You have probably heard about stem cells in the popular media, about how they hold so much promise for human disorders and the ethical concerns about how scientists obtain them, even about how they have become a political issue. All of this information may have you wondering: what's the big deal about stem cells anyway, and how are they different from any other cell in your body?
Earlier in this module we talked about how the way a given cell divides, and whether or not it can keep on dividing, are important characteristics for categorizing different cell types. Then, what is a stem cell? A stem cell is an unspecialized (undifferentiated) cell, which is capable of dividing and reproducing for long periods of time. Most importantly, stem cells can give rise to, through cell division, other types of specialized (differentiated) cells. It is for this reason that stem cells are hailed for their potential medicinal purposes, such as repairing damaged heart tissue.
Stem cells divide asymmetrically, meaning that instead of giving rise to two daughter cells with the same fate, stem cells give rise to cells with two different fates. One of the daughter cells takes on a differentiated state, meaning that it becomes a heart cell, for example, while the other daughter cell remains an undifferentiated stem cell capable of dividing again. In this way stem cells are capable of both regenerating tissues and replacing themselves.
How does the stem cell divide asymmetrically? In some cases, other cells surrounding the stem cell signal to the different daughter cells, which in turn decides which way they'll progress. It is kind of like becoming whom you date, or like those dog owners that suddenly start looking like their dog—creepy. Alternatively, the cells themselves seem to 'know'. During cell division certain proteins, RNA and other biological molecules are relocated to one half of the cell. When the cell divides, these molecules are only contained in one of the daughter cells, meaning that the two daughters have their own unique start on life. Voila. An identity crisis averted (if only it were that easy for us).
Asymmetric division is actually important for the development of many different species. This importance can be easily viewed in C. elegans, a transparent nematode worm, where scientists have mapped every cell division that comprises the organism. An adult worm has 959 somatic cells plus germ cells. You can even watch all of these divisions at http://www.bio.unc.edu/faculty/goldstein/lab/celdev.mov. Basically, asymmetric divisions in C. elegans helps determine which end of the worm becomes the head, and which end becomes the, well, other end. Yeah, we know: life isn't fair sometimes.
Another place where asymmetry is important is during oogenesis. The diploid primary oocyte divides asymmetrically during its first meiotic division, giving rise to two cells, one known as the secondary oocyte, which contains most of the cytoplasm from the parent cell, and one known as a polar body. The polar body divides symmetrically during meiosis II, but the secondary oocyte again divides asymmetrically, giving rise to the ovum and another polar body. At the end of meiosis, you end up with one ovum and three much smaller polar bodies, which eventually degenerate. This is different to spermatogenesis, where the primary spermatocyte divides evenly to produce four spermatids at the end of meiosis II, which then mature into functional spermatozoa.
Now that you understand what stem cells are, you can begin to think about the ethics question. Where do we get stem cells from? We can harvest embryonic stem cells from aborted fetuses, the umbilical cord after a baby's delivery, and from discarded embryos created for in vitro fertilization. Alternatively, scientists have been able to induce some adult cells, which are called induced pluripotent stem cells (iPSCs), to change and so generate other types of stem cells. While there are some questions about the efficiency of iPSCs, iPSCs are generally thought to sidestep the ethical concerns of using stem cells derived from embryos. The potential therapeutic promise of iPSCs is also intriguing because an individual's own cells could be used, therefore bypassing the worry about transplant rejection. Don't forget, though, that adults have their own natural stem cells too, like the ones in bone marrow that produce all of your blood cells. In fact, bone marrow transplants are a type of stem cell transplant, even though most people don't realize it.
The ethical concerns related to how human stem cells are obtained have significantly impacted on how scientists go about stem cell research. The Bush administration limited federally funded stem cell research to currently existing stem cell lines. In 2009, President Obama repealed the restrictions on stem cell funding guidelines set forth by President Bush. Today these issues are still hot subjects, and will likely remain so for years to come. The debate still rages on – but what do you think?