Stem Cells Comprehensive Notes
Overview of Stem Cells
• Stem cells are unique cells fundamentally characterized by two core properties: they both self-renew (undergo cell division to produce more stem cells, thus maintaining a stable undifferentiated pool) and generate differentiating progeny (cells that are committed to a specific lineage and will eventually specialize).
• The broad spectrum of derivatives illustrated in lecture images encompasses a wide array of specialized cell types, including but not limited to: highly contractile cardiac cells, electrically active neurons, diverse epithelial cells forming linings and coverings, chondrocytes responsible for cartilage formation, enterocytes of the intestinal lining, adipocytes for fat storage, red blood cells for oxygen transport, and many others.
• This key property—their capacity to form "many different cell types"—is orchestrated under appropriate cues, which can be internal (cell-intrinsic genetic programs) or external (signals from the microenvironment).
Cell Potency & Differentiation Concepts
• Totipotent – This represents the highest level of potency, where a single cell possesses the intrinsic capability to create all cell types of a complete organism. This includes both the embryonic tissues that will form the fetus and crucial extra-embryonic tissues like the placenta and umbilical cord (e.g., the zygote immediately after fertilization, and the blastomeres up to the morula stage).
• Pluripotent – Cells at this stage can generate all cell lineages that constitute the embryo (ectoderm, mesoderm, endoderm) except the extra-embryonic tissue. This is the hallmark characteristic of true ESCs and properly reprogrammed iPSCs.
• Multipotent – These cells have a more restricted developmental potential. They can form multiple (but limited) lineages, typically within one specific tissue or organ system. They are crucial for lifelong tissue regeneration and repair throughout an organism's life (e.g., hematopoietic stem cells, which can differentiate into all blood cell types but not other tissue types).
• "Differentiation" = The terminal process by which a less specialized cell becomes a more specialized cell type with a distinct structure and function, reaching an end-point, terminally specialized phenotype.
• "Cell commitment" = A fundamental, stepwise restriction of a cell's future developmental potential. This process occurs prior to any visible morphological differentiation, meaning the cell has made an irreversible, or very difficult to reverse, decision about its future fate.
• Commitment can involve:
– Autonomous effects (cell-intrinsic genetic/epigenetic program): This refers to a cell's internal machinery, where its fate is determined by factors already present within the cell, often due to asymmetric distribution of cytoplasmic determinants during cell division.
– Non-autonomous effects (signals from neighbors / niche): This involves external signals from the surrounding microenvironment, including cell-to-cell contact, secreted signaling molecules (growth factors, morphogens), and components of the extracellular matrix, which collectively form the stem cell niche.
• Reversibility? Historically, the differentiation process was considered largely irreversible once a cell reached its specialized state. However, modern groundbreaking reprogramming techniques, such as Somatic Cell Nuclear Transfer (SCNT) and induced Pluripotent Stem Cells (iPSCs), have definitively shown that a differentiated fate can be reversed or reprogrammed under precisely defined and controlled laboratory conditions, demonstrating cellular plasticity.
Embryonic Stem Cells (ESCs)
• Derived from the inner cell mass (ICM), a small cluster of cells within the pre-implantation blastocyst, typically 5-7 days post-fertilization. The ICM is the part of the blastocyst that will eventually form the embryo proper.
• Can be cultured indefinitely in vitro while retaining their pluripotent state, provided they are maintained in an optimized culture medium containing specific factors. For mouse ESCs, Leukemia Inhibitory Factor (LIF) is crucial, while human ESCs require factors like ACTIVIN and FGF2 (Fibroblast Growth Factor 2).
• Demonstrated potential to generate entire embryo in:
– Retinoic acid (RA)–induced differentiation assays: Exposure to specific signaling molecules like Retinoic Acid in vitro can trigger ESCs to differentiate into various cell types, allowing for studies of germ-layer formation.
– Embryoid body formation: When cultured in suspension without feeder layers or specific inhibitory factors, ESCs spontaneously aggregate to form 3-D spherical structures called embryoid bodies. These structures mimic early embryonic development and can spontaneously differentiate into derivatives from all three germ layers (ectoderm, mesoderm, and endoderm).
• Martin Evans (Nobel Prize in Physiology or Medicine 2007, shared with Oliver Smithies and Mario Capecchi) – a pioneer in the field, credited with the initial isolation and successful culture of mouse Embryonic Stem Cells in 1981, laying the groundwork for genetic engineering in mice.
• Gold-standard functional tests for pluripotency and developmental potential:
– Teratoma assay: This involves injecting pluripotency-tested ESCs into an immunodeficient mouse. If the cells are truly pluripotent, they will form a benign tumor called a teratoma, which characteristically contains disorganized but identifiable tissues derived from all three embryonic germ layers (e.g., bone, cartilage, neural tissue, gut epithelium, muscle, skin), serving as direct evidence of pluripotency.
– Chimera formation: ESCs, typically genetically marked (e.g., with GFP), are injected into a host blastocyst. The injected ESCs integrate into the host embryo, leading to an organism (chimera) where some tissues are derived from the host and some from the donor ESCs. This assay demonstrates the ability of ESCs to contribute to somatic tissues and, importantly, to the germline (sperm or egg), meaning they can pass on their genetic material to future generations.
– Tetraploid complementation: This is considered the most stringent test for pluripotency. Diploid ESCs () are combined with a tetraploid host embryo () (created by electrofusing two-cell stage embryos). The unique aspect is that tetraploid cells are generally restricted to contributing only to extra-embryonic tissues (like the placenta), while the ESCs are expected to form the entire embryo proper. If live, fertile offspring are born derived solely from the ESC genome, it provides definitive proof of full pluripotency and developmental competence.
