Adult Stem Cells

Adult stem cells are undifferentiated cells that can be found in individuals of all multicellural organisms, including humans. Their primary role is to replenish dying cells and to repair damaged tissues and organs. In humans, and other animals, adult stem cells can be found in a great number of tissues/organs including bone marrow, blood, adipose tissue and liver. Sometimes the term "somatic stem cells" is used instead.

Adult stem cells, should not be confused with induced pluripotent stem cells, as they are two totally different things. Adult stem cells can be found in an organism naturally, whereas induced pluripotent stem cells are artificial stem cells derived from normal, differentiated cells.

Picture of adult stem cells taken from adipose tissue
Adult stem cells from fat

Unlike human embryonic stem cells, the use of human adult stem cells typically doesn't raise any ethical issues and concerns. This is because, human adult stem cell extraction doesn't require the destruction of embryonic bodies. For more info on the topic, check our Stem Cell Controversy article.

Adult stem cell properties
There are two characteristics that distinguish all stem cells, including adult stem cells, from the rest of the normal cells an organism has:
  • Self Renewal
  • Potency

Self renewal, is the capacity of a stem cell to undergo numerous cycles of cell division, without losing its undifferentiated state. There are two different types of cell division, that an adult stem cell may employ, both of which ensure that the total stem cell population remains stable:
  • Symmetric Division
  • Asymmetric Division

During symmetric division one adult stem cell, gives  rise to two identical daughter cells, both of which share the same stem cell properties.

During asymmetric division, the father stem cell gives rise to one identical stem cell, and one progenitor stem cell. Progenitor stem cells are cells with a tendency to differentiate into a specific type of cell. They are the "intermediate cell" between a stem cell and the final cell target. Unlike stem cells that can replicate indefinitely, they can only divide for a limited number of times.

Potency,  refers to the ability of a stem cell to develop into other types of specialised cells, for instance neurons, platelets, muscle cells and epitheleal cells. The higher the number of distinct cell types a stem cell can develop into, the higher its potency. For example,  hematopoietic stem cells have the capacity to develop into a great number of blood cells (e.g. red blood cells or platelets) and are categorised as multipotent. On the other hand, vascular stem cells can only develop into either endothelial or smooth muscle cells, and are listed as oligopotent. An adult stem cell can have one the following potency degrees:
  • Multipotent, which are adult stem cells that can differentiate into many types of cells, that all however belong to the same family
  • Oligopotent, which are adult stem cells that can only differentiate into a very limited number of very closely related cells. For instance, vascular stem cells have the capacity to become either endothelial or smooth muscle cells.
  • Unipotent, which are adult stem cells with the ability to produce only one type of cell. Their existence is still hypothesised, as a true example of unipotent stem cell has yet to be found.
 For more info about self renewal and all the degrees of potency, check out our what are stem cells article. 

Adult stem cells, where are they found?
Human adult cells have been discovered in many different types of organs and tissues, and there are probably more, yet to be discovered from researchers places, where adult stem cells may reside. Some of these  tissues and organs are:
  • blood vessels
  • bone marrow
  • gut
  • heart
  • including brain
  • liver
  • ovarian epithelium
  • peripheral blood
  • skeletal muscle
  • skin teeth
  • testis
Usually, only a very small number of stem cells reside in a tissue/organ, and they can't be found throughout its in entirety. Instead, they usually reside in a very limited area of that tissue or organ which  is called "stem cell niche".

Types of adult stem cells
Adult stem cells, are categorised by the differentiation pathway they are programmed to follow. Some, but not all, types are the following:
  • Hematopoietic stem cells, they are multipotent stem cells found in the bone marrow. They give rise to all blood cell types like erythrocytes, macrophages, t-cells, b-cells and other.
  • Mammary stem cells, they are found in the human breast. They are the source of cells responsible for the growth of the mammary gland during puberty and gestation, and can develop into luminal and myoepithelial cells.
  • Intestinal stem cells, they produce the cells lining the surface of the small and large intestines. They can be found in the "crypts of Lieberkuhn", small intestinal glands located in the epithelial lining of the small intestine and colon
  • Mesenchymal stem cells, they are multipotent stem cells with the capacity to differentiate into a great number of cell types,  like osteoblasts, chondrocytes and adipocytes (fat cells). They can be extracted from many sources including: developing teeth, umbilical cord blood, adipose tissue and bone marrow
  • Endothelial stem cells, they are multipotent stem cells that can also be found in the bone marrow. They can develop into endothelial cells which create the thin-walled endothelium that lines the inner surface of blood vessels and lymphatic vessels
  • Neural stem cells, multipotent stem cells that develop into neural cells (neurons) and two categories of non neural cells—astrocytes and oligodendrocytes.
  • Olfactory stem cells, they are multipotent stem cells, found in the olfactory mucosa, the upper region of the nasal cavity. They can develop into heart, liver, kidney, muscle, brain and nerve cells.
  • Skin stem cells, they are found in the basal layer of the epidermis and at the base of hair follicles. The have the capacity to develop  into keratinocytes.

Adult Stem Cell Transdifferentiation
Certain lab experiments, have shown that it is possible to differentiate certain adult stem cells into cell types of other organs or tissues, which are unrelated to the one the father stem cell originates from. For example researchers have managed  to develop a blood cell from a brain stem cell! This phenomenon is called transdifferentiation.

The phenomenon has been reported to occur naturally in certain vertebrate animals, whether it actually occurs naturally in humans or not, is still debatable and unclear.

Human adult stem cell treatments and applications 
Adult stem cells have been under excessive research, for their potential therapeutic applications, for more than a decade. Unlike embryonic stem cells, there are very few, if any, that oppose their use for therapeutic purposes. This is due to the fact that they can be extracted from the patient himself (or another donor) without requiring the destruction of a human embryo.

Bone marrow transplantation (Hematopoietic stem cell transplantation), is probably the best known example of a stem cell-related therapy. It has been performed on thousands of patients, mainly ones suffering from leukemia and related bone/blood cancers.

However, with the exception of bone marrow transplants, adult stem cell use is largely limited on a research or clinical trial (human or animal). There are a lot of stem cell clinical trials running at the moment, and there are some published clinical data available. Some representative examples are:
  • Wakitani, has published a report with nine knee chondral defects, that were surgically treated with mesenchymal stem cells. Later examination showed coverage of the treated defects.
  • Centeno et al.  have reported high field MRI evidence of cartilage and meniscus growth in individual human clinical studies.
  • Korean researchers reported on November 25, 2003, that they had transplanted multipotent adult stem cells from umbilical cord blood to a patient suffering from a spinal-cord injury. Supposedly the patient managed to walk after 19 years.
  • Since 2003, researchers successfully use transplanted corneal stem cells to repair/return vision in patients with damaged eyes.
  • A few small studies ,carried out in humans, have demonstrated that stem cells injected into injured heart tissue improves cardiac function and/or induces the formation of new capillaries.
In 2008, Paolo Macchiarini  transplanted the first human organ, a trachea, fully grown from stem cells. The operation was carried out at the Hospital ClĂ­nic of Barcelona on Claudia Castillo, on a Colombian female who had her trachea collapsed due to tuberculosis.

Video about the first human transplantation of an organ fully grown from stem cells

It should be mentioned that there are many stem cell clinics exclusively or mainly performing stem cell related treatments and operations. However in most cases the efficacy and safety of the offered treatments, despite what they claim, is not approved by the FDA or other similar agencies, check our list of stem cell clinics for more info.

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