Essentially, stem cells are special types of cells in multicellular organisms that are capable of self-renewal as well as the ability to differentiate into various types of cells for specific functions. As cells that originate from a fertilized egg, they influence the formation of various intracellular and extracellular tissues during growth and development.
Depending on the organism and tissue type, stem cells have also been shown to play an important role in internal repair where they continually divide in order to replace affected/damaged cells.
Compared to other types of cells, stem cells have the following unique characteristics:
With regards to stem cells, potency refers to their ability to differentiate into distinctive types of cells. Whereas some stem cells are capable of generating more types of specialized cells (those with greater potency) others can only generate few specialized cells and thus have lower potency.
Totipotent- A zygote is a good example of a totipotent stem cell. These cells are characterized by high potency which means that they are capable of differentiating into any type of cell in an organism.
For instance, the zygote formed following fertilization of the egg gives rise to types of cells that form particular tissues and organs.
* The word toti means whole.
* Totipotent stem cells are located in the early cells of a fertilized egg.
Pluripotent- Examples of pluripotent stem cells include embryonic stem cells. Compared to totipotent stem cells, pluripotent stem cells can only progress to cells from any of the three germ layers (mesoderm, endoderm, and ectoderm).
* Unlike the totipotent stem cells, pluripotent stem cells are located in the inner mass of the blastocyst.
* Pluri means many.
Multipotent- Multipotent stem cells are the type of stem cells that only produce closely related cells. As such, they do not differentiate into a wide variety of cells as is the case with totipotent and pluripotent stem cells.
Hematopoietic stem cells, for instance, can only produce red and white blood cells (including platelets). Because these types of stem cells yield to very specific type of cells (closely related) they are found in many tissues in the body where they perform this function.
Oligopotent stem cells- Stem cells like myeloid stem cells are referred to as oligopotent because they produce a few types of cells (e.g. neutrophils and eosinophils)
Unipotent- Unipotent stem cells give rise to cells of their own type along a single lineage. As such, unipotent stem cells have the lowest differentiation potential compared to other types of stem cells. Skin cells are a good example of unipotent stem cells.
Classification Based on the Source
Based on their sources, stem cells are widely divided into two main categories that include:
Embryonic Stem Cells
As the name suggests, embryonic stem cells are the type of stem cells derived from embryos (4 to 5 days old). In particular, they are located within an inner cell mass of the blastocyst.
Because of their ability for self-renewal and the fact that they differentiate into every type of cell in the body, these stem cells significantly contribute to the growth and development of a fetus in the womb. These are only present during the first stage of development.
* At about the third day of division (following fertilization), at the eight-cell stage the cells are totipotent. At this stage, the embryo is capable of developing into a fully developed organism.
* The blastocyst is formed about the 5th day. At this point, the inner cell mass that contains the embryonic stem cells (pluripotent cells) is fully formed.
Mature/Adult Stem Cells
Also known as somatic stem cells, adult stem cells are a type found in children and adults. Like other stem cells, these are capable of self-renewal as they can make new copies of themselves (long term self-renewal).
Various types of mature specialized cells that perform specific functions are developed. Unlike embryonic stem cells located that are found/derived from a mass of cells of the blastocyst, adult stem cells can be found in different parts of the body. This makes it possible to replenish and regenerate tissues in cases where cells die or are damaged.
* In some mature tissues, the origin of adult stem cells is yet to be fully understood.
* In adults, stem cells are very few in number. However, they can still be found in various tissues where they serve to replace cells that die due to diseases or injuries. As such, adult stem cells also contribute to homeostasis.
Adult stem cells include:
Hematopoietic Stem Cells (HSCs)
Hematopoietic stem cells only make up a small percentage of cells in the bone marrow (about 1 in 10,000 cells). In addition to the ability to renew themselves (self-renewal), hematopoietic stem cells in the bone marrow also contribute to the entire spectrum of blood cells (red cells, white cells, platelets). Because these help to produce only specific related cells (blood cells) they are described as multipotent stem cells.
* Some hematopoietic stem cells have also been identified in blood.
Also referred to as skeletal stem cells in some books, mesenchymal stem cells are not related to blood stem cells (hematopoietic stem cells) in adults.
Although they are, for the most part, found in the bone marrow, mesenchymal stem cells have also been identified in a number of other tissue including the liver and lung of the fetus, and cord blood among others.
