1: How can stem cells be classified?
Stem cells are classified based on their potency:
• Totipotent: Can differentiate into all cell types, including extraembryonic tissues.
• Pluripotent: Can differentiate into all cell types of the body (e.g., ES cells).
• Multipotent: Can differentiate into a limited range of cell types (e.g., hematopoietic stem cells).
• Unipotent: Can produce only one cell type but have self-renewal capacity.
2: What are the differences between ES, EC, and EG cells? What do they have in common?
• ES (Embryonic Stem) cells: Derived from the inner cell mass of the blastocyst; pluripotent.
• EC (Embryonal Carcinoma) cells: Cancerous cells with some pluripotent traits; less stable.
• EG (Embryonic Germ) cells: Derived from primordial germ cells; pluripotent but more challenging to culture.
Commonality: All are pluripotent and can differentiate into various cell types.
3: What is a teratoma?
A teratoma is a benign tumor composed of multiple cell types, often derived from all three germ layers, typically forming when pluripotent cells differentiate irregularly.
4: How can you generate genetic chimeras in mammals?
Genetic chimeras are generated by introducing pluripotent cells (e.g., ES cells) into a developing blastocyst, which is then implanted into a surrogate mother.
5: What is meant by “Pluripotency Transcription Factors”?
These are transcription factors (e.g., Oct4, Sox2, Nanog) that regulate the gene expression necessary for maintaining pluripotency and self-renewal in stem cells.
6: What is the current model on how in evolution a small set of transcription factors became potent to control pluripotency?
A few transcription factors evolved from ancestral regulatory networks to control pluripotency, gaining unique binding motifs and cooperative functions over time, ensuring robust control of stem cell states.
7: What are the main aspects of the pluripotency transcriptional network?
The network involves core factors (Oct4, Sox2, Nanog) that regulate one another and activate genes for pluripotency while repressing differentiation pathways.
8: How can animals be cloned?
By somatic cell nuclear transfer (SCNT), where the nucleus of a somatic cell is transferred into an enucleated egg, followed by activation and development.
9: What are the problems encountered during somatic cloning?
Inefficiencies, abnormal epigenetic reprogramming, developmental defects, low success rates, and premature aging.
10: How do chromosomes in somatic cells “age” with each cell cycle?
Telomeres shorten with each cell division, leading to reduced chromosomal stability and cellular aging.
11: What kind of defects are typical for cloned animals?
Abnormal development, large offspring syndrome, immune deficiencies, and premature aging due to incomplete epigenetic reprogramming.
12: What is imprinting?
Imprinting is an epigenetic mechanism where certain genes are expressed in a parent-of-origin-specific manner, regulated by DNA methylation and histone modifications.
13: What are iPS cells and how can they be generated?
iPS (induced pluripotent stem) cells are reprogrammed somatic cells, induced by expressing key factors like Oct4, Sox2, Klf4, and c-Myc (Yamanaka factors).
14: Please explain the steps iPS cells undergo during reprogramming?
1. Loss of somatic identity.
2. Activation of pluripotency genes.
3. Epigenetic remodeling.
4. Establishment of a stable pluripotent state.
15: Is it possible to generate iPS cells without interfering directly with the genome?
Yes, using non-integrative methods like mRNA, proteins, or small molecules to avoid direct genetic modifications.
16: What are the sequential steps of regeneration?
1. Wound healing.
2. Dedifferentiation or activation of progenitor cells.
3. Proliferation.
4. Differentiation and tissue remodeling.
17: What are the sources of the different cell types during regeneration of the amphibian limb?
Cells originate from dedifferentiated tissues, resident stem/progenitor cells, and reprogrammed mature cells near the injury site.
18: What is transdifferentiation - "direct conversion"?
Transdifferentiation is the direct conversion of one differentiated cell type into another without reverting to a pluripotent state.
19: Is there anything special about umbilical cord stem cells?
They are rich in hematopoietic and mesenchymal stem cells, are immunologically immature, and have a high proliferation capacity.
20: What are tissue-specific stem cells?
These are multipotent stem cells residing in specific tissues, responsible for tissue repair and regeneration (e.g., hematopoietic stem cells).
