Understanding Pluripotent Stem Cells: A Patient-Friendly Guide to the Science, Promise, and Progress

Table of Contents

• Key Points
• Background: What Are Pluripotent Stem Cells?
• How This Review Was Conducted
• Key Milestones in Stem Cell History
• The Five Basic Categories of Stem Cells
• How Scientists Test Pluripotency
• Embryonic Stem Cells (ESCs)
• Very Small Embryonic-Like Stem Cells (VSELs)
• Nuclear Transfer Stem Cells (NTSCs) and Cloning
• Reprogrammed Stem Cells (RSCs) and Induced Pluripotent Stem Cells (iPSCs)
• A Closer Look: Urine-Derived iPSCs
• Clinical Implications and Future Applications
• Limitations and Ethical Considerations
• Additional Topics in the Original Review
• Recommendations and Hope for Patients
• Frequently Asked Questions
• Source Information

Key Points

• Pluripotent stem cells can become any of the three germ layers: ectoderm, endoderm, and mesoderm.
• Urine-derived iPSCs offer a non-invasive, simple way to create stem cells from a 30-ml sample.
• Stem cell therapies carry risks including teratoma formation, genetic instability, and possible immune rejection.
• Only nuclear transfer stem cells have generated a complete organism, such as cloned monkeys in 2018.
• Some stem cell treatments, like retinal and spinal cord transplants, are already in early clinical trials.

Background: What Are Pluripotent Stem Cells?

Pluripotent stem cells are the body’s “master cells.” They have two defining properties: self-renewal (the ability to divide and create more stem cells) and potency (the ability to mature into specialized cell types).

Specifically, these cells can differentiate into any of the three primary germ layers that form all human tissues: ectoderm (skin, brain, and nervous system), endoderm (lungs, liver, and digestive tract), and mesoderm (muscle, bone, and blood). This incredible flexibility makes PSCs a powerful tool for medicine.

How This Review Was Conducted

The article is a systematic review, meaning the authors—researchers from Rush University Medical College in Chicago—collected, analyzed, and summarized findings from numerous published scientific studies. Instead of performing one new experiment, they reviewed the existing evidence to provide a “big picture” of stem cell research. The review focuses on 15 key domains, covering everything from the sources of PSCs to current ethical controversies.

Key Milestones in Stem Cell History

The journey began more than half a century ago. In 1961, Drs. James A. Till and Ernest A. McCulloch at the University of Toronto in Canada first described stem cells. They discovered that cells from mouse bone marrow could differentiate into various cell types, and they named these pluripotent stem cells.

Several decades later, in 1996, Dolly the sheep became the first mammal cloned from an adult cell using a technique called somatic cell nuclear transfer (SCNT). This was carried out by Keith Campbell, Ian Wilmut, and colleagues at the Roslin Institute in Scotland. Dolly’s creation proved that the genetic material of a mature cell could be “reset” to an embryonic state.

In 1998, the first human embryonic stem cells (hESCs) were isolated by James Thomson in the United States. Then, in 2006, a major breakthrough occurred: Shinya Yamanaka and colleagues derived induced pluripotent stem cells (iPSCs) from adult cells by reprogramming them with just four basic transcription factors—Oct4, Sox2, Klf4, and c-Myc. This achievement was remarkable because only 24 candidate factors were originally tested, and just four were essential. In 2012, Yamanaka and John Gurdon received the Nobel Prize in Physiology or Medicine for their discovery that mature cells can be reprogrammed into a pluripotent state.

Since then, stem cell research has blossomed into an exciting and promising field, with scientists finding innate adult stem cells in several organs and developing ever more advanced technologies.

The Five Basic Categories of Stem Cells

After systematically reviewing the research, the authors propose that all stem cells can be classified into five basic categories. Each type has a unique origin, method of production, and potential use in medicine:

1. Embryonic Stem Cells (ESCs): Derived from early-stage embryos (blastocysts).
2. Very Small Embryonic-Like Stem Cells (VSELs): Tiny, primitive cells found in adult tissues.
3. Nuclear Transfer Stem Cells (NTSCs): Created by transferring a donor cell nucleus into an egg cell whose own nucleus has been removed.
4. Reprogrammed Stem Cells (RSCs): Adult cells that have been genetically “rewound” in a laboratory (includes iPSCs).
5. Adult Stem Cells (ASCs): Tissue-specific stem cells found in the body.

According to the review, only NTSCs have been used to generate a complete organism—monkeys were grown from NTSCs in China in 2018. ESCs, iPSCs, and adult stem cells have so far been used to generate tissues and organs, not whole animals.

