Stages of Stem Cell Differentiation Potential (Totipotent → Pluripotent → Multipotent)
Let’s summarize the stages of stem cells as seen in embryonic development. Up to the early cleavage stage—around the 8-cell or 16-cell stage after fertilization—the embryo consists of totipotent stem cells that have the potential to generate an entire organism. As the embryo develops into the blastocyst stage, its cells divide into an inner cell mass (a cluster of inner cells) and an outer layer (the balloon-like trophoblast). Only the inner cell mass has the potential to differentiate into any of the three germ layers in the future, and thus the stem cells at this stage are called pluripotent embryonic stem cells.
These embryonic stem cells, which are cultured in the laboratory from surplus embryos donated from in vitro fertilization procedures, are the main subjects of research at this stage. Because of ethical issues, however, such research is strictly regulated, and this discussion will not cover that topic further.
After differentiation progresses into the three germ layers, the developmental fate of the cells becomes increasingly determined, since the differentiation paths of ectodermal cells differ from those of the other germ layers. Within each germ layer, the specific location of a cell and the signals it receives determine which tissue it will differentiate into. Cells in this stage, which are committed to a particular tissue lineage, are classified as multipotent stem cells.
Adult Stem Cells
Having examined the stem cells that appear during embryonic development, let’s now ask: What are mesenchymal stem cells (MSCs)—which can differentiate into osteoblasts—and hematopoietic stem cells (HSCs), which give rise to blood cells? They both belong to the category of adult stem cells, which are found not in embryos but in mature organisms. Here, “adult” refers to a fully developed organism in which all organs have already differentiated. Adult stem cells vary widely. Depending on whether they can differentiate into multiple lineages (multipotent) or only into a single lineage with self-renewal capacity (unipotent), they can be classified as shown in the table below.
Properties and Division Patterns of Stem Cells
The most important characteristic of stem cells is their ability to self-renew. Through this self-replicating ability, they can maintain their undifferentiated state while also producing cells that can differentiate into specific types. If a stem cell were to differentiate every time it divides, the total number of stem cells would decrease, and it would not be possible to maintain a stable stem cell pool. However, because stem cells can self-renew, they can generate identical copies of themselves to maintain this pool while still producing progenitor cells that will differentiate into somatic cells as needed. In other words, they replicate themselves—keeping an “original copy”—while also producing a progenitor cell that can further differentiate into various types of body cells. Let’s examine how stem cell division occurs.
Asymmetric Division vs. Symmetric Division
When a stem cell divides, the two resulting daughter cells can arise in two different ways. Depending on whether the purpose is to rapidly expand the number of stem cells or to produce differentiated cells, the mode of division changes. The most common division pattern is one that produces one stem cell and one progenitor cell. This allows the maintenance of the stem cell pool while supplying the necessary somatic cells for tissue needs. This type of division is called asymmetric division.
Another mode of division produces two identical daughter cells. If a tissue is damaged and needs rapid regeneration or if the organism is in a growth phase that requires massive cell proliferation, stem cells divide into two stem cells to expand their population. Conversely, when there are enough stem cells and the tissue is stable, the cell may divide into two progenitor cells to replenish differentiated cells only. These progenitor cells then mature into various specialized cells depending on the needs of the tissue. This process—where two identical daughter cells are formed—is called symmetric division.(Perhaps the term sounds a bit too grandiose...)
Adult Stem Cells
Now, let’s explore adult stem cells that continue to generate bone and blood even after development. When the three germ layers are formed early in embryonic development, they become the foundation for nearly all tissues and organs of the human body. Stem cells with self-renewal ability show remarkable potential for diverse differentiation at this stage. However, even after this special developmental period ends, undifferentiated stem cells from the three germ layers remain and continue to regenerate cells, rebuilding and maintaining tissues. These are called adult stem cells. They reside in specialized microenvironments called niches, where they stay in a dormant, quiescent state. When they receive activation signals—such as external stimuli—they self-renew and differentiate into specific cell types required by the situation, replacing damaged cells and maintaining tissue and organ homeostasis so that the body functions properly. However, because adult stem cells are remnants of tissue-specific stem cells that did not fully differentiate during embryonic development, they do not retain the full range of functions of embryonic stem cells. Therefore, their differentiation capacity is much narrower and limited.
Among adult stem cells, mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) have been the most extensively studied. HSCs, first identified in bone marrow transplants, are the best-established and most clinically used stem cells today, obtained from bone marrow or umbilical cord blood. MSCs, on the other hand, are found in many tissues—bone marrow, fat, umbilical cord blood, dental pulp, perivascular cells, and more. They play crucial roles in tissue repair due to their strong immunosuppressive and immunomodulatory properties, and animal experiments have shown that they can differentiate into bone and cartilage, making them key targets in regenerative medicine and clinical trials.
Both stem cell types originate from the mesoderm during embryonic development. HSCs derive from the lateral plate mesoderm, which gives rise to the blood and vascular systems, while MSCs originate from the somitic, intermediate, and lateral mesoderm regions. The relationship between MSCs and HSCs is particularly special: MSCs provide the supportive niche environment required for HSC survival. Before we look more closely at hematopoietic stem cells, I’d like to take a detailed and careful look at mesenchymal stem cells.
