Premed · Premed · Cell Biology
Lecture 25: Stem Cells and Tissue Renewal
Cell Biology
Learning Objectives
By the end of this lecture, students will be able to:
- Define stem cells and explain the properties of self-renewal and potency
- Compare embryonic stem cells, adult stem cells, and induced pluripotent stem cells
- Describe the stem cell niche and how it regulates stem cell behavior
- Explain how stem cells maintain tissue homeostasis in the intestine, skin, and blood
- Discuss the therapeutic potential and ethical considerations of stem cell biology
Lecture Content
I. Defining Stem Cells
Two defining properties: Self-renewal: ability to divide and produce at least one daughter cell that remains a stem cell. Differentiation: ability to produce specialized cell types. Potency hierarchy: Totipotent: can give rise to all cell types including extraembryonic tissues (zygote, early blastomeres) Pluripotent: can give rise to all cell types of the embryo (all three germ layers) but not extraembryonic tissues. Embryonic stem cells (ESCs); inner cell mass of the blastocyst. Multipotent: can give rise to multiple cell types within one lineage. Hematopoietic stem cells (HSCs): all blood cell types. Neural stem cells: neurons, astrocytes, oligodendrocytes. Oligopotent: few cell types (e.g., myeloid progenitor -> granulocytes, monocytes) Unipotent: one cell type only (e.g., spermatogonial stem cells) Division strategies: Asymmetric division: one daughter remains a stem cell, the other becomes a committed progenitor (cell-intrinsic mechanism) Symmetric division: both daughters become stem cells (expansion) or both differentiate (depletion) Population asymmetry: at the population level, self-renewal and differentiation are balanced even if individual divisions are stochastic.
II. Embryonic Stem Cells (ESCs)
Derived from the inner cell mass (ICM) of the blastocyst (day ~5 in humans) Properties in culture: Indefinite self-renewal. Pluripotent: can differentiate into all three germ layers (ectoderm, mesoderm, endoderm) Demonstrated by: teratoma formation (in vivo), embryoid body formation (in vitro), chimera contribution (mouse) Key transcription factors maintaining pluripotency: Oct4 (Pou5f1): POU-domain transcription factor; essential for ICM identity. Sox2: HMG-box transcription factor; partners with Oct4. Nanog: homeodomain protein; blocks differentiation. These form a core regulatory circuit: they activate each other's expression and co-occupy promoters of pluripotency genes while repressing lineage-specific genes. Epigenetic state: open chromatin, bivalent domains (H3K4me3 + H3K27me3) at developmental gene promoters -> poised for activation or silencing upon differentiation. Signaling pathways maintaining pluripotency: Mouse ESCs: LIF/STAT3 pathway + BMP4; or 2i conditions (MEK inhibitor + GSK3 inhibitor) Human ESCs: FGF2 