How Scientists Test Pluripotency

To confirm that a stem cell is truly pluripotent, researchers use three main in vivo (within a living organism) tests in mice, as summarized by Aoi (2016):

• Teratoma formation assay: Stem cells are transplanted into immunocompromised mice, and scientists check whether the cells spontaneously form differentiated tissues from all three germ layers. This proves the cells can become many cell types.
• Chimera formation assay: Stem cells are injected into early-stage embryos (called 2N blastocysts). The resulting “chimeras” are then bred. If the donor cells contribute to the germline—meaning they can produce functional sperm or eggs—and maintain normal chromosomes, the cells have “functional pluripotency.”
• Tetraploid (4N) complementation assay: This is the most rigorous test. Stem cells are injected into 4N embryos (tetraploid blastocysts). If the tested cells can generate the entire organism—including the embryo itself, while the 4N embryo contributes only to extra-embryonic tissues—the cells are truly pluripotent.

These assays are essential for determining whether a new stem cell line is safe and useful for future therapies.

Embryonic Stem Cells (ESCs)

Human embryonic stem cells (hESCs) are harvested from early-stage blastocysts—about 4 to 5 days after fertilization—by destroying the source blastocyst. They can also be obtained from later-stage tissues (up to 3 months gestational age or less). hESCs were the first stem cells used in research and remain commonly used in clinical trials today, as listed on clinicaltrials.gov.

However, because their collection requires the destruction of human embryos, hESCs raise significant ethical concerns. These concerns are a major reason why scientists have worked to develop alternative sources of pluripotent cells.

Very Small Embryonic-Like Stem Cells (VSELs)

VSELs were identified in 2006 by Ratajczak and colleagues. Since then, more than 20 independent laboratories have confirmed their existence, although a few groups still question it.

As their name suggests, these cells are very small—about 3 to 5 micrometres in mice and 5 to 7 micrometres in humans (slightly smaller than red blood cells). They are found in adult tissues and share some markers with embryonic stem cells, such as SSEA, nuclear Oct-4A, Nanog, and Rex1. They also express markers typical of migrating primordial germ cells (cells that give rise to eggs or sperm), such as Stella and Fragilis.

According to Ratajczak’s 2019 model, VSELs originate from primordial germ cells (PGCs) and can differentiate into three potential fates:

• Mesenchymal stem cells (MSCs) – support tissue repair
• Hemangioblasts – which include hematopoietic stem cells (HSCs) and endothelial progenitor cells (EPCs), helping form blood and blood vessels
• Tissue-committed stem cells (TCSCs) – cells ready to become specific tissues

Because VSELs are found in adult tissues, they might avoid the ethical issues linked to embryonic stem cells, and they may also be less likely to form teratomas (tumors) than iPSCs. For these reasons, they are considered a promising alternative for future stem cell treatments.

Nuclear Transfer Stem Cells (NTSCs) and Cloning

The somatic cell nuclear transfer (SCNT) technique was first proven possible with Dolly the sheep in 1996. The process begins by taking a fully differentiated adult cell (like a fibroblast) and placing its nucleus—which contains the DNA—into an egg cell that has had its own nucleus removed. The egg then “reprograms” the donor nucleus to behave like an early embryo. After several divisions, the cell develops into a blastocyst of about 100 cells, and eventually can grow into an organism that is almost an identical clone of the original donor.

This technique can be used for two purposes:

• Reproductive cloning: creating a whole living animal.
• Therapeutic cloning: generating stem cells for medical treatments.

Since Dolly, about two dozen other species have been cloned. In January 2018, scientists in Shanghai, China, announced that they had successfully used fetal fibroblasts to clone two female macaque monkeys—the first primates ever cloned using SCNT.

Why does this matter for patients? Primate cloning could help create genetically uniform animal models that closely mimic human diseases. This would make it easier to study disease mechanisms and test new drugs, reducing the number of animals needed. It could also be combined with CRISPR-Cas9 gene editing to create primate models of human disorders such as Parkinson’s disease and various cancers. Pharmaceutical companies have already shown high interest in cloned monkeys for drug testing, and the city of Shanghai is funding an International Primate Research Center to produce these animals for use worldwide.

One special advantage of NTSCs is that they can generate a complete living body, whereas ESCs and iPSCs are typically used to grow sheets of cells, tissues, or pieces of organs. From a biophysiological viewpoint, this gives SCNT a unique edge in basic research and clinical applications.

Reprogrammed Stem Cells (RSCs) and Induced Pluripotent Stem Cells (iPSCs)

Since Yamanaka’s landmark discovery in 2006, reprogramming technologies have advanced dramatically. Researchers can now directly convert one adult cell type into another using lineage-restricted transcription factors (proteins that turn on specific genes), RNA signal modifications, and small molecules or chemicals. This is called “direct reprogramming.”

