What Are The Different Types Of Stem Cells?
Stem cell biology aims to clarify how stem cells are identiﬁed, isolated, and characterized. The methodology used also helps to define the different types of stem cells. Stem cells are generally classified to one of the two main types – 1) adult stem cell are isolated from various body tissues and 2) embryonic stem cells, which are harvested from blastocysts’ inner cell mass. Actually, stem cells isolated from umbilical cord blood or fetal tissue are usually considered as adult stem cells. Because of this, the term “tissue stem cells” is used for all other than embryonic stem cells. Stem and progenitor cells can replenishing tissues in adult organisms, operating as a whole body repairing system. In a growing embryo, stem cells can turn into all different types of cells (like ectoderm, endoderm and mesoderm), but also can participate in the regeneration of some tissues and organs, such as skin, blood, or intestinal tissues.
Embryonic Stem Cells
Embryonic stem (ES) cells can be very easy isolated and purified. They are acquired from the inner cell mass of blastocysts of pre-implantation stage embryos. In humans, the blastocyst is formed up to 5 days after the formation of a zygote (fertilized egg cell). Under normal conditions, these cells can differentiate into every cell type, which form the different body tissues and organs, but not umbilical cord or placenta. This means that embryonic stem cells are pluripotent cells. When used for scientific purposes, human ES cells can be grown in the lab for years without transforming into more specialized cell types. Still, they are capable of forming every kind of cell in the body, under the right conditions. To do so, embryonic stem cells need specific signals to differentiate to the required cell type. If they are directly injected, without a targeted treatment, these cells will differentiate into cells from many different types. The result is a teratoma – a tumor produced by this irregular pluripotent cell development.
Some of the main characteristics of an embryonic stem cells are:
- able to self-renew
- clonogenic capability
- have normal karyotype and able to differentiate into different cell types (multi-lineage) both in vitro and in vivo
- extensive proliferation in vitro under well-defined culture conditions
- can be freezed (and thawed, respectively)
In theory, embryonic stem cells can multiply in large quantities and differentiate into any type of tissue. Embryonic stem cells are extremely valuable because they can serve as unlimited source of cell cultures that can be used for research on normal and disease related development processes, and, ultimately, to discover an appropriate treatment.
Adult Stem Cells
An adult stem cell is an undifferentiated cell, that can be usually found among other differentiated cells in body tissue or organ. Adults cells can self-renew and could produce different types of differentiated cells and thus to maintain and repair the tissues and organs. The terms “adult stem cells” and “somatic stem cells” are interchangeable in case we don’t speak of germ cells. Adult stem cells can be isolated from both juvenile and adult organisms.
Adult Stem Cell Lineage Maintenance
Adult stem cells undergo two types of cell division. With the symmetric division, stem cells produce two identical daughter cells, which both the same stem cell features. As the name suggests, the resulting cells from the asymmetric division of stem cells is not equal – it produces just one stem cell and a progenitor cell. The progenitor cell is limited in terms of self-renewal potential and can undergo a limited number of cell division before turning into a mature differentiated cell.
Adult Stem Cells Plasticity
Plasticity or in other words “stem cell trans-differentiation” is called the capability of stem cells to differentiate into different cell types compared to the germ they originate. For example stem cells that originate normally from the ectoderm, like neural stem cells, can differentiate to any ectoderm, mesoderm, and endoderm cell type. This effect could be achieved by adding specific stimulating growth factors to the cell media, which can direct the development of the stem cells to the desired direction. This allows isolated cells from germ line (either ectoderm, mesoderm or endoderm) to be induced to differentiate into a specific cell type, while they are cultivated in vitro and then to be transplanted to a body organ which is different from the one they are isolated from.
Types Of Adult Stem Cells
Based on the type of the tissue from which cells are harvested, they could be classified to the following types of adult stem cells:
- Endothelial stem cells are one of the 3 types of multi-potent stem cells that can be harvested from bone marrow. They can differentiate into the cells the cells that line blood vessels, which are a type of endothelial cells.
- Mesenchymal stem cells have stromal lineage (isolated from adipose tissue, placenta, lungs, bone marrow and blood, even teeth!). Mesenchymal stem cells are mostly used in clinical therapies because of their potential to differentiate, participate in trophic activities and regulate innate immune response.
- Intestinal stem cells descendant build the lining of the surface of the intestines (both small and large). They are harvested from formation called “the crypts of Lieberkuhn”. Scientific research suggests that intestinal stem cells might be the cause of some cancers of the colon and small intestine.
- Hematopoietic stem cells produce all different types of blood cell. They are extracted from the umbilical cord blood and bone marrow.
- Mammary stem cells produce cells for required for the development of mammary gland during puberty and also gestation. Thus. they act as a fundamental factor in carcinogenesis of the breast. Mammary stem cells can differentiate into both the myoepithelial and luminal gland cells. Studies with mice have demonstrated that mammary stem cells have the power to replenish the entire organ.
- Neural stem cells are cultured in vitro in the form of large aggregates of stem cells called neurospheres. They have the ability to differentiate into neuronal and glia cell types and as most of the stem cells neural stem cells can be propagated for long time periods.
In the last years, treatments based on adult stem cell are applied for the treatment of leukemia, bone or blood cancers, in the form of bone marrow transplants. In contrast with the embryonic stem cells, which require wipe-out of the embryo to harvest them, the application of adult stem cells in scientific research and clinical therapy is not so highly debated and criticized.