Types of Stem Cells

Origins of Stem Cells

The term “stem cell” encompasses a wide variety of biological cells, unified by their ability to turn into different specialized cells, or differentiate. Some stem cells can only turn into a select few cell types, while others can turn into any kind of cell. The different types of stem cells are grouped into several categories; some are found in young organisms, while others can be present in adult tissues. Here are just a few of the types of stem cells that are known to exist, with points of interest for each kind;

Embryonic Stem Cells

As their name implies, embryonic stem cells (ESCs) are present in embryos – the start of an organism is an embryo, which is technically a stem cell. ESCs can turn into any other cell type, which makes them the most versatile stem cells out there. However, ESCs are known to form tumors when simply injected into an organism, and they do so precisely because they can turn into any other cell type; there is nothing to tell them that they shouldn’t turn into a tooth or a kidney.

ESCs are also problematic for ethical reasons – because ESCs come from an unborn embryo, their use in research necessarily comes at the expense of a potentially viable individual. Because of such moral and medical dilemmas, other stem cell types are also being investigated for therapeutic applications. Nonetheless, stem cell banks (which cryogenically store stem cells for future use) are popular, and parents can purchase storage plans for cord blood stem cells obtained during birth. This is usually done to ensure that a child has compatible stem cells for future surgeries, if necessary. For example, traumatic injuries and long-term therapies can be complimented by a stem cell injection.

One of the more ethically complicated uses of embryonic stem cells occurs when they are harvested from an aborted fetus; either due to maternal considerations or medical reasons, the embryo may be discarded. In such cases, harvesting of embryonic stem cells does not come at the expense of a viable individual, though there are moral objections to such practices as well.

Embryonic stem cells grown on a layer of fibroblasts, which feed the stem cells with nutrients and other vital compounds for proper growth. When cryogenically stored, the stem cells do not grow, and hence do not differentiate. This allows them to be used for therapies conducted decades after the cells were harvested.

Cord Blood Stem Cells

When an individual is born, the umbilical cord is severed, and usually the contents therein (cord blood mainly) are discarded. Yet cord blood contains within itself a comparatively large concentration of stem cells that can turn into a wide variety of other cell types. Cord blood stem cells are usually called umbilical cord mesenchymal stem cells (UC-MSCs) and they are used very frequently in homologous injections, as such injections do not require clinical trials under FDA regulations. Moreover, because cord blood stem cells are not ESCs, they do not come at the expense of an individual, and hence their approved use (by the donor) is ethically sound. Better yet, UC-MSCs do not form tumors in surrogate organisms as ESCs do; because UC-MSCs cannot turn into all cell types, and hence they are far less likely to result in medical complications from direct injections.

UC-MSCs are the subject of Altogen’s current research into a homologous injection therapy, and they are extracted from cord blood obtained with the consent of mothers during birth. The stem cells are collected after the umbilical cord is severed, which means the collection does not affect the health of the baby or its mother. The cord blood samples are immediately shipped to our laboratory for further processing and diagnostics, as even though the mother is screened for infectious diseases, further testing of immune characteristics is necessary for histocompatibility (so that the stem cells are not rejected by a patient’s body). Although this procedure is not currently available for purchase, in the future it may become a more affordable, and less-painful stem cell therapy.

Amniotic Stem Cells

The amniotic fluid is found in the amniotic sac, which encloses the developing fetus. The fluid contains large amounts of stem cells involved in fetal development, and its extraction is generally considered to be ethically sound, as the procedure does not harm the fetus. Currently, amniotic stem cell banks offer storage of amniotic stem cells for individuals that may need them in the future. The stem cells can be extracted during the later stages of development, and frozen in liquid nitrogen thereafter.

Although Altogen’s homologous therapy research efforts are focused on cord blood derived stem cells, amniotic stem cell therapies are also popular, though they usually provide a far lower dosage of stem cells, which can lower the effectiveness of stem cell therapy. Many institutions will conduct autologous stem cell injections using amniotic stem cells that have been cryogenically stored. Some families decide to invest in cryogenic storage of amniotic stem cells, but the practice is recent, and older generations (so far) only have homologous implantation options for stem cell therapies, as well as autologous therapies such as the one Altogen offers.