Direct reprogramming can bypass the iPSC stage entirely, producing cells that are already close to the target cell type. One example is the creation of induced neural progenitor cells (iNPCs), which can then become neurons and motor neurons. These cells are formed by expressing transcription factors important for neural development, offering a faster and potentially safer route to making cells for treating spinal cord injuries or neurological diseases.

Reprogrammed stem cells (RSCs) include all stem cells created by manual laboratory methods that “rewrite” the genetic signals of primary cells—except for SCNT, which is classified separately.

A Closer Look: Urine-Derived iPSCs

One of the most patient-friendly advances is the ability to create iPSCs from urine. This method was first described by Zhou and colleagues in July 2011, with a more detailed protocol published a year later. It has several major advantages:

• Non-invasive: No needles or surgery are needed.
• Simple and fast: The entire process takes about 2 weeks of cell culturing followed by 3–4 weeks of reprogramming.
• Cheap and universal: It works with just a 30-ml sample of urine and is suitable for patients of all ages, genders, and racial/ethnic backgrounds.
• High yield and excellent differentiation potential: In a study using 200 ml of clean midstream urine and the Sendai virus delivery system, urine-derived iPSCs showed a normal karyotype (normal chromosome structure) and the ability to differentiate into all three germ layers in a teratoma assay.

Urine also contains a subpopulation of cells with progenitor features, as reported by Zhang and colleagues. These cells express markers such as c-Kit, SSEA4, CD105, CD73, CD91, CD133, and CD44, which can help distinguish among bladder cell lineages (urothelial, smooth muscle, endothelial, and interstitial cells). This means urine could serve as a valuable and easy-to-access source of cells for building bladder tissue or other urological therapies.

Clinical Implications and Future Applications

Stem cells—especially ESCs and iPSCs—have enormous potential in four major fields:

1. Regenerative and transplant medicine: Replacing damaged tissues or organs, such as heart muscle after a heart attack, retinal cells in macular degeneration, or neural cells in spinal cord injury.
2. Disease modeling: Creating cells in a dish that carry the genetic mutations of a patient, allowing scientists to study diseases like Parkinson’s or cancer more accurately.
3. Drug discovery and screening: Testing new medications on patient-specific cells to identify which drugs are most effective and safe before human trials.
4. Human developmental biology: Understanding how a single fertilized egg develops into a complex organism, which can reveal the causes of birth defects and miscarriages.

The review emphasizes that these applications are already moving from “fundamental research” to “pre-clinical research” and, in some cases, into early clinical trials—such as retinal cell transplants and spinal cord transplants.

Limitations and Ethical Considerations

Despite the immense promise, several challenges remain:

• Teratoma risk: Both ESCs and iPSCs can form teratomas (tumors) if undifferentiated cells remain after transplantation. This is a major safety barrier that must be overcome before widespread clinical use.
• Genetic instability: iPSCs can acquire genetic mutations during reprogramming or long-term culture. Maintaining genomic stability is critical for patient safety.
• Immune rejection: Even with patient-derived iPSCs, there may be subtle immune differences. However, because iPSCs can be harvested from the patient themselves, the risk of rejection is greatly reduced compared to donor cells.
• Ethical concerns: The use of human embryos to obtain ESCs remains ethically controversial. Cloning primates also raises serious ethical questions about animal welfare and the potential future application of cloning to humans. The review stresses that ethical discussions must keep pace with scientific advances.
• Scientific skepticism: As with VSELs, some findings in stem cell research are still debated. Replication and independent confirmation are essential.

Additional Topics in the Original Review

The full review also covers several advanced topics that were highlighted in the abstract but not detailed in the summary text provided here. These include:

• Next-generation in vivo direct reprogramming: The concept of directly converting cells inside the body rather than in a laboratory dish.
• Cell types derived from PSCs and genetic memory: How differentiated cells retain “memory” of their original state, which can affect their behavior.
• Induction of pluripotency with genomic modifications: Using genetic engineering to help drive cells back to a pluripotent state.
• Construction of vectors with reprogramming factor combinations: Building safer and more efficient delivery systems (like viruses or episomes) to carry the Yamanaka factors.
• Enhancing pluripotency with small molecules and genetic signaling pathways: Using drugs or chemicals that affect pathways like ERK, MAPK, MEK, GSK3, BMP, TGF, and FGF to improve reprogramming.
• Induction of cell reprogramming by RNA signaling: Using microRNAs or other RNA molecules to trigger reprogramming without altering the DNA itself.
• Induction and enhancement of pluripotency with chemicals: Identifying chemical cocktails that can replace genetic factors entirely.
• Maintenance of pluripotency and genomic stability: Culturing iPSCs in ways that keep them stable and healthy.
• Feeder-free and xeno-free culture environments: Growing stem cells without mouse cells (feeders) or animal products, which is necessary for producing cells suitable for human transplantation.
• Biomaterial applications: Using scaffolds and other materials to support stem cell growth and organization.
• Three-dimensional (3D) cell technology and 3D bioprinting: Growing cells in 3D structures and using printers to create complex tissues layer by layer.
• Current ethical issues: Ongoing debates about embryo use, cloning, and how to responsibly translate stem cell science into medicine.

