This comprehensive review highlights major breakthroughs in stem cell research over the past two decades, showing how scientists can now reprogram adult cells into pluripotent stem cells capable of becoming any cell type in the body. The article covers five main stem cell types—embryonic, very small embryonic-like, nuclear transfer, reprogrammed, and adult stem cells—each with unique sources and clinical potential. Key advances include improved reprogramming methods using viruses, RNA, and chemicals; better culture systems avoiding animal products; and emerging 3D bioprinting technologies that could eventually generate transplantable tissues and organs.
Recent Breakthroughs in Stem Cell Research: From Lab Discoveries to Patient Applications
Table of Contents
- Introduction
- Sources of Pluripotent Stem Cells
- Embryonic Stem Cells (ESCs)
- Very Small Embryonic-Like Stem Cells (VSELs)
- Nuclear Transfer Stem Cells (NTSCs)
- Reprogrammed Stem Cells (RSCs)
- Adult Stem Cells
- Clinical Applications and Future Directions
- Ethical Considerations
- Source Information
Introduction
Stem cell research has undergone revolutionary changes over the past 20 years, with particularly rapid advancements in the last decade. This field began in 1961 when Canadian researchers Drs. James A. Till and Ernest A. McCulloch first discovered stem cells in mouse bone marrow that could differentiate into various cell types, establishing the concept of pluripotent stem cells (PSCs)—cells capable of becoming any cell type in the body.
The field achieved several critical milestones: Dolly the sheep was cloned in 1996 using somatic cell nuclear transfer (SCNT), the first human embryonic stem cells (hESCs) were isolated in 1998, and induced pluripotent stem cells (iPSCs) were created in 2006 by reprogramming adult cells with just four transcription factors. The importance of these discoveries was recognized when Shinya Yamanaka and John Gurdon received the 2012 Nobel Prize for their work showing that mature cells could be reprogrammed to a pluripotent state.
Researchers have identified five main categories of stem cells through systematic review: embryonic stem cells (ESCs), very small embryonic-like stem cells (VSELs), nuclear transfer stem cells (NTSCs), reprogrammed stem cells (RSCs), and adult stem cells. Each type offers unique advantages and challenges for clinical applications. Only NTSCs have been used to generate a complete organism (monkeys in China, 2018), while other types have been used to generate tissues and organs.
Stem cells, particularly ESCs and iPSCs, show tremendous promise in four major areas: regenerative and transplant medicine, disease modeling, drug discovery screening, and human developmental biology. The field continues to evolve from initial discoveries to expanding clinical applications, though challenges remain—especially regarding cell proliferation and differentiation control as iPSC reprogramming technology is still relatively new.
Sources of Pluripotent Stem Cells
Pluripotent stem cells (PSCs) are characterized by two essential properties: self-renewal (ability to proliferate) and potency (ability to differentiate into specialized cell types derived from one of three primary germ layers: ectoderm, endoderm, or mesoderm). Researchers use three main assays to test pluripotency in mouse models.
The teratoma formation assay evaluates spontaneous generation of differentiated tissues from all three germ layers after transplanting cells into immunocompromised mice. The chimera formation assay tests whether stem cells contribute to development by injecting them into early embryos (2N blastocysts) and checking if donor cells have germline transmission capacity, generate functional gametes, and retain chromosomal integrity. The tetraploid (4N) complementation assay determines the capacity of pluripotent cells within an entire organism by injecting cells into 4N embryos and monitoring growth stages for extra-embryonic lineages resulting from the transplanted stem cells rather than the embryo itself.
Embryonic Stem Cells (ESCs)
Human embryonic stem cells (hESCs) are harvested from early-stage blastocysts (4-5 days post-fertilization) either by destroying the source blastocyst or by harvesting later stage tissues (up to 3 months gestational age). These were the first stem cells applied in research applications and remain commonly used in clinical trials today (as tracked on clinicaltrials.gov).
hESCs represent the gold standard for pluripotency but come with ethical concerns regarding embryo destruction and potential immune rejection issues when transplanted into patients. Despite these challenges, they continue to provide valuable insights into developmental biology and serve as important comparators for newer stem cell technologies.