Adult Stem Cells

ESCs and UC-MSCs go away after birth, turning into other necessary cells in the body, but some stem cells remain – they are broadly classified as adult stem cells. Usually they are present in various tissues in the body to aid in repair mechanisms if there is any damage to tissues; the stem cells are guided to the damaged site and there they replace necessary cells that have been injured or destroyed. Such stem cells are usually only able to turn into just a few other cell types, meaning that an adult stem cell found in the heart will not be able to turn into a neuron to be present in the brain. This limitation has led to a widespread effort to find ways of turning adult stem cells into more competent ones, which would be able to turn into a variety of different tissues, just like embryonic stem cells.

Adult stem cells are an area of active research, as they can be derived from patients, grown in the laboratory, and reimplanted back into the patient where the cells can do their helpful work. Adult stem cells differ by nature of where they are found and what kind of cells they can turn into. Different locations of adult stem cells contribute to their differing characteristics; stem cells located in the bone marrow (which is where Altogen’s stem cell therapy obtains stem cells from) are able to regenerate bone and connective tissues, but not more specialized cell types, such as dendrites. Here are a few of the broad categories of adult stem cells and how they can work in biological tissues;

Bone Marrow

There are actually several kinds of stem cells found in the bone marrow; some are responsible for the replenishing of blood cells, while others can lead to the formation of bones and and cartilage. Stem cells from bone marrow can be isolated in a procedure known as bone marrow aspiration, when a needle is inserted into a bone (usually the hip). Such stem cells have been tested in clinical trials for the treatment of leukemia, and the associated stem cell therapy is the first one approved by the FDA. There are several kinds of stem cells found in the bone marrow, here are some common kinds;

  • Endothelial Stem Cells
    • Endothelial stem cells, commonly abbreviated as ECs, are the progenitors (precursors) of many cell types, but are restricted somewhat in that they cannot entirely reform an organ. They are called endothelial because they are found on the inner surface of bone – the boundary between bone marrow and bone itself.
  • Hematopoietic Stem Cells
    • Abbreviated as HSC, hematopoietic stem cells lead to the formation of blood cells, and have found clinical applications in the treatment of leukemia; after cancerous blood cells have been killed by chemotherapy, normal blood cells are reformed by injected HSCs, which then get rid of remaining leukemic cells. HSCs were part of the first FDA-approved stem cell therapy, and have been commonly used ever since in hospitals for leukemia treatment. HSCs can be found floating around in the bloodstream, and one of the processes by which they can be isolated involves the centrifugation (spinning) of blood to filter out such stem cells, while pumping back regular blood into the body.
  • Mesenchymal Stem Cells
    • Although technically they can be found in a variety of other tissues (including adipose and cord blood), mesenchymal stem cells (MSCs) are highly concentrated in the bone marrow and are often derived from it. These cells are called mesenchymal because they come from the connective tissues found in early-stage development, resembling the vitreous fluid within the eye or the jelly-like substance in the umbilical cord (called Wharton’s jelly). MSCs are some of the most commonly used stem cells in the medical field, as they have shown relevance in the treatment of osteoporosis and other age-related diseases affecting bones or joints. MSCs can be derived from adipose tissue (fat) and then be injected into a malfunctioning joint or bone (although this procedure is not yet FDA approved). MSCs can also be derived from the umbilical cord, at which point they are more apt to be named cord blood stem cells.


  • Adipose tissue, consisting of connective tissues and fats, also has stem cells within itself that can turn into cartilage, bone, and a few other cell types. Adipose-derived stem cells (ADSCs) are a subject of active research, as they can be easily obtained from many patients, and can be grown in laboratories for reimplantation during treatment of several bone-related diseases. Despite their promises, the FDA has not approved any ADSC-based stem cell therapy that has passed all three clinical trials, though there are ADSC-based stem cell therapies that are nearing completion of clinical trials, with early promising results. In other countries outside of the United States, ADSC-based stem cell therapies have been allowed for several years now, and in due time they will likely be allowed in the United States as well. The procedure for deriving stem cells from adipose tissue (which is itself obtained through liposuction) can be complex, and has important effects on the functionality of ADSCs when injected back into a patient.


