Stem cells are the body’s repair system, acting as a reservoir of unspecialized cells capable of replenishing damaged or aged tissue. These cells can divide to either remain stem cells or transform into various specialized cell types. The term “adult stem cell,” also called “somatic stem cell,” refers to these cells found within mature organs and tissues. They allow the body to maintain its complex structures throughout life.
Defining Adult Stem Cells
Adult stem cells are undifferentiated cells defined by two main characteristics. They possess the capacity for self-renewal, meaning they can divide repeatedly to maintain their own population. They also have the ability to differentiate, or mature, into specialized cells specific to the tissue where they reside.
These cells are classified as multipotent, meaning their differentiation potential is limited to a specific lineage of cells. For example, a hematopoietic stem cell in the bone marrow can only generate various types of blood cells, such as red blood cells, white blood cells, and platelets. It cannot differentiate into a liver cell or a nerve cell, which illustrates their tissue-specific nature.
Adult stem cells are maintained in a specialized microenvironment called a niche. The niche provides support cells and signaling molecules that control the stem cell’s behavior. This environment keeps the stem cells in a mostly dormant, or quiescent, state until they are needed for maintenance or repair. When damage or cell loss occurs, the niche prompts the stem cells to activate, divide, and begin differentiation.
Key Differences from Embryonic Stem Cells
The primary distinction between adult stem cells and embryonic stem cells (ESCs) lies in their developmental flexibility, known as potency. Adult stem cells are multipotent, restricted to forming only the cell types of their tissue of origin. In contrast, embryonic stem cells are pluripotent, meaning they can differentiate into nearly every cell type in the human body.
Adult stem cells are sourced from mature tissues, such as bone marrow, fat tissue, or the brain, where they function as the tissue’s repair system. Embryonic stem cells are derived from the inner cell mass of the blastocyst, an early-stage embryo formed shortly after fertilization.
This difference in origin impacts their use and research. Obtaining adult stem cells involves a standard medical procedure from a consenting individual and does not raise the same level of ethical consideration. Isolating embryonic stem cells involves the destruction of an early-stage embryo. This distinction has made adult stem cell research and therapy generally less controversial and more readily accepted for clinical application.
Primary Roles in the Body
The primary function of adult stem cells is to maintain tissue balance, a process known as homeostasis. They continuously replace cells lost through normal wear and tear, ensuring the health and stability of organs. This daily maintenance is evident in tissues that experience high turnover, such as the lining of the gut and the blood system.
Hematopoietic Stem Cells (HSCs) exemplify this continuous function by producing billions of new blood cells daily to replace old ones. Similarly, stem cells within the intestinal crypts ensure the rapid renewal of the epithelial lining, which faces constant damage from digestion. This constant replenishment is necessary for tissue function.
Adult stem cells also respond to injury by activating from their quiescent state to facilitate immediate repair. In the skin, epidermal stem cells proliferate and migrate to the wound site to reconstruct the protective barrier. Satellite cells, found in muscle tissue, generate new muscle fibers to repair damage following strain or trauma. These repair mechanisms are localized and specific to the surrounding tissue.
Therapeutic Applications
The established therapeutic use of adult stem cells centers on hematopoietic stem cell transplantation (HSCT), commonly known as a bone marrow transplant. This procedure uses adult stem cells, collected from the bone marrow or circulating blood, to restore the body’s ability to produce blood cells following high-dose chemotherapy or radiation. HSCT is a standard treatment for various cancers, such as leukemia and lymphoma, as well as several immune and genetic disorders.
Mesenchymal Stem Cells (MSCs) are another major category, sourced from tissues like bone marrow and fat. MSCs are the subject of numerous clinical trials for regenerative medicine. They are being investigated for conditions ranging from orthopedic injuries to autoimmune diseases, due to their ability to differentiate into bone, cartilage, and fat, and their capacity to modulate the immune system. While the use of MSCs in regenerative therapies is expanding, HSCT remains the most widely established clinical application of adult stem cells.