Very Small Embryonic-Like Stem Cells (VSELs)
A novel type of pluripotent stem cell called Very Small Embryonic-Like Stem Cells (VSELs) has shown promise since their identification in 2006. Over 20 independent laboratories have confirmed their existence, though some groups have questioned their validity. These cells are small, early development stem cells found in adult tissues that express pluripotency markers.
VSELs measure approximately 3-5 micrometers in mice and 5-7 micrometers in humans (slightly smaller than red blood cells). They express ESC markers including SSEA, nuclear Oct-4A, Nanog, and Rex1, along with markers for migrating primordial germ cells such as Stella and Fragilis. Their developmental origin may be associated with germline deposits in developing organs during embryogenesis.
According to a 2019 proposed model, VSELs originate from primordial germ cells and differentiate into three potential fates: mesenchymal stem cells (MSCs), hemangioblasts (including hematopoietic stem cells and endothelial progenitor cells), and tissue-committed stem cells. As pluripotent stem cells, VSELs may hold the advantage of being able to differentiate across germ layers in adult animals or humans, potentially functioning as an alternative to monopotent tissue-committed stem cells in adults.
VSELs may overcome several problems associated with other stem cell types: the ethical controversies of ESCs and the teratoma (tumor) formation risk of iPSCs. This makes them particularly promising for future stem cell studies and clinical applications where these concerns present significant barriers.
Nuclear Transfer Stem Cells (NTSCs)
Originally discovered in 1996, the somatic cell nuclear transfer (SCNT) technique has gradually evolved to generate nuclear transfer stem cells (NTSCs). This process begins by implanting a donor nucleus from a fully differentiated somatic cell (like a fibroblast) into an enucleated oocyte (egg cell with nucleus removed).
The new host egg cell then triggers genetic reprogramming of the donor nucleus. Numerous mitotic divisions of this single cell in culture develop a blastocyst (about 100 cells at early-stage embryo), ultimately generating an organism with almost identical DNA to the original organism—a clone of the nuclear donor. The process can produce both therapeutic and reproductive cloning.
Dolly the Sheep (1996) was the first successful reproductive clone of a mammal. Since then, approximately two dozen other species have been cloned. In January 2018, Chinese scientists in Shanghai announced successfully cloning two female macaque monkeys using fetal fibroblasts via SCNT—the first primates cloned by this method.
Creating cloned primates could revolutionize human disease research. Genetically uniform non-human primates could serve as valuable animal models for primate biology and biomedical research, helping investigate disease mechanisms and drug targets while reducing genetic variation confounders and the number of laboratory animals needed. This technology could combine with CRISPR-Cas9 genomic editing to create genetically engineered primate models of human disorders like Parkinson's disease and various cancers.
Pharmaceutical companies have expressed high demand for cloned monkeys for drug testing. enthused by this prospect, Shanghai has prioritized funding for establishing an International Primate Research Center to produce cloned research animals for international use. SCNT is unique among stem cell approaches as it can generate an entire living body rather than just sheets of cells, tissues, or organ pieces, giving it biophysiological functional advantages over ESCs and iPSCs for both basic research and clinical application.
Reprogrammed Stem Cells (RSCs)
Since 2006 when Yamanaka and colleagues first generated induced pluripotent stem cells (iPSCs), reprogramming technologies have significantly progressed. This is especially true for direct reprogramming methods both in lab settings (in vitro) and within living organisms (in vivo) to produce specific tissue lineages using lineage-restricted transcription factors, RNA signal modifications, and small molecules or chemicals.
These direct approaches skip the iPSC step, yielding more precise cells like induced neural progenitor cells (iNPCs) that are closer to target cell lineages such as neural cells and subsequent motor neurons. Reprogrammed stem cells (RSCs) are derived by applying any laboratory methods to reprogram genetic signals of primary cells, excluding the SCNT technique.
To overcome ethical and immunogenic challenges associated with hESCs, iPSCs have emerged as a promising alternative since they're derived from adult somatic tissues. Human iPSC sources—including blood, skin, and urine—are plentiful. Because hiPSCs can be harvested from individual patients, immune rejection can be avoided when they're transplanted back into the same patient (autologous transplantation).
