What are stem cells?

Author: Sharon Jang , Vanessa Mok | Reviewer: Peggy Assinck | Published: 13 May 2020 | Updated: ~

Stems cells have the ability to self-renew (blue arrow), and turn into different cell types such as muscle cells, skin cells, and nerve cells (pictured).¹

All tissues and organs in the body originate from stem cells. Stem cells are unspecialized, blank cells that have the ability to turn into specialized cells (e.g., neural cells, muscle cells, etc) through chemical signals in the body. These cells are responsible for maintaining and repairing the tissue they are found in. Stem cells are defined by two characteristic features: 1) their ability to self-renew (i.e., make copies of themselves) and 2) their ability to differentiate into functioning cell types (i.e., they can turn into specialized cells). Some stem cells are only found in certain parts of the body or at certain stages of life. There are four main types of stem cells that are currently used in stem cell therapy or research.

Neurons extending from a group of embryonic stem cells.²

Embryonic stem cells are sourced from embryos that are about a week old. In Canada, embryos cannot be specifically created for research purposes. Instead, these stem cells originate from unused embryos from fertility clinics. Their use in research raises ethical controversies as the embryos are destroyed during the harvesting process. These cells are pluripotent which means that they can turn into any cell in the body except for the umbilical cord and placenta.

Adult stem cells (also known as somatic stem cells or tissue-specific stem cells) are sourced from organ tissues (e.g., the heart, bone marrow, fat) found in the bodies of infants, children, and adults. They can also be produced from discarded placentas and umbilical cords. They have less capability to self-divide and specialize into different cell types compared to embryonic stem cells. Rather, adult stem cells more commonly produce tissue types in which they are found, so are multipotent. For instance, blood stem cells only have the potential to generate red blood cells, white blood cells, and platelets.

Advancements in technology have allowed researchers to reprogram specialized adult cells back into their pluripotent state to resemble embryonic stem cells. These cells are known as induced pluripotent stem cells. These cells are often obtained from taking a small sample from adult tissue, such as the skin. Because this process does not harm the donor nor does it involve embryos, the same ethical issues facing above-discussed cells are not encountered with induced pluripotent stem cells. Induced pluripotent stem cells can also reduce the risk of rejection by the body because the adult cells are derived from the person’s own body. Despite this exciting discovery, there are lots of obstacles to overcome before they are used in human studies.