Epithelial Cells vs. Mesenchymal Cells
First, what is “mesenchymal”? To understand the mesenchyme, it’s helpful to understand epithelial cells as well. Epithelial cells cover the surfaces of the body and line the inner cavities of hollow organs (skin, breast, lung, colon, prostate), providing protection and performing absorption or secretion functions. Depending on shape, epithelial cells are classified as squamous (flat), cuboidal, or columnar. They can form simple (single-layered) or stratified (multi-layered) structures. Some appear multi-layered despite being single-layered, known as pseudostratified epithelium. Regardless of shape or layer structure, epithelial cells share key characteristics: they possess apical-basal polarity, form strong cell–cell junctions, have little extracellular matrix (ECM) between them, and exhibit limited mobility. The surfaces of our organs and body are made of sheets of epithelial cells. For example, in the intestinal epithelium, the apical surface faces the intestinal lumen while the basolateral surface contacts the basement membrane leading to blood and lymph vessels. Thus, the upper side absorbs nutrients, and the lower side transfers them to the bloodstream—this is cell polarity. Intestinal epithelial cells are tightly connected to their neighbors, forming a barrier that strictly separates the intestinal lumen from the bloodstream, allowing only selective transport of nutrients. Their tight junctions act as a physical barrier against bacteria and toxins, serving as an important immune defense mechanism. When these junctions are weakened or damaged, gaps form through which intestinal bacteria, endotoxins, and partially digested proteins can enter the bloodstream. The immune system perceives these as “threats” and launches an attack, leading to chronic inflammation and autoimmune responses. This phenomenon is known as Leaky Gut Syndrome, which can have serious consequences.
So, what are Mesenchymal Cells? The term refers to cells or tissues characterized by a loosely organized structure involving cytoskeletal and ECM remodeling. Morphologically, mesenchymal cells are spindle-shaped, elongated, and irregular. They have front–rear polarity (not top–bottom like epithelial cells), form loose intercellular contacts, and are highly motile. The contrast between epithelial and mesenchymal cell types is thus quite distinct.
Epithelial–Mesenchymal Transition (EMT) vs. Mesenchymal–Epithelial Transition (MET)
To understand “mesenchyme,” we must briefly revisit early embryonic development discussed in the previous article. During the transformation of the bilaminar disc(epiblast and hypoblast) into the trilaminar structure(ectoderm, mesoderm, endoderm), the primitive streak forms. Cells of the epiblast migrate inward through this streak—this “mass migration” of cells marks the formation of mesoderm and endoderm. How do tightly connected epithelial cells of the epiblast acquire this mobility? By transforming into mesenchymal cells. This process, where epithelial cells lose their polarity and adhesion and acquire migratory, invasive properties, is called the Epithelial–Mesenchymal Transition (EMT). Conversely, when dispersed mesenchymal-like cells reaggregate and form epithelial layers again, it is called the Mesenchymal–Epithelial Transition (MET). Importantly, EMT is also a key mechanism in cancer metastasis, as tumor cells detach and invade blood or lymphatic vessels through this process.
Epithelial–Mesenchymal Transition (EMT), in which epithelial cells—originally attached in layers—change their shape and characteristics to become migratory and invasive mesenchymal cells, is triggered by multiple signals. Among them, TGF-β (Transforming Growth Factor-β) plays a central role. Other inducers include HGF (Hepatocyte Growth Factor), which promotes cell migration and tissue regeneration; EGF(Epidermal Growth Factor), which stimulates cell proliferation; and Wnt signaling, which regulates cell fate determination. Additionally, hypoxia (through activation of HIF-1α), ECM components such as Type I collagen, and various inflammatory cytokines (e.g., TNF-α, IL-6) can also promote EMT.[1]
These signals activate transcription factors such as SNAIL, SLUG, TWIST, and ZEB. These factors suppress expression of E-cadherin (an adhesion protein that tightly binds epithelial cells) while upregulating N-cadherin (favorable for mobility), vimentin (a cytoskeletal protein), and fibronectin (an ECM protein). As a result, cells lose their original polarity, weaken their cell–cell adhesion, reorganize their cytoskeleton, and begin producing ECM themselves, achieving a freely migratory state. This process plays an essential role during development (e.g., germ layer formation, organogenesis) and in adults during wound healing. However, excessive or pathological EMT can lead to fibrosis (scarring and organ dysfunction) or be exploited by cancer cells during metastasis, making it a key subject of biomedical research.
After breaking adhesion and remodeling their cytoskeleton, migrating mesenchymal cells replace the hypoblast to form the endoderm and quickly re-epithelialize to create epithelial sheets, which then differentiate into organs such as the digestive tract, liver, and pancreas. In contrast, mesodermal cells retain their mesenchymal properties for longer periods, migrating widely and differentiating into various connective tissues, blood vessels, and muscles.
Mesenchyme, as seen in embryonic development, refers to a loosely connected population of cells surrounded by abundant ECM—similar to connective tissue. Indeed, many connective tissues arise from the mesenchyme, making it a collective term for cells of connective-tissue origin. So, what are mesenchymal stem cells (MSCs)? Even after embryogenesis, multipotent stem cells capable of differentiating into osteocytes, chondrocytes, adipocytes, and certain muscle cells are found in adult tissues such as bone marrow, umbilical cord blood, teeth, and perivascular regions. These are MSCs. They are thought to be residual, tissue-specific stem cells that did not fully differentiate during embryonic development and persisted into adulthood—thus their origins trace back to the embryonic period. However, since they do not retain the full functional spectrum of embryonic mesodermal stem cells, their differentiation potential is limited to a narrower range of tissues. MSCs exist as loosely scattered undifferentiated cells within connective tissue, characterized by abundant intercellular matrix and space. They differentiate into cartilage, bone, fat, vascular endothelium, vascular smooth muscle, and lymphoid tissues (lymph nodes, spleen, tonsils, thymic stroma, lymphatic vessels, etc.), supporting normal proliferation and enabling rapid repair and regeneration when these tissues are damaged—serving as a reservoir of precursor cells for maintenance and healing.
[Reference]
[1] The epithelial-mesenchymal transition and the cytoskeleton in bioengineered systems
https://doi.org/10.1186/s12964-021-00713-2
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