Adult stem cells (ASCs) are one of the three major types of stem cells. They are somatic (or body) cells which are multipotent in nature, meaning that they can differentiate into some but not all cell types, and they can be found throughout the body within stem cell niches. Stem cell niches are environments designed specifically for the growth and development of certain types of stem cells. While some of these niches are located in common organs, such as in the brain, the bone marrow, or the small intestine, other niches are much harder to locate.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Unlike ESCs, adult stem cells can be harder to locate, as they must be found within a larger body. Adult stem cells are often categorized for their limited self-renewal capabilities: while they can continue to renew themselves, they cannot do so indefinitely. The loss of self-renewal, along with other factors, is part of the reason that aging occurs.

​

The benefits of using adult stem cells over other types of stem cells include their non-controversial nature (as donors can give their consent), the fact that patients can be treated with their own ASCs, thereby avoiding immune rejection, and their ease of access and availability (as they come from most somatic tissues found within the body). As with any other type of stem cell, adult stem cells also come with their limitations. For example, they are only multipotent in nature, meaning that their differentiation capacity is limited to a few cell types. As mentioned above, they are incapable of indefinite self-renewal, meaning that an ASC line cannot go on forever.

​

Although each type of ASC has a specific function, together they aid in regeneration of the body, a response initiated by age, damage, or death in order to help regulate internal conditions. Almost every type of cell in our bodies is replaced at some point in our lives, and it is the job of adult stem cells to do this replacing. Although adult stem cells usually stay quiet, keeping to themselves within their niches, they can be activated by the correct signals. These signals tell the adult stem cells to divide asymmetrically to maintain their own multipotent line while also producing differentiated cells to contribute to the body as necessary.

​

 

 

 

 

 

 

 

 

 

 

 

Adult stem cells can be harvested from a number of sources, including fetal tissues and placenta, amniotic and umbilical fluid, the brain and spinal cord, the cornea, the liver, the lungs, skeletal muscle, blood vessels, the pancreas, bone marrow, and epithelia of both the skin and the digestive system. There are many different types of adult stem cells, including mesenchymal stem cells, hematopoietic stem cells, neural stem cells, epithelial stem cells, and skin stem cells. We highlight these cells below.

 

Mesenchymal Stem Cell (MSC):

Mesenchymal stem cells, also known as stromal stem cells, generate cells which eventually make up the bone, cartilage, fat, muscle, tendons, ligaments, and connective tissue of our bodies. Because they can differentiate into so many different types of cells, MSCs are located throughout the body in a number of stem cell niches, including within the bone marrow, fat, and cord blood within fetuses.

​

 

 

 

 

 

 

 

 

 

 

 

Hematopoietic Stem Cell (HSC):

Hematopoietic stem cells are the stem cells of the blood, generating cells which will eventually become all blood cell types, including those of both the myeloid (red blood cell) and lymphoid (white blood cell) lineages. The myeloid cell line gives rise to monocytes, macrophages, neutrophils, basophils, eosinophils, erythrocytes, platelets, and dendritic cells, while the lymphoid cell line gives rise to T-cells, B-cells, and NK-cells. In addition to oxygen delivery and gas exchange, all of these blood cell types are involved in the immunoregulation of the body, serving as one of the main lines of defense when it comes to foreign bodies entering the blood flow. Blood cells are the first responders to injuries which cut through the skin, and they are also the reason for us having strong (or in some cases weak) immune systems. HSCs are found most often in the bone marrow, although they can also be harvested from umbilical cord blood and placental tissue. Of all types of adult stem cells, HSCs are by far the most understood by scientists as they were the first type discovered mentioned here.

​

 

 

 

 

 

 

 

 

 

 

Neural Stem Cell:

Neural stem cells are the stem cells of the brain and spinal cord, generating cells which will eventually become neurons and glial cells. While neurons are the major communicators of the nervous system, transmitting electrical impulses which allow us to process and interact with our surroundings, glial cells serve as the support system for neurons, keeping them healthy and enhancing their transmission ability so that signals are sent through the brain at a faster rate than they might otherwise move. Neural stem cells form neurons, oligodendrocytes, and astrocytes. While their existence is known, neural stem cells are one of the less utilized adult stem cell types due to their relative inaccessibility; often, it is impractical for these stem cells to be harvested given their location within the brain, making them challenging to reach and therefore challenging to utilize.

​

 

 

 

 

 

 

 

 

 

 

Epithelial Stem Cell:

Epithelial stem cells generate the epithelial cells which will eventually make up the linings of the body. Not surprisingly, epithelial stem cells are responsible for the creation of up to 60% of the mature, differentiated cells of the body, as most of our body parts are lined by cells in order to aid in compartmentalization and increase functionality. Epithelial cells line both the external and internal surfaces of our body. The external lining they make is otherwise known as our skin. Internal linings include those surrounding glands, vessels, parts of the intestines, and other body cavities. Additionally, epithelial stem cells can be found at the bulge region of hair follicles.

​

 

 

 

 

 

 

 

 

 

 

 

 

 

​Skin Stem Cell:

Skin stem cells, also known as epidermal stem cells, generate the cells which will eventually produce the different types of specialized skin cells found within the epidermis. Different from epithelial stem cells, epidermal stem cells are more specific, dedicated entirely to the external lining of the body known as our skin. This skin is responsible for waterproofing us, protecting the body from outside threats as the first line of defense, and allowing us to sense the world around us through sense receptors. Unlike some of the other adult stem cells mentioned here, skin stem cells are extremely active, with regeneration of various parts of the skin occurring every day. In addition to the epidermal cells often associated with skin stem cells, hair follicle cells and melanocyte cells are also generated, ensuring regeneration of hair, oil glands, and pigmented cells.