About
STEM CELLS

Your body has many different types of cells (more than 200 to be more exact) each geared toward specific functions. You have skin cells and blood cells, and you have bone cells and brain cells. All your organs comprise specific cells, too, from kidney cells to heart cells.

Your cells didn’t start out knowing how to come together to form your bones, heart or blood; they begun with more of a blank slate. These completely undifferentiated cells can be found during gestation, or the time the baby is in the womb, and are called embryonic stem cells. These early stage stem cells are master cells that have the potential to become any type of cell in the body.

First isolated in 1998, there is a lot of controversy around acquiring embryonic stem cells. Thankfully, we can also acquire stem cells that form just a little bit later down the road, like in the umbillical cord. These stem cells, known as adult stem cells, stay with us for life. (Later, we will learn why not all adult stem cells are equal.) Adult stem cells are more limited in the types of cells they can become, something known as being tissue-specific, but share many of the same qualities. Hematopoietic stem cells (Greek “to make blood” and pronounced he-mah-toe-po-ee-tic) found in the umbilical cord's blood, for instance, can become any of the different types of blood cells found in the body and are the foundation of our immune systems. Another example is mesenchymal (meh-sen-ki-mal) stem cells, which can be found in the umbilical cord tissue and can become a host of cells including those found in your nervous system, sensory organs, circulatory tissues, skin, bone, cartilage, and more.

To recap, we have certain types of stem cells that can become a variety of different cells—they are like the renaissance men of cells—but there is one more thing that makes stem cells special. This has to do with how they replicate themselves.

The body has two ways to create more cells. The first is usually taught in middle school science. Known as cell division, it’s where a cell replicates within its membrane before dividing into two identical cells. Cells do this as needed for regeneration, which we will touch on in a second.

The other way the body creates more cells is through its stem cells, and stem cells do things a little differently. They undergo what is called asymmetric division, forming not one but two daughter cells: one cell often an exact replica of itself, a new stem cell with a relatively clean slate, and another stem cell that is ready to turn into a specific type of cell. This trait is known as self-renewal and allows stem cells to proliferate, or reproduce rapidly.

Through these two means, we are always producing more cells. In fact, much of your body is in a state of constant renewal because many cells can live for only certain period of time. The lifespan for a cell in the stomach lining is about two days. Red blood cells, about four months. Nerve and brain cells are supposed to live forever. This is why these cells rarely regenerate and take a long time if they do.

The immune system has a way to identify foreign cells; it’s what allows the body to defend itself. So although transplants were proving successful after the first in 1956, they were limited to twins because their shared genetic makeup made them 100 percent compatible. This took a turn in 1958, when scientists discovered a protein present on the surface of almost all cells that lets the body know if the cell is one of its own cells or a foreign cell. In 1973, we finally learned enough about these compatibility markers (called human leukocyte antigens or HLAs) to perform the first unrelated bone marrow transplant.

The proteins stem from three HLA genes, and you inherit one HLA from each parent, or half your HLA markers from your mother and half from your father. This gives siblings a 25 percent chance of being a perfect match, a 50 percent chance of being a partial match and another one-in-four chance of not being a match at all. Unfortunately, about seven out 10 patients who need a transplant don’t have a suitable donor in their family. They can either rely on their own stem cells, isolated before treatment or previously preserved, or try to find a match through a public donor.

An HLA-match helps ensure the body accepts the new cell and the transplant is successful. It also reduces the risk of graft-versus-host disease (GvHD), which is when the transplanted cells attack the recipient’s body. GvHD occurs in 30%–40% of recipients when they aren’t a perfect match but the donor is still related. If the donor and recipient are not related, it increases to a 60%–80% risk. The better the match, the more likely any GvHD symptoms will be mild, if they suffer from GvHD at all. Unfortunately, GvHD can also be deadly.

Further advancements were made in 1978, when stem cells were discovered in cord blood and in 1988, when cord blood stem cells were first used in a transplant. Stem cells extracted from the umbilical cord blood or tissue have since been shown to be more advantageous than those extracted from other sources such as bone marrow. In many ways, this is because stem cells from the umbilical cord can be considered naïve and immature compared to stem cells from other sources. Cord stem cells haven’t been exposed to disease or environmental pollutants, and they are more accepting of foreign cells. In this case, inexperience makes them stronger.