There are two main sources of stem cells:
Embryonic stem cells:
Pluripotent cells that exist in the early stages of embryonic development, specifically in the inner mass of blastocysts, and go on to form all the cells of the adult body. These cells no longer exist after five days of development.
Adult stem cells:
SCs that can be found in almost all body tissues including blood, brain, liver, intestine, or skin. They are more tissue specific cells committed to become a cell from their own tissue of origin, but cannot go to form all tissues of the body.
Based on the differentiation potentiality of SCs, they can be categorized into hematopoietic stem cells (HSCs) and non-hematopoietic stem cells (NHSCs). HSCs are cells that give rise to blood cell lineages and are capable of differentiating to white blood cells (WBCs), red blood cells (RBCs), and platelets. NHSCs are cells that give rise to tissues and organs other than blood.
Sources of Adult Stem Cells
Stem cell sources vary depending on the type of SCs. Of special interest, are the adult stem cells (ASCs) which are found in the following body sites:
Bone Marrow (BM):
The factory responsible for manufacturing different blood components, white blood cells, red blood cells, and platelets in a process called hematopoiesis. Bone marrow is classified into red bone marrow (hematopoietic) and white bone marrow. The red bone marrow is responsible for the blood supply, i.e. supplying SCs for the generation of different body tissues, and for repair and regeneration of injured or wasted organs. The white bone marrow is responsible for replacing the red bone marrow and providing stem cells for the generation of other body tissues including bone, cartilage, and fat.
Peripheral Blood (PB):
The human peripheral blood contains several types of cells that include RBCs contributing with 4.5-5.4 x106 cells/mm3, and WBCs with a count of 4-10 ×103cells/ mm3. WBCs are divided to granulocytes and agranulocytes.
Granulocytes:
Neutrophils: 60-65% of WBCs
Eosinophils: 1-2% of WBCs
Basophils: 0-1% of WBCs
Agranulocytes:
Monocytes: 5-8% of WBCs
Lymphocytes: 30-35% of WBCs (subdivided to B and T lymphocytes; T cells are then subdivided to T helper (Th), and T cytotoxic (Tc) cells).
Human PB contains stem cells that make up less than 0.1% of total WBCs. PB SCs are also called “circulating SCs”, because they are able to migrate out of the niche into the peripheral blood and back to the niche in the BM again after migration and homing. The major types of SCs that are found in PB are CD34+ and CD133+ stem cells.
Tissue-Resident Stem Cells:
These cells can be found in various body tissues and organs. Their capabilities include the normal regeneration and repair of venial damages of that organ. Tissue resident stem cells have been found in various organs and tissues across the human body including the liver, spleen, adipose tissue, heart, and neuronal tissues including the brain.
Cord Blood Stem Cells (CB-SC):
Recent studies have shown the ability and the capability of SCs extracted from the umbilical cord blood to differentiate into blood cells (granulocytes, agranulocytes), bone cells (osteoblasts, osteocytes), and several other cell types. Evidence to date has indicated that cord blood contains an abundant source of hematopoietic cells (HSCs) in particular. Interestingly, no significant differences were found between SCs obtained from BM, PB, or CB.
Adipose Tissue (Fat):
Fat tissue has been shown to contain a large number of stem cells that have high regeneration capacities, anti-inflammatory effects, and can differentiate along multiple lineage pathways. The advantage in using adipose derived stem cells (ASC) is that they can be obtained with a minimally invasive procedure and their collection and purification takes only minutes. ASCs have been shown to be very effective in the treatment of heart diseases, chronic wounds, rheumatism and arthritis.