– Ploidy diagram: donor ESCs, the cells being tested for pluripotency, have a diploid set of chromosomes (). The host embryo, specifically prepared for this assay, is tetraploid (), meaning it has double the normal number of chromosomes. This distinction is critical for the assay's mechanism.
Naïve vs. Primed Pluripotent States
• Comparative properties (mouse paradigm extrapolated to human, although human states are still a subject of active research):
– Naïve (analogous to the pre-implantation ICM): Cells in this state exhibit a compact, dome-shaped colony morphology. They show no lineage bias, meaning they have unrestricted developmental potential. They contribute robustly to chimera formation and are typically dependent on LIF (Leukemia Inhibitory Factor) for self-renewal. In female cells, both X-chromosomes are active (), and the genome exhibits global hypomethylation, indicating a highly plastic and open chromatin state.
– Primed (analogous to the post-implantation epiblast): These cells form flatter colonies, are more elongated, and often exhibit a subtle differentiation bias towards specific lineages. Crucially, they typically show no chimera competence in rodents, meaning they cannot integrate into a developing embryo to form a full organism. They are dependent on ACTIVIN and FGF2 for maintenance. In female cells, one X chromosome is already inactivated (), and the genome is more hypermethylated, reflecting a more restricted and partially committed chromatin state.
• Understanding these distinct pluripotent states is paramount as it guides the optimization of culture conditions, refinement of reprogramming strategies (e.g., aiming for naïve iPSCs for certain applications), and the development of more efficient and specific differentiation protocols.
Somatic Cell Nuclear Transfer (SCNT) & Cloning
• Technique: SCNT involves the precise insertion of a diploid nucleus, typically isolated from a differentiated somatic donor cell (e.g., a skin cell), into an enucleated oocyte (an egg cell from which its own nucleus has been physically removed). The cytoplasm of the oocyte contains powerful reprogramming factors that can effectively reprogram the somatic nucleus to an embryonic, totipotent or pluripotent state.
• Outcome options:
– Implant into surrogate → cloned organism (reproductive cloning): If the reprogrammed embryo is implanted into the uterus of a surrogate mother, it can develop into a new organism that is genetically identical or nearly identical to the somatic cell donor.
– Culture to blastocyst → derive patient-matched ESCs (therapeutic cloning): Alternatively, the reprogrammed embryo can be cultured in vitro to the blastocyst stage from which ESCs can be derived. These ESCs would be genetically matched to the patient, offering a source for cell therapies without immune rejection issues.
• Landmark: Dolly the sheep (1996) – the first mammal successfully cloned from an adult somatic cell. This achievement required attempts, signifying the immense technical challenges but definitively proving the principle of epigenetic reprogrammability of differentiated cells.
• 2018: Two macaque monkeys, Zhong Zhong and Hua Hua, were cloned via SCNT, demonstrating the feasibility of SCNT in non-human primates. This notable success involved 21 surrogates, resulting in 6 pregnancies, 6 births, and ultimately 2 live pups.
• Limitations: Despite success, SCNT often suffers from low efficiency, leading to many failed attempts. A key genetic limitation is that mitochondrial DNA (mtDNA) is derived solely from the egg donor, not the nucleus donor, which can result in mitochondrial heteroplasmy if the donor mtDNA is different. There are also significant technical and ethical hurdles associated with SCNT, including the demand for many oocytes and the debates surrounding reproductive cloning, as well as potential interspecies applications which present unique challenges.
Induced Pluripotent Stem Cells (iPSCs)
• Paradigm-shifting 2006 discovery (Shinya Yamanaka; Nobel Prize in Physiology or Medicine 2012, shared with John Gurdon): This groundbreaking work demonstrated that transient, forced expression of just 4 specific transcription factors (TFs) could reprogram differentiated mouse fibroblasts back to a pluripotent state, resembling ESCs.
• The canonical Yamanaka factors:
– Oct4 & Sox2: These are crucial core pluripotency factors. They act synergistically to activate the Nanog gene and other components of the endogenous pluripotency network, while simultaneously suppressing genes associated with cell differentiation.
– c-Myc: This factor is an oncogene and broadly functions to open chromatin structure, making genes more accessible for transcription, and significantly increases cellular proliferation rates. Its oncogenic potential is a safety concern for therapeutic applications.
– Klf4: This factor plays roles in cell survival, promoting cell growth, and notably suppresses p53-mediated apoptosis (programmed cell death), thus aiding the survival of reprogrammed cells.
• Only transient expression of these exogenous factors is required during the reprogramming process; once the endogenous pluripotency network is established and self-sustaining, the artificial expression of these factors can be discontinued.
• Resulting iPSCs closely resemble ESCs in their characteristic gene expression profiles, epigenetic marks (DNA methylation patterns and histone modifications), and their broad differentiation capacity across all three germ layers.
Reprogramming Efficiency & Safety Challenges
• Original efficiency: The initial reprogramming protocols yielded extremely low efficiencies, with only approximately of the starting differentiated cells successfully converting into iPSCs.
• Common issues:
– Genomic integration or residual expression of TF transgenes (insertional mutagenesis): Early methods often involved viral vectors that integrated the reprogramming factor genes into the host genome, leading to potential insertional mutagenesis or uncontrolled, persistent expression of oncogenic TFs.
– Oncogenic potential (notably c-Myc): The inclusion of c-Myc, a proto-oncogene, raises concerns about the potential for tumor formation if iPSCs are used therapeutically.
– Epigenetic / transcriptional "incomplete" reprogramming → residual somatic memory: Even after successful reprogramming, iPSCs can sometimes retain an iPSCs can sometimes retain an epigenetic / transcriptional "memory" of their original somatic cell type. This "residual somatic memory" can make them less efficient at differentiating into certain lineages or predispose them to differentiate into their original cell type, posing a challenge for consistent and broad therapeutic applications.