Adult Stem Cells
An adult stem cell is considered to be an undifferentiated cell, found amongst a microenvironment or niche consisting of differentiated cells in a tissue or organ. The adult stem cell has the ability to renew itself and to differentiate in order to yield some or all of the major specialized cell types with characteristic shapes and functions of that particular tissue or organ. Hence the primary functions of adult stem cells in a living organism are maintenance and repair of the tissue in which they are found.
Scientists sometimes also use the term somatic stem cell, where somatic refers to cells of the body (not the germ cells). Unlike embryonic stem cells, which are defined by their origin, the existence of adult stem cells in some mature tissues is still under investigation.
Commonly, there is a very small amount of adult stem cells present in each tissue and once it is removed from the body, their capacity to divide becomes limited, therefore making the production of large quantities of stem cells difficult.
Some instances of the potential treatments using adult stem cells include regenerating bone using cells cultured from bone marrow stroma, development of insulin-producing cells for type 1 diabetes, and repairing damaged heart muscle with cardiac muscle cells.
Location of Adult Stem Cells (ASCs)
Researchers In the 1950s discovered that the bone marrow contains at least two kinds of stem cells:
- Hematopoietic stem cells. They give rise to all types of blood cells in the body.
- Bone marrow stromal stem cells (also known as mesenchymal stem cells/skeletal stem cells). These are non-hematopoietic stem cells that make up a small portion of the stromal cell population in the bone marrow. They can generate bone, cartilage, as well as the fat cells that support the constitution of blood and fibrous connective tissue.
Adult stem cells can be situated in many organs and tissues, including the brain, bone marrow, blood vessels, peripheral blood, skeletal muscle, heart, gut, liver, skin, teeth, ovarian epithelium, and testis. They inhabit a specific area of each tissue (called a "stem cell niche"). Current research suggests that there are some types of stem cells called pericytes that compose the outermost layer of small blood vessels. In the 1990s, scientists agreed that even the adult brain does, in fact, contain stem cells that have the capability to generate the brain's three major cell types—astrocytes and oligodendrocytes (which are non-neuronal cells), and neurons, or nerve cells.
Adult stem cells can be activated by a necessity for more cells to maintain the tissues, or in the event of disease or tissue injury. Otherwise, they may remain quiescent (non-dividing) for long periods of time.
Some major types of ASC’s
- Hematopoietic stem cells (HSC's) are the progenitors to all of the types of cells present in the blood such as red blood cells, B and T lymphocytes, natural killer cells, neutrophils, eosinophils, basophils, monocytes, and macrophages.
- Mesenchymal stem cells are commonly found in the bone marrow. They have also reportedly been present in the cord blood, peripheral blood, fallopian tube, and fetal liver and lung tissues. Those from the bone marrow (bone marrow stromal stem cells, skeletal stem cells) have the ability to give rise to a variety of cell types: bone cells (osteoblasts and osteocytes), cartilage cells (chondrocytes), fat cells (adipocytes), and stromal cells that assist in the differentiation of stem cells to form different types of blood cells.
- Neural stem cells present in the brain give rise to three major cell types: nerve cells (neurons) and two categories of non-neuronal cells—astrocytes and oligodendrocytes.
- Epithelial stem cells present in the lining of the entire digestive tract are located in deep crypts and give rise to several cell types: absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells.
- Skin stem cells occur in the basal layer of the epidermis and also in the base of hair follicles.
- Epidermal stem cells give rise to keratinocytes, which then migrate to the surface of the skin and form a protective layer. The follicular stem cells can produce both the hair follicle and the epidermis.
Induced Pluripotent Stem Cells (iPSCs)
Induced pluripotent stem cells (iPSCs) are normal adult cells that have been genetically reprogrammed to enter into an embryonic stem cell-like state which is forced to express genes and factors that are important for maintaining the characteristic properties of embryonic stem cells. It also has fewer ethical concerns compared to other sources of stem cells.
Human iPSCs can also express stem cell markers and are capable of generating cells characteristic of all three germ layers and also have the ability to differentiate and become various different fetal cell types. They are very valuable in the study of disease modeling, disease pathology, treatment, and drug development, and reports have suggested their remarkable potential in transplantation and regenerative medicine. Their utilization in transplantation medicine is especially useful as the usage of post-transplant immune suppression can be avoided since the cells are derived from the patients themselves and hence are genetically identical.