Mesenchymal stem cells develop into a number of specialized cells including chondrocytes, adipocytes, and osteoblasts among others.
Like many other types of stem cells, mesenchymal stem cells are also capable of self-renewal.
* While this type has been studied for a long time, the term "mesenchymal stem cells" has generated some controversy as it has been expanded to include different types of cells from various tissue. The term has been labeled as misleading because it refers to a population of heterogeneous cells some of which are not stem cells.
Neural stem cells
Neural stem cells are found in the neural tissue of adults.
In the fetal brain, these cells can be found in the following areas:
Cerebral cortex and cerebellum
As some of the most primordial stem cells in adults, neural stem cells are also capable of self-renewal and multipotency. As multipotent stem cells, neural stem cells give rise to both functional cells (neurons) and glial cells. In addition to neurons, neural stem cells also give rise to astrocytes and oligodendrocytes.
* Neural stem cells produce progeny cells that ultimately differentiate giving rise to neurons, astrocytes and oligodendrocytes.
As compared to various multipotent stem cells, epithelial stem cells have been shown to be regional-specific. Through stem cell markers, epithelial stem cells have been identified in such regions as the basal layer, sebaceous gland, and kidney papilla among others.
Like other stem cells, epithelial stem cells are also capable of self-renewal while continually developing into various mature cells that serve different functions. In the gut, epithelial stem cells produce several types of cells including goblet cells, paneth cells and absorptive cells among others.
* Generally, epithelial stem cells have been described as cells of origin for various epithelial cells. For some tissues, epithelial cells are lost at a high rate (e.g. squamous cells, etc). Here, epithelial stem cells are highly involved in the replenishment of these cells. This maintains homeostasis as well as the normal functioning of various tissue and organs such as the lungs.
Skin Stem Cells
Skin stem cells are found in the basal layer of the epidermis as well as the base of hair follicles.
Some of the skin stem cells include:
Follicle multipotent stem cells
Epidermal stem cells
Mesenchymal stem cells
Dermal sheath stem cells
Endothelial stem cells, and
Melanocyte stem cells among others
The different types of stem cells are also located in different areas of the skin. Whereas the follicle multipotent stem cells and epidermal stem cells are located in the epidermis, melanocyte stem cells can be found in the hair follicle bulge region.
In their location, skin stem cells are capable of self-renewal as well as giving rise to different types of cells.
Given that cells of the skin have to be continually replaced (throughout life) skin cells found in these locations continually progress to specialized cells that replace those lost thereby maintaining skin homeostasis as well as hair regeneration.
Induced pluripotent stem cells
As compared to the other types of stem cells in the body, induced pluripotent stem cells (iPSCs) are unique in that they are made in the laboratory. Here, embryonic genes are introduced into the somatic cells which convert them into stem-like cells.
Therefore, induced pluripotent stem cells are different from a majority of stem cells that are produced in various parts of the body. However, like the others, these are capable of giving rise to various specialized cells.
Functions, Uses and Research
As mentioned, stem cells are very few in number in adults as compared to a fetus. However, they are present in various regions of the body.
Epithelial cells found in the lungs (hepatic stem cells) progress to hepatoblasts which are the precursor to hepatocytes. Stem cells, therefore, function to maintain cell homeostasis in the body. Although matured, specialized cells also divide to maintain this cell balance, and are particularly important during damage and injuries to various cells.
Apart from the functions they perform in different organisms (both the fetus and adults) the ability of stem cells to renew themselves and contribute to other more specialized cells have made them particularly important in various aspects of medicine.
Currently, stem cells are being used for the following:
Tissue regeneration- It has become possible to repair, by generating new cells, various tissues in the body. Given that some patients have to wait for a long period of time for donors to donate given body organs/tissue to replace damaged/non-functional ones, tissue regeneration promises to be of great value because scientists can now use stem cells to grow specific tissue and even organs for such patients.
Treatment- Scientists in medicine have reported successful treatment of cardiovascular, blood diseases and brain diseases using stem cells in the past decade.
* Currently, stem cell research is being geared towards the treatment of genetic defects, the testing of new drugs and to understanding different types of cancers.
There are various ethical issues revolving around stem cell research, particularly embryonic stem cell research.
According to researchers, embryonic stem cells can be used to treat the following diseases:
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