The Secret Life of Stem Cells

Stem cells have the unique ability to renew themselves and develop into various cell types during growth and development. Researchers study different types of stem cells, mainly divided into pluripotent stem cells (embryonic and induced pluripotent stem cells) and non-embryonic or somatic stem cells (commonly called adult stem cells). Pluripotent stem cells can differentiate into any cell type in the adult body, while adult stem cells are found within specific tissues or organs and can give rise to specialized cells of that particular tissue.

Pluripotent Stem Cells

Early mammalian embryos at the blastocyst stage consist of two main cell types: the inner cell mass and the trophectoderm. The trophectoderm forms the placenta, while the inner cell mass develops into all the specialized cells, tissues, and organs of the body. In 1998, researchers developed a method to isolate stem cells from the inner cell mass of preimplantation human embryos, leading to the creation of human embryonic stem cells (hESCs) grown in the lab. Later, in 2006, scientists discovered how to reprogram mature adult human cells back into an embryonic-like state, producing induced pluripotent stem cells (iPSCs).

Adult Stem Cells

Throughout an organism’s life, adult stem cells act as an internal repair system, replacing cells lost to normal wear, injury, or disease. These stem cells have been found in many tissues and organs and are usually located in specific anatomical niches. Often, adult stem cells remain inactive (quiescent) for long periods but can be activated when new cells are needed to maintain or repair tissues.

Stem cells possess two unique properties: the ability to self-renew and to generate functional tissues. Unlike mature cells such as muscle, blood, or nerve cells, which rarely divide, stem cells can replicate many times. When a stem cell divides, it can produce two stem cells, one stem cell and one specialized cell, or two specialized cells. The mechanisms that regulate this balance to maintain the appropriate number of stem cells in a tissue are not yet fully understood. Unlocking the secrets of self-renewal could shed light on how cell fate decisions are controlled during embryonic development, throughout life, and in diseases like cancer. This knowledge also helps researchers culture stem cells more effectively in the lab. Of particular interest are the factors and conditions that keep pluripotent stem cells undifferentiated, a challenge that has required years of research. Pluripotent stem cells themselves are undifferentiated and lack tissue-specific traits but can give rise to all cell types in the body, including heart muscle, blood, and nerve cells. In contrast, adult stem cells are more specialized, generating only the cell types of their resident tissue or organ, often showing unique morphology and gene expression patterns. Stem cells vary in potency—the range of cell types they can form—and usually pass through several stages of increasing specialization during differentiation. Scientists are actively studying the signals that trigger each step of this process, which include molecules secreted by neighboring cells, physical contact, and environmental factors within the stem cell niche.

Mesenchymal stem cells (MSCs), also called mesenchymal stromal or medicinal signaling cells, are multipotent cells found in various adult tissues such as muscle, liver, bone marrow, and adipose tissue. MSCs primarily provide structural support in these organs and help regulate the movement of substances within them. They can differentiate into multiple cell types, including adipocytes (fat cells), osteocytes (bone cells), and chondrocytes (cartilage cells), all derived from the mesodermal layer—the middle layer of embryonic tissue responsible for forming skeletal elements like bone and cartilage. The term “mesenchymal” comes from the Greek “meso,” meaning middle, reflecting their origin between the ectoderm and endoderm layers during early embryonic development. This mobility allows MSCs to fill spaces and play a crucial role in wound repair in adult tissues such as skin, bone, and muscle.

MSCs are vital to regenerative medicine and are widely studied in clinical trials due to their ease of isolation, high yield, and remarkable plasticity. They facilitate inflammation control, promote cell growth and differentiation, and aid tissue repair through immunomodulation and immunosuppression. Bone marrow is a common source of MSCs, but isolating high-quality cells can be challenging and varies with donor age. Interestingly, MSCs are more abundant in the bone marrow stroma than in aspirates. These cells are heterogeneous and express high levels of pluripotency markers compared to other stem cell types, like embryonic stem cells. MSC injections promote wound healing mainly by stimulating new blood vessel formation (angiogenesis).