Stem cell Study Guide

📖 Core Concepts Stem cell – undifferentiated (or partially differentiated) cell that can both self‑renew indefinitely and give rise to other cell types. Potency – range of cell fates a stem cell can generate (totipotent → unipotent). Self‑renewal – ability to undergo repeated divisions while retaining the undifferentiated state. Niche – specialized microenvironment that supplies signals keeping a stem cell undifferentiated; loss of niche cues triggers differentiation. Asymmetric vs. symmetric division – asymmetric yields one stem + one differentiated daughter; symmetric yields either two stem cells (expansion) or two differentiated cells (depletion). Telomerase activity – maintains telomere length in stem cells, allowing a long replicative lifespan. 📌 Must Remember Potency hierarchy: Totipotent > Pluripotent > Multipotent > Oligopotent > Unipotent. Embryonic stem cells (ESCs) are pluripotent and derived from the inner cell mass of a blastocyst (≈50‑150 cells). Adult stem cells are multipotent (or unipotent) and reside in niches (bone marrow, adipose, menstrual fluid, umbilical cord). Induced pluripotent stem cells (iPSCs) are reprogrammed adult somatic cells (Oct4, Sox2, Nanog, Lin28). Hematopoietic stem cell transplantation (HSCT) = only widely established clinical stem‑cell therapy (treats leukemia, blood cancers). Teratoma risk – undifferentiated ESCs or iPSCs can form tumors if transplanted without controlled differentiation. Ethical core: ESC derivation destroys a blastocyst → major regulatory restrictions. 🔄 Key Processes Self‑renewal via division type Decide division mode (asymmetric → maintain pool; symmetric → expand pool). Reprogramming to iPSC Introduce transcription factors (Oct4, Sox2, Nanog, Lin28) → epigenetic reset → pluripotency. Hematopoietic stem cell transplantation Harvest → conditioning (radiation/chemo) → infusion → engraftment & immune reconstitution. Lineage tracing Label a single cell (reporter gene) → follow progeny through divisions → map developmental pathways. 🔍 Key Comparisons Embryonic vs. Adult Stem Cells Potency: ESC = pluripotent; Adult = multipotent/unipotent. Source: ESC = inner cell mass of blastocyst; Adult = niche tissues (bone marrow, adipose, etc.). Ethics: ESC require embryo destruction; Adult cells avoid this issue. iPSC vs. ESC Origin: iPSC = reprogrammed somatic cell; ESC = derived directly from embryo. Epigenetics: iPSC retain some donor‑cell memory; ESC are naïve. Tumor risk: Both can form teratomas, but iPSC may carry additional genetic abnormalities from reprogramming. Asymmetric vs. Symmetric Division Outcome: Asymmetric → 1 stem + 1 differentiated; Symmetric → 2 stem cells (expansion) or 2 differentiated (depletion). Regulation: Niche signals bias toward asymmetric division to preserve pool size. ⚠️ Common Misunderstandings “All stem cells divide indefinitely.” → Only true stem cells (not progenitors) have unlimited self‑renewal; progenitor/precursor cells have limited divisions. “iPSCs are ethically identical to ESCs.” → iPSCs avoid embryo destruction but still raise concerns about genetic manipulation and tumorigenicity. “Mesenchymal stem cells directly replace damaged tissue.” → Their primary benefit is paracrine (angiogenesis, immunomodulation), not robust tissue replacement. 🧠 Mental Models / Intuition Potency ladder – imagine a ladder: bottom rung = totipotent (can build whole house), each step up loses a few “rooms” (germ layers). Niche as a “home base” – stem cells stay “at home” (undifferentiated) while the home supplies food, shelter, and signals; leaving home forces them to “grow up.” Division decision switch – think of a traffic light: green (symmetric expansion) when tissue needs more stem cells; amber (asymmetric) for maintenance; red (symmetric differentiation) when replacement is required. 🚩 Exceptions & Edge Cases Oligopotent cells (e.g., lymphoid‑restricted progenitors) sit between multipotent and unipotent; they can still generate several related lineages. Adult stem‑cell debate – some adult cell populations may not meet strict self‑renewal criteria; they behave more like long‑lived progenitors. Telomerase re‑activation – not all adult stem cells express high telomerase; some rely on niche protection instead. 📍 When to Use Which Therapeutic choice Use HSCT for hematologic malignancies or bone‑marrow failure. Consider mesenchymal stem cell trials for immunomodulation or wound healing. Opt for iPSC‑derived cells when patient‑specific, immune‑compatible tissue is required (e.g., disease‑modeling, personalized drug screening). Research model Use ESCs for studying early development and pluripotency mechanisms. Use adult stem cells for niche‑specific studies and tissue‑specific regeneration. 👀 Patterns to Recognize Potency ↔ Germ‑layer contribution – pluripotent → all three germ layers; multipotent → one germ layer (usually mesoderm for MSCs). Division mode ↔ niche status – intact niche → predominance of asymmetric division; niche loss → surge of symmetric differentiation. Teratoma formation ↔ undifferentiated pluripotent cells – any transplantation of ESC/iPSC without pre‑differentiation signals high tumor risk. 🗂️ Exam Traps “All stem cells are pluripotent.” – Only ESCs and iPSCs are pluripotent; most adult stem cells are multipotent or less. Confusing progenitor vs. stem cell – Progenitors have limited self‑renewal; stem cells do not. “Mesenchymal stem cells become any tissue type.” – MSCs are mesoderm‑derived and differentiate mainly into adipocytes, osteocytes, chondrocytes; they do not become neurons or hepatocytes directly. “iPSC generation requires only one factor.” – Reprogramming typically needs a cocktail (Oct4, Sox2, Nanog, Lin28) or equivalent. “Stem‑cell tourism is safe if the clinic is abroad.” – Unregulated clinics have caused severe complications (spinal tumors, death); geography does not guarantee safety.