent cells, meaning they each have the potential to create every cell of the body but not the necessary placenta and extra-embryonic tissues, and thus cannot form a human being.  Pluripotent cells can be isolated from embryos and the germ line cells of fetuses.

Multipotent Cells:  Pluripotent cells soon undergo further specialization into multipotent cells (sometimes referred to as adult stem cells, multipotent adult progenitor cells, or MAPCs), which can give rise to a limited number of other particular types of cells.  For example, hematopoietic cells (blood cells) in the bone marrow are multipotent and give rise to the various types of blood cells, including RBCs, WBCs, and platelets.  Multipotent cells are found in both developing fetuses and fully developed human beings.  There are certain limitations to using multipotent cells, however.  Scientists have not identified multipotent cells for every type of mature body cell; so far, private research has isolated about 60 different types.  Unlike pluripotent cells, multipotent cells are often in minute quantities and their numbers can decrease with age.  Multipotent cells from a specific patient may take time to mature in culture in order to produce adequate amounts for treatment.  They can and often do contain DNA damage due to aging, sunlight (radiation), toxins, and random DNA mutation during replication.  Spontaneous mutations are more likely to show up in older multipotent cells than younger pluripotent cells.  In addition, multipotent cells may or may not offer the same level of plasticity as pluripotent cells, although this is presently an unresolved issue.  Research on the early stages of cell specialization may not be possible with multipotent cells because they are further along the specialization pathway.  Thus, study of both pluripotent and multipotent stem cells is vital to fully understand cell specialization and potentially develop new treatments or even cures for diseases.

Source:  National Institutes of Health, http://www4.od.nih.gov/stemcell/fig2.gif

Concept Check:

1. In vivo, the embryonic stage (both pre-implantation and post-implantation) lasts from the second cellular division until 8-12 weeks after conception.  Then, the fetal stage begins.  Is it possible to extract totipotent cells at any period of the embryonic stage of development?  What about pluripotent cells?
2. If a scientist were to implant only a pluripotent cell into the uterus of Lois Lane, could she carry a baby to term?  Why or why not?
3. Besides hematopoietic cells, can you think of other multipotent cells in the body?
4. Why might there be ethical issues surrounding the manipulation of pluripotent stem cells?
5. What are a few limitations to using multipotent stem cells?  Can you think of other possible limitations not mentioned?
6. What is one advantage of multipotent cells over pluripotent cells?  (HINT: it involves immunosuppression).

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