Researchers have developed methods for obtaining hiPSCs from renal tubular cells present in urine. A protocol requiring only a 30-ml urine sample is simple, relatively fast, cost-effective, and universal (applicable to patients of all ages, genders, and racial/ethnic backgrounds). The total procedure involves just 2 weeks of cell culturing and 3-4 weeks of reprogramming, producing high iPSC yields with excellent differentiation potential.
Urine-derived iPSCs collected from 200 mL clean midstream urine samples via the Sendai virus delivery system showed normal karyotype (chromosomal structure) and exhibited potential to differentiate into all three germ layers in teratoma assays. A subpopulation of cells isolated from urine displayed progenitor cell features, including cell-surface expression of c-Kit, SSEA4, CD105, CD73, CD91, CD133, and CD44 markers that can distinguish among bladder cell lineages (urothelial, smooth muscle, endothelial and interstitial), making them a promising alternative cell source.
Adult Stem Cells
Adult stem cells represent another important category of stem cells found in various tissues throughout the body. Unlike pluripotent stem cells, these are typically multipotent—able to differentiate into a limited range of cell types specific to their tissue of origin.
Common sources include bone marrow, adipose (fat) tissue, dental pulp, and various organs. Mesenchymal stem cells (MSCs) are among the most studied adult stem cells and have shown promise in treating inflammatory conditions, promoting tissue repair, and modulating immune responses.
While less versatile than pluripotent stem cells, adult stem cells offer advantages including fewer ethical concerns, lower tumor formation risk, and established clinical use in procedures like bone marrow transplantation. Research continues to explore their full potential and mechanisms of action.
Clinical Applications and Future Directions
Stem cell research has progressed through fundamental research, pre-clinical studies, and now clinical trials across multiple application areas. Advances in reprogramming factor combinations, experimental methods, and signaling pathway elucidation have contributed to the first clinical trials for retinal cell transplants and spinal cord transplants.
The field continues to address challenges related to cell proliferation and differentiation control. Researchers are systematically reviewing methodological topics including: induction of pluripotency by genomic modifications; construction of novel vectors with reprogramming factors; promotion of iPSC pluripotency with small molecules and genetic signaling pathways; enhancement of reprogramming with microRNAs; induction and enhancement of iPSC pluripotency with chemicals; generation of specific differentiated cell types; and maintenance of iPSC pluripotency and genomic stability.
These topics are crucial for maximizing efficacy of iPSC generation and differentiation in preparation for clinical translation. Advances in cell culture include feeder-free culture, xeno-free media (avoiding animal products), and various biomaterial-augmented techniques. Three-dimensional (3D) cellular and bioprinting technologies represent particularly promising directions, along with PSC resources and second-generation direct cellular reprogramming in living organisms.
Long-term stem cell research and clinical goals focus on developing safe, effective treatments for conditions including neurodegenerative diseases, spinal cord injuries, heart disease, diabetes, and many other conditions where cell replacement or tissue regeneration could provide therapeutic benefits.
Ethical Considerations
Stem cell research continues to navigate important ethical considerations, particularly regarding embryonic stem cells and cloning technologies. The destruction of human embryos for hESC research remains controversial in many societies and is regulated differently across countries.
Emerging technologies like iPSCs help address some ethical concerns by providing alternative pluripotent cell sources without embryo destruction. However, new ethical questions emerge regarding genetic manipulation, consent for cell donation, and equitable access to resulting therapies.
The international research community continues to develop guidelines and regulations to ensure ethical progress in stem cell research while maximizing potential benefits for patients suffering from various diseases and conditions.
Source Information
Original Article Title: Advances in Pluripotent Stem Cells: History, Mechanisms, Technologies, and Applications
Authors: Gele Liu, Brian T. David, Matthew Trawczynski, Richard G. Fessler
Publication: Stem Cell Reviews and Reports (2020) 16:3–32
DOI: https://doi.org/10.1007/s12015-019-09935-x
This patient-friendly article is based on peer-reviewed research and aims to make complex scientific information accessible while preserving all essential findings and data from the original publication.