  • The heart contains cardiac stem cells, which are responsible for keeping cardiac tissue in good shape and replacing damaged or older cells. Cardiac stem cells aren’t easy to obtain, and they have not found widespread application in stem cell therapy. However, the characteristics of cardiac stem cells may be valuable as they can be conferred to other stem cells, so that they can also regenerate heart tissue.

Neural Current research into neural stem cells is mainly focusing on finding alternate sources (such as adipose tissue) and modifying those cells to fit the criteria for neural stem cells. As they are usually located in the brain and spinal cord, the extraction of neural stem cells can be perilous. Neural stem cells are crucial to the development of the brain, and might hold the key to Alzheimer’s cures in the near future. Neural stem cells, if they can be efficiently made from other sources (like cord blood stem cells) will be able to be injected into the spinal cord or brain to replenish vital tissues necessary for cognitive function and abilities. Although there is currently no guaranteed method of creating neural stem cells from more primitive stem cell types, the addition of several chemical compounds may help them turn into neurons while being grown in the laboratory. Such techniques may end up passing clinical trials and becoming a standard in the treatment of neuroblastomas (brain cancer) and neurodegenerative diseases.


  • Limbal stem cells are found in the corneal limbus, and are responsible for ocular regeneration. In simpler terms, limbal stem cells keep the eye functioning, as the cells responsible for vision are highly specialized, and require their own kind of precursor stem cells. They are not considered to be viable candidates for stem cell therapy because they can be difficult to extract; limbal stem cells number few, and surgery to remove them from the eye is costly and difficult. Like neural stem cells, limbal stem cells are known to have several defining features that can be replicated in laboratory settings, which could help with cures to eye diseases in the future.

Oogonial and Spermatogonial

  • Oogonial and spermatogonial stem cells give rise to gametes, which are the cells involved in sexual reproduction. They are stem cells because they eventually lead to the formation of different cell types, but by themselves they are incapable of regenerating organs. Current research into these stem cells is focused on the genetic aspects of regenerative medicine, and how early-stage genetic abnormalities can affect the ability of cells later on to repair damaged organs and tissues.


  • Renal stem cells are key to the integrity of the kidney, and can lead to specialized cell types involved in blood filtration. Renal stem cells are an area of active research, as in the future they may potentially be used to regrow entire kidneys in laboratory settings for reimplantation into those who need them. Although renal stem cells are not used for stem cell therapies, they are highly important in the current fight against kidney diseases.

Dental Pulp

  • Stem cells are found in the soft tissue within teeth, and can turn into a variety of cell types. They are currently being looked into as possible sources of stem cells for growth in the laboratory; during tooth extractions, the teeth are usually discarded, but the dental pulp could potentially be further grown in a laboratory for autologous stem cell therapies.

Induced Pluripotent Stem Cells

One of the more interesting questions facing researchers is why normal cells cannot be stem cells; after all, every cell in the body contains the same DNA. Induced pluripotent stem cells (IPSCs) represent our rising understanding of how stem cells function and how they can be made from already specialized cells. The area is one of active research, and in the future it may be possible to take any cell from the body, modify it, and have it function as a stem cell – meaning that it will be able to replace all other kinds of cells in the body. Current knowledge is limited, but preliminary results show that it is possible to get certain cells into an undifferentiated state, potentially helping out with therapeutic applications in the future.

By definition, IPSCs are normal cells that have been “induced”, or turned into stem cells. There are several chemical factors that can affect this process, and some studies have successfully shown how to generate IPSCs from normal tissues. These IPSCs are not like embryonic stem cells in that they cannot turn into every other cell type, but they are still able to regenerate damaged tissues that are near in origin; for example, an IPSC made from fat tissue may be able to regenerate bone, cartilage, and connective tissues when re-implanted back into the corresponding tissue. With rising understanding of how stem cells differ from normal ones, IPSCs may become an efficient starting source for stem cell therapies.