Stem Cells Explained: The Body Renewable Resource

What Makes a Cell a Stem Cell

Two properties distinguish stem cells from the roughly 200 other cell types in the human body. The first is self-renewal, the ability to undergo cell division and produce daughter cells that remain stem cells rather than differentiating. This allows stem cell populations to be maintained over the lifetime of an organism. The second is potency, the ability to differentiate into one or more specialized cell types. The degree of potency varies among different stem cell types and is a key factor in their biological role and medical potential.

When a stem cell divides, it can do so symmetrically, producing two identical stem cells, or asymmetrically, producing one stem cell and one cell that begins the path toward differentiation. Asymmetric division is the more common mode in adult tissues, as it simultaneously maintains the stem cell pool and generates the progenitor cells needed for tissue replenishment. The molecular mechanisms that control symmetric versus asymmetric division involve the unequal distribution of cell fate determinants, proteins and mRNA molecules that are partitioned to one daughter cell but not the other during division.

Types of Stem Cells by Potency

Totipotent stem cells have the highest differentiation potential: they can give rise to every cell type in the organism, including the extraembryonic tissues (placenta and umbilical cord). Only the zygote (fertilized egg) and the cells of the very early embryo (up to about the 8-cell stage in humans) are truly totipotent. After this point, the cells begin to specialize and lose their ability to form all tissue types.

Pluripotent stem cells can differentiate into any cell type of the body proper but cannot form extraembryonic tissues. Embryonic stem cells (ESCs), derived from the inner cell mass of the blastocyst (a roughly 5-day-old embryo), are the best-known pluripotent cells. When cultured under appropriate conditions, ESCs can be maintained indefinitely in the laboratory while retaining their ability to differentiate into cell types from all three embryonic germ layers: ectoderm (skin, nervous system), mesoderm (muscle, bone, blood), and endoderm (gut lining, lungs, liver).

Multipotent stem cells can differentiate into a limited range of cell types, typically within a single tissue or organ system. Hematopoietic stem cells (HSCs) in the bone marrow are multipotent: they give rise to all blood cell types, including red blood cells, white blood cells, and platelets, but they cannot produce nerve cells, muscle cells, or other non-blood cell types. Neural stem cells in certain brain regions can produce neurons, astrocytes, and oligodendrocytes but not blood cells or skin cells.

Unipotent stem cells, or progenitor cells, can produce only one cell type but retain the ability to self-renew. Satellite cells in skeletal muscle, for example, can only generate new muscle fibers, but they persist throughout life and are activated in response to muscle injury, enabling tissue repair.

Adult Stem Cells

Adult stem cells (also called somatic stem cells) reside in specific locations within tissues called niches, microenvironments that provide the signals needed to maintain stem cell identity and regulate the balance between self-renewal and differentiation. The bone marrow niche houses hematopoietic stem cells and mesenchymal stem cells. The intestinal crypt niche contains the stem cells that regenerate the gut lining every 3 to 5 days. The hair follicle bulge region contains epithelial stem cells that drive hair growth and can contribute to skin repair after injury.

Hematopoietic stem cells are the best-characterized adult stem cells and the basis of bone marrow transplantation, a therapy that has been used since the 1960s to treat blood cancers (leukemias and lymphomas), bone marrow failure, and certain genetic blood disorders like sickle cell disease. In a transplant, the patient diseased bone marrow is destroyed by chemotherapy or radiation, then replaced with healthy HSCs from a matched donor or, in some cases, from the patient own previously collected and stored cells.

Mesenchymal stem cells (MSCs) are multipotent cells found in bone marrow, adipose tissue, umbilical cord blood, and several other tissues. They can differentiate into osteoblasts (bone cells), chondrocytes (cartilage cells), and adipocytes (fat cells). MSCs have generated considerable clinical interest not only for their differentiation potential but also for their immunomodulatory properties: they secrete factors that suppress inflammation and promote tissue repair, making them candidates for treating conditions ranging from graft-versus-host disease to osteoarthritis.

Induced Pluripotent Stem Cells

In 2006, Shinya Yamanaka and his colleague Kazutoshi Takahashi at Kyoto University demonstrated that ordinary adult cells could be reprogrammed into a pluripotent state by introducing just four transcription factors: Oct4, Sox2, Klf4, and c-Myc (collectively known as the Yamanaka factors). The resulting cells, called induced pluripotent stem cells (iPSCs), behave remarkably like embryonic stem cells, capable of self-renewal and differentiation into virtually any cell type. This discovery earned Yamanaka the Nobel Prize in Physiology or Medicine in 2012.

iPSCs have several advantages over embryonic stem cells for research and therapy. Because they can be generated from a patient own cells, they are genetically matched to that patient, potentially eliminating the immune rejection problems associated with using donor cells. They also avoid the ethical concerns surrounding the destruction of human embryos that is necessary to derive embryonic stem cells. iPSC technology has been used to create patient-specific cell models of diseases including Parkinson disease, amyotrophic lateral sclerosis, and cardiac arrhythmias, enabling researchers to study disease mechanisms in the relevant cell types and screen potential drugs in the laboratory.

Challenges remain before iPSC-based therapies become routine. The reprogramming process is inefficient, typically converting fewer than 1 percent of treated cells. The original Yamanaka method used retroviruses to deliver the reprogramming factors, raising concerns about insertional mutagenesis and the oncogenic potential of c-Myc. Newer methods using non-integrating vectors, mRNA transfection, and small molecule cocktails have addressed some of these safety concerns. Additionally, iPSCs can accumulate genetic and epigenetic abnormalities during reprogramming and subsequent culture, requiring rigorous quality control before clinical use.

Stem Cells in Medicine

Beyond bone marrow transplantation, stem cell therapies are advancing through clinical trials for a variety of conditions. Limbal stem cell transplantation has been approved in Europe and Japan for treating corneal blindness caused by chemical burns or other damage to the limbal stem cells that normally regenerate the corneal epithelium. In Japan, iPSC-derived retinal pigment epithelium cells have been transplanted into patients with age-related macular degeneration in early-phase clinical trials.

Organoids, three-dimensional structures grown from stem cells that recapitulate the architecture and function of real organs, represent another frontier in stem cell application. Researchers have grown organoids resembling the brain, intestine, kidney, liver, and lung from both ESCs and iPSCs. While not suitable for transplantation in their current form, organoids provide powerful models for studying development, modeling disease, and screening drugs in a context that more closely mimics real organ biology than traditional two-dimensional cell cultures.

The concept of in vivo reprogramming, converting cells directly within the body without removing and culturing them, is an emerging area of research. Scientists have demonstrated that certain cell types can be directly converted into other cell types by introducing specific transcription factors in living animals. For example, researchers have converted cardiac fibroblasts (scar-forming cells in the heart) into functional cardiomyocytes (heart muscle cells) in mice after a heart attack, reducing scar tissue and improving heart function. If translated to humans, such approaches could offer therapeutic benefits without the complexity of cell transplantation.