Types and Sources of Stem Cells
The human body is composed of 220 different types of cells. A stem cell can be coaxed into developing into any one of those. Everyday, stem cell research is opening new doors and creating new opportunities in the fields of reparative and regenerative medicine.
What are Stem Cells?
As an embryo develops into a fetus, stem cells, also called undifferentiated precursor cells, replicate into different types of cells to build the organs and structures of a human being. All humans retain some of these cells throughout their life in varying concentrations throughout the body. These precursor, or stem, cells do not have the ability to perform any functions on their own, however, they do have two unique abilities:
- Differentiation: Stem cells can be manipulated into becoming a specialized cells, such as a blood cell, nerve cell or a liver cell.
- Proliferation: Stem cells can divide more times than other kinds of cells, giving one cell the ability to produce millions of other cells.
Types of Stem Cells
There are four categories of stem cells, divided by developmental potential:
These stem cells can transform into any type of human cell. They can also differentiate into the cells of the extra-embryonic membranes, such as the amniotic fluid. These cells are only produced during a fertilized egg’s first few divisions and are not capable of replicating.
Arise from totipotent stem cells and can become any human cell, except for a totipotent cell. The three types of pluripotent cells are:
- Embryonic stem cells, present in the inner cell mass of the blastocyst – the embryonic mass of cells that forms before implantation.
- Embryonic germ cells, which are present in the fetal precursors to testicles or ovaries
- Embryonic carcinoma cells, found in a specific kind of testicular tumor.
- Multipotent stem cells are more specialized forms of pluripotent cells. These cells can only differentiate into specific types of cells. For instance, hematopoietic stem cells, found in bone marrow, can become all of the cells that make up blood, such as platelets and white and red blood cells.
- Progenitor cells, also called unipotent stem cells, can only become one, specific type of cell.
- Adult stem cells: This encompasses stem cells taken from umbilical cord blood, as well as stem cells taken from the blood and bone marrow of adults. The scientific term for these cells is somatic stem cells.
- Human embryonic stem cells: These stem cells are taken from donated human embryos that are not needed for in vitro fertilization. These are only used with the permission of the parents. The blastocyte (inner cell mass of an embryo prior to implantation) grows in a petri dish for six months, producing a cell line, which contains millions of stem cells.
The Importance of Stem Cells
Stem cells are the cornerstone of the immune system. The stem cells in the basal layer of the skin, and at the base of hair follicles, form a protective barrier to prevent against infections when our skin gets cut. Stem cells in bone marrow create white blood cells, which fight off internal infections and virii. Stem cells in the brain can regenerate brain cells and neural tissues.
Unfortunately, there are limits to what the stem cells retained by the body can do. They cannot regenerate a pancreas or a spinal cord and they cannot fix a damaged heart muscle. The hope is that stem cells harvested from embryos and cord blood can one day be used to heal those things, in addition to helping to heal cancers, Parkinson’s disease and other diseases.
Sources of Stem Cells
Stem cells used in research are generally harvested from one of two sources:
Differences Between Cord Blood and Adult Stem Cells
Despite both falling under the heading of somatic stem cells, cord blood is unique from stem cells harvested from adults in the following ways:
- Primitive: Stem cells found in cord blood are young, giving them a greater ability to become many different kinds of specialized cells. Young cells are less likely to be rejected or cause graft vs. host disease (GvHD) in the recipient. They are also more likely to be healthy since the have had less exposure to the environmental toxins that having a damaging affect on DNA.
- Availability: By banking cord blood, the immediate availability of stem cells when they are needed is ensured.
- Stem Cell Rich: Cord blood is one of the richest sources of stem cells. Adult stem cells are present in much smaller numbers than cord blood stem cells, and are more difficult to separate.