What are the different types of stem cells?
There are many types of stem cells, but broadly they are classified into 3 types:
Embryonic stem cells
Embryonic stem cells (ESCs) are stem cells derived from the undifferentiated inner mass cells of mammalian embryos in the blastocyst stage on the fifth and sixth day after fertilization. They are derived from embryos that have been fertilized in vitro (in laboratory) in an in vitro fertilization clinic. Embryonic stem cells are distinguished by two distinctive properties: their pluripotency i.e their ability to form any cells of the body and their ability to multiply indefinitely. Because of these properties, embryonic cell therapies have been proposed for regenerative medicine and tissue replacement after injury or disease. These cells are grown in culture medium in the laboratory at a suitable temperature and humidity where they divide and multiply indefinitely.
Some of the advantages of embryonic stem cells are:
Embryonic stem cells are able to form any cell type of the body, and are more versatile than other stem cells
These cells are considered immortal as they can be maintained in undifferentiated state for a very long time in controlled conditions.
Some of the disadvantages of embryonic stem cells are as follows:
These cells can sometimes divide uncontrollable, forming tumors and growths madeof unwanted tissues.
Human embryos are destroyed in the process of collecting these cells.
These cells are related with many immunological rejections that results from the expression of MHC proteins.
It is very expensive.
Umbilical cord stem cells
Umbilical cord is a rich source of stem cells which are either obtained from umbilical cord blood or Wharton Jelly. There are a variety of stem cells which can be obtained from the Umbilical cord namely hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), endothelial progenitor cells (EPCs), and unrestricted somatic stem cells (USSCs) and other non- hematopoietic stem cells. Use of these cells has been currently approved for treatment of leukemias, lymphomas, anemias and inherited metabolic disorders. Numerous clinical trials are being conducted to study their potential in disorders like cerebral palsy, autism, ALS, etc.
The umbilical cord is collected and stored in a cord blood bank which is either public or private bank.
The advantages of using umbilical cord derived stem cells are:
It is a non-invasive source and can be obtained from the umbilical cord immediately after birth.
Available in vast abundance; thousands of babies are born each day and the umbilical cord and placenta are discarded as waste.
Despite its high content of immune cells, it does not produce strong graft-versus- host disease
Therefore, cord blood grafts do not need to be as rigorously matched to a recipient as bone marrow grafts.
Along with these advantages there are also few disadvantages of using umbilical cord stem cells such as:
Engraftment of transplanted cells is slow as compared to other cell sources
The volume of cells obtained from one umbilical cord is limited and low. Also, additional cell dose cannot be obtained.
Few occurrences of infection have been recorded on use of these cells.
There may be storage related issues such as loss of viability due to long term storage, Cost related to long term storage, etc
Adult stem cells
Stem cells are found in our body during all stages of life, number of stem cells and their ability to differentiate into different cell types differs as per the age of the person. Different types of stem cells are available in the body. There are stem cells that are tissue specific and are committed to differentiate only in one type of tissues. But adult stem cells can also be multipotent, with ability to be able to differentiate into different cell types.
Niches are the specific areas from where these cells can be procured. Adult stem cell niches have been found in most organs of the human body, eg. liver, brain, bone marrow, fat tissue, heart, nasal ensheathing cells, tooth pulp tissue etc. Whenever there is an injury or wear tear in the body these cells initiate the repair process. These are not only the cells from that particular tissue but the stem cells from bone marrow are mobilized to the peripheral blood and to different organs. This repair process is their primary role. Adult stem cells are a non-controversial and safe option for use in human subjects as they can be used autologously.
To date, published scientific literature indicates that adult stem cells have been derived from brain, bone marrow, peripheral blood, dental pulp, spinal cord, blood vessels, skeletal muscle, epithelia of the skin and digestive system, cornea, retina, liver, and pancreas; thus, adult stem cells have been found in tissues that develop from all three embryonic germ layers. However they cannot be procured easily from all of these sources; most common sources are bone marrow, abdominal fat and dental pulp.
Most of the beneficial effects of adult stem cells are also because of the paracrine effects of these cells on surrounding tissues. Paracrine effects are the changes induced in the functioning of cells surrounding these stem cells in the body. These are mediated through different chemicals secreted by the tissue. These paracrine effects are reduction in inflammation, neo-angiogenesis, immunomodulation and stimulation of local resident stem cells.
Bone Marrow Derived Cells
Different types of stem cells have been found to be present in the bone marrow, which differ in their potential to differentiate and form cells from one or more germ layers.
Initially, the bone marrow was thought to contain only hematopoietic stem cells. However, increasingly, evidence is pouring in regarding the heterogenous population of cells having varying plasticity. Mononuclear cells obtained from the bone marrow are a mixture of heamatopoietic stem cells, mesenchymal stem cells, very small embryonic like stem cells etc. Therefore these is a mixture of different types of stem cells in this tissue fraction.
Adipose tissue derived cells:
The adipose tissue (AT) is a highly complex tissue. It consists of mature adipocytes, preadipocytes, fibroblasts, vascular smooth muscle cells, endothelial cells, resident monocytes/macrophages and lymphocytes. Adipose tissue provides a rich source of pluripotent stromal cells.
Adult stem cells have no ethical issues as associated with embryonic- and fetal-derived cells and are less susceptible to malignant changes and genetic abnormalities.