HMM103 – Stem Cells and Therapy (Vocabulary Flashcards)

Lecture Learning Outcomes

• Describe the different types of stem cells (embryonic, induced pluripotent, adult).
• Explain the molecular process of stem-cell differentiation and commitment.
• Outline therapeutic applications and current research strategies in stem-cell technology.

Cell Differentiation

• All somatic cells possess identical genomic DNA yet display diverse morphology & function.
  • Differential gene expression is the determinant; genes irrelevant to a given lineage are permanently silenced once commitment occurs (epigenetic locking).
• Differentiated cells express only the subset of genes required for their specialised role.
• Approximate number of distinct human cell types: $\approx 220$.
• Significance:
  • Demonstrates that information is necessary but not sufficient—regulation drives phenotype.
  • Provides the conceptual basis for re-programming (iPSC) and directed differentiation protocols.

Stem Cells – Historical & Conceptual Overview

• First experimentally described by Ernest McCulloch & James Till (early 1960s).
• First therapeutic success: 1958 bone-marrow transplant cured radiation-induced marrow failure in criticality-accident victims (all patients survived).
• Defining characteristics:
  • Undifferentiated/“blank” state.
  • Self-renewal: unlimited symmetrical or asymmetrical mitoses.
  • Potency: ability to generate one or more mature lineages.
• Embryogenesis: fertilised egg → embryonic cells—the starting pool of stem cells.

Stem-Cell Division & Renewal

• Symmetric division → two identical stem cells (expands stem pool).
• Asymmetric division → one stem cell + one progenitor/differentiating cell (maintains pool while permitting tissue generation).
• Cellular mechanisms:
  • Partitioning of cytoplasmic cell-fate determinants.
  • Mitotic spindle orientation & microtubule dynamics.
  • Sister chromatid segregation (template versus newly synthesized DNA—“immortal strand” hypothesis).

Potency‐Based Classification

• Totipotent: can form every embryonic & extra-embryonic tissue (e.g. placenta). Only fertilised ovum & first $\le 4$ blastomeres (day $1\text{–}3$ post-fertilisation).
• Pluripotent: capable of generating any of the three germ-layer derivatives but not extra-embryonic structures; derives from inner cell mass (blastocyst, day $5\text{–}14$).
• Multipotent: forms multiple specialised types within a related family (e.g. hematopoietic stem cells produce all blood elements).
• Oligopotent: restricted to a few lineages (e.g. myeloid vs lymphoid progenitors).
• Unipotent: single lineage potential; retains self-renewal (e.g. epidermal basal cells, hepatic progenitors).
• Terminally differentiated: mature, no proliferative capacity.

Embryonic & iPSC Differentiation Pathways (Key Signals)

• Core self-renewal cues: LIF, Activin, basic FGF, Wnt.
• Directed germ-layer induction:
  • Ectoderm: BMP-4 inhibition (Noggin, Dkk-1), FGF basic → neural; EGF → epidermal.
  • Mesoderm: BMP-4 (low), FGF basic, Wnt-3a, Nodal.
  • Endoderm: Activin A high, FGF basic, Wnt-3a.
• Lineage-specific progenitors & examples:
  • Neural, intestinal, hepatic, pancreatic, cardiac, hemangioblast, lung, etc., each requiring combinatorial cytokines (IL-3, IL-6, VEGF, SCF/c-kit, Sonic Hedgehog, KGF, PDGF, IGF-1 …).
• Practical upshot: by modulating growth-factor milieu, researchers recapitulate in-vivo embryology in a dish to generate cells for therapy/drug screening.

Three Germ Layers – Developmental Foundations

• Ectoderm → nervous system, epidermis, neural crest derivatives.
• Mesoderm → muscle, bone, cartilage, blood, heart, kidneys.
• Endoderm → gut epithelium, liver, pancreas, lungs.
• Stem-cell manipulation aims to mimic germ-layer patterning for targeted differentiation.

Embryonic Stem Cells (ESC)

• Isolated from blastocyst inner cell mass.
• Combine totipotent/pluripotent subpopulations; indefinite self-renewal in vitro.
• Major ethical debate: retrieval entails destruction of developing embryo.
• Risks: teratoma formation if transplanted undifferentiated; immune rejection if non-autologous.