21: Where do we find tissue-specific stem cells?
In specialized niches, such as the bone marrow, skin, brain, intestine, and skeletal muscle.
22: How can one demonstrate experimentally the existence of stem cells and their progeny?
Use lineage tracing, clonal analysis, or transplantation assays to observe differentiation and self-renewal.
23: What is a stem cell niche?
A specialized microenvironment that supports and regulates stem cell behavior, maintaining their self-renewal and differentiation balance.
24: How can one isolate stem cells?
By using techniques like flow cytometry (FACS), magnetic-activated cell sorting (MACS), or specific surface marker selection.
25: What are the most important stem cell niche components?
Extracellular matrix, signaling molecules (e.g., Wnt, Notch), neighboring cells, and mechanical cues.
26: Explain different mechanisms by which stem cells are regulated in their niche (example neural or hematopoietic stem cells).
Signaling pathways (Wnt, Notch, BMP), adhesion molecules, metabolic cues, and local cytokines regulate quiescence, proliferation, and differentiation.
27: Evaluate advantages and disadvantages of ES/iPS cells and tissue-specific stem cells for tissue regeneration.
• ES/iPS cells: High plasticity, but ethical issues (ES), tumor risk, and immune rejection (ES).
• Tissue-specific stem cells: Safer, but limited differentiation capacity and accessibility.
The ability of neurons to regenerate varies significantly depending on their location and the conditions surrounding the injury. Here’s an explanation focusing on the differences between neurons in the spinal cord and the arm:
1. Structure and Type of Neurons
• Spinal Cord Neurons: These are part of the central nervous system (CNS), which includes the brain and spinal cord. Neurons here are primarily involved in processing and transmitting information.
• Arm Neurons: These are part of the peripheral nervous system (PNS), which includes nerves outside the brain and spinal cord. Peripheral neurons transmit signals between the CNS and the rest of the body.
2. Environment and Regenerative Capacity
• Central Nervous System (Spinal Cord):
• Inhibitory Environment: After an injury, the CNS creates a hostile environment for regeneration. The formation of glial scars and the release of inhibitory molecules (like chondroitin sulfate proteoglycans) block axonal regrowth.
• Limited Intrinsic Capacity: CNS neurons have a limited ability to regrow their axons due to reduced intrinsic growth programs.
• Myelin Inhibition: Myelin in the CNS contains molecules (e.g., Nogo-A) that actively inhibit regeneration.
• Peripheral Nervous System (Arm):
• Supportive Environment: The PNS environment promotes regeneration. Schwann cells in the PNS guide axonal regrowth by releasing growth factors and forming regeneration-friendly pathways.
• Intrinsic Growth Ability: Peripheral neurons retain a stronger ability to activate regenerative programs after injury.
• Wallerian Degeneration: This process clears debris more effectively in the PNS, allowing for faster regrowth.
3. Why Peripheral Neurons Can Regenerate
• Schwann Cells: These cells produce neurotrophic factors and create a pathway for axonal regrowth through a process called “Bands of Büngner.”
• Clearance of Debris: In the PNS, immune cells quickly clear damaged myelin and other debris, facilitating regeneration.
• Plasticity: Peripheral neurons can adapt and rewire themselves more effectively than CNS neurons.
4. Why CNS Neurons Struggle to Regenerate
• Astrocyte Activity: Astrocytes in the CNS form scars that prevent axon regrowth.
• Chronic Inflammation: Persistent inflammation in the CNS can exacerbate damage and further inhibit recovery.
• Lack of Growth Factors: The CNS lacks the regenerative cues and support present in the PNS.
Future Directions
Research into promoting CNS regeneration includes:
• Blocking inhibitory molecules (e.g., Nogo-A antibodies).
• Enhancing intrinsic growth programs in CNS neurons.
• Using stem cells and tissue engineering to bridge damaged areas.
• Modifying the glial response to reduce scar formation.
In summary, the regenerative capacity of neurons depends heavily on their environment, with peripheral neurons having the support and conditions needed for regrowth, while central neurons face inhibitory barriers that currently limit recovery after injury.
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