Recommendations and Hope for Patients

For patients and families interested in stem cell therapies, this review offers several key takeaways:

• Ask about clinical trials: Some stem cell-based treatments, such as retinal cell transplants and spinal cord transplants, are already in early clinical trials. Always ask your doctor about legitimate, approved options.
• Be wary of unproven clinics: The review notes that while the field is exciting, many products are still in the experimental stage. Seek information from reputable medical centers and official registries like clinicaltrials.gov.
• Consider donating biological samples: Simple samples like blood or urine can be used to create iPSCs for research. Participating in research biobanks may help accelerate discoveries.
• Stay informed: Stem cell science is moving quickly. Follow updates from peer-reviewed journals and patient advocacy groups.

The authors of the review believe that with continued research, stem cell technologies will eventually offer new treatments for conditions that currently have no cure. The path forward requires collaboration between scientists, clinicians, regulators, and patients—and a commitment to both innovation and ethical responsibility.

Frequently Asked Questions

What are pluripotent stem cells?

Pluripotent stem cells are the body's master cells. They can self-renew, meaning they divide to create more stem cells, and they have potency, meaning they can mature into many specialized cell types. Specifically, they can become any of the three primary germ layers that form all human tissues: ectoderm, endoderm, and mesoderm.

How are induced pluripotent stem cells (iPSCs) made from urine?

Urine-derived iPSCs are created from a small urine sample, typically 30 ml. The process is non-invasive, simple, and fast: about 2 weeks of cell culturing followed by 3 to 4 weeks of reprogramming. This method works for patients of all ages, genders, and backgrounds, and the cells can differentiate into all three germ layers.

What is the difference between embryonic stem cells and induced pluripotent stem cells?

Embryonic stem cells (ESCs) come from early-stage blastocysts, about 4 to 5 days after fertilization, and their collection destroys the embryo. Induced pluripotent stem cells (iPSCs) are adult cells reprogrammed in a laboratory, for example with four transcription factors. iPSCs avoid the need to destroy embryos and can be made from a patient's own cells.

What are the risks of stem cell treatments?

Both embryonic and induced pluripotent stem cells can form teratomas, which are tumors, if undifferentiated cells remain after transplantation. Also, iPSCs may acquire genetic mutations during reprogramming or long-term culture. There can also be subtle immune rejection, though using patient-derived iPSCs greatly reduces this risk. Many treatments are still experimental.

Are stem cell therapies available for patients now?

Some stem cell-based treatments, such as retinal cell transplants and spinal cord transplants, are already in early clinical trials. Always ask your doctor about legitimate, approved options and seek information from reputable medical centers or official registries like clinicaltrials.gov. Be wary of unproven clinics, as many products are still experimental.

What is somatic cell nuclear transfer (SCNT) and why does it matter?

Somatic cell nuclear transfer is a technique where the nucleus of an adult cell is placed into an egg cell that has had its own nucleus removed. The egg reprograms the donor nucleus to act like an embryo. It was first proven with Dolly the sheep in 1996 and can be used for reproductive cloning or therapeutic cloning.

How do scientists test if a stem cell is truly pluripotent?

Scientists use three main tests in mice. The teratoma formation assay checks if transplanted cells form tissues from all three germ layers. The chimera formation assay tests if cells contribute to the germline in early embryos. The tetraploid complementation assay is the most rigorous: if tested cells generate the entire organism, they are truly pluripotent.

Source Information

This patient-friendly article is based on the following peer-reviewed scientific paper:

Original title: Advances in Pluripotent Stem Cells: History, Mechanisms, Technologies, and Applications

Authors: Gele Liu, Brian T. David, Matthew Trawczynski, Richard G. Fessler

Journal: Stem Cell Reviews and Reports (2020) 16:3–32

Published online: 23 November 2019

DOI: https://doi.org/10.1007/s12015-019-09935-x

Affiliation: Department of Neurosurgery, Rush University Medical College, Chicago, IL, USA

Note: This patient-friendly article is based on peer-reviewed research and includes direct references to data and findings from the original publication. For full details, please consult the original article.