Induced Pluripotent Stem Cells (iPSC)

• Adult somatic cells reprogrammed to ESC-like state via Yamanaka factors: OCT4, SOX2, KLF4, CMYC (2006; Nobel 2012).
• Applications:
  • “Disease-in-a-dish” modelling for pathogenesis & drug discovery.
  • Autologous regenerative medicine after gene editing (see schematic below).
• Generic workflow:
  1. Skin biopsy → fibroblasts.
  2. Viral/non-viral transduction of reprogramming genes.
  3. Expansion → disease gene correction (CRISPR, homologous recombination).
  4. Directed differentiation → functional cells.
  5. Transplant back to patient (histocompatibility preserved).
• Limitations: insertional mutagenesis, oncogenic KLF4/CMYC, reproducibility of differentiation.

Adult Stem Cells

• Typically multipotent or oligopotent; reside in specialised micro‐environments (niches) providing growth-factor gradients & extracellular-matrix cues.
• Renewal is finite (exhaustion possible) – contrast with ESC/iPSC.

Mesenchymal Stem Cells (MSC)

• Synonym: bone-marrow stromal cells.
• Differentiate into osteocytes, chondrocytes, adipocytes, tenocytes, myoblasts.
• Therapeutic uses: Crohn’s disease, bone defects, cardiac & tracheal repair, osteoarthritis, tissue engineering scaffolds.

Hematopoietic Stem Cells (HSC)

• Supply all blood lineages; daily turnover requirement $\approx 1\times10^{11}$ cells.
• Sources: bone marrow, peripheral blood (after G-CSF mobilisation), umbilical-cord blood.
• Standard of care for leukemias, lymphomas, immunodeficiencies.

Neural Stem Cells (NSC)

• Generate neurons, astrocytes, oligodendrocytes.
• Sourced from fetal/embryonic tissue or derived from ESC/iPSC.
• Investigated for Parkinson’s, Alzheimer’s, Amyotrophic Lateral Sclerosis.

Comparative Summary – Pros & Cons

Stem Cell Therapy – Modalities

• Autologous transplantation

  • Source: patient’s own HSC/MSC (bone marrow, peripheral blood, adipose).
  • Advantages: rapid engraftment, minimal graft-vs-host disease (GVHD), lower rejection.
  • Common for blood cancers (after high-dose chemo) & autoimmune conditions.

• Allogeneic transplantation

  • Matched donor (HLA-identical sibling/unrelated registry).
  • Greater graft-versus-leukemia effect but higher GVHD & immunosuppression burden.

• Gene-modified cell therapy

  • Ex vivo transduction with therapeutic gene (e.g. $\beta$-globin for sickle-cell) via retro/lentivirus → re-infusion.
  • Alternative: in vivo viral delivery directly to patient.

• Regenerative medicine & tissue engineering

  • Directed differentiation to generate organoids, grafts (e.g. skin‐equivalent), bio-printed tissues.
  • Example: IPSC-derived cardiomyocytes for heart repair; scaffold-seeded MSC for tracheal replacement.

Current Research & Future Directions

• Correction of monogenic disorders via CRISPR-Cas9 + iPSC (e.g. cystic fibrosis, β-thalassemia).
• Elucidation & prevention of congenital birth defects by modelling early development in vitro.
• Large-scale biobanking of patient-specific iPSC lines for pharmacogenomic screening.
• Xenogeneic scaffolds repopulated with human stem cells → whole-organ bioengineering.
• Synthetic biology: programming stem cells with “suicide genes” to control proliferation post-transplant.

Challenges & Limitations

• Ethics: embryo destruction (ESC), chimeric embryo research controversies.
• Tumorigenicity: uncontrolled differentiation → teratoma/oncogenesis.
• Immune rejection: even autologous cells may express neoantigens (post-culture epigenetic drift).
• Knowledge gap: incomplete map of signalling networks; difficulty in precise fate control.
• Cost & logistics: GMP manufacturing, cryostorage, patient-specific customisation.
• Long-term safety: paucity of data beyond >10 years; need for registries.
• Targeting & delivery: homing to specific tissue, survival in hostile micro-environment.
• Cell-number limitation: some adult tissues yield scant stem cells—necessitating expansion protocols that may alter phenotype.

Lecture Conclusions (Key Takeaways)

• Stem cells = undifferentiated units capable of $\rightarrow$ any lineage under appropriate signals.
• Potency hierarchy: totipotent > pluripotent > multipotent > oligopotent > unipotent > terminal.
• Main categories: Embryonic, Induced Pluripotent, Adult; each with distinct advantages, risks & ethical standings.
• Adult stem-cell subtypes (MSC, HSC, NSC) already underpin established or emerging therapies.
• Therapeutic formats include autologous, allogeneic, gene-edited, and tissue-engineered approaches.
• Ongoing hurdles: ethical, immunological, technical & economic; however, advances in genomic editing and biomaterials herald transformative potential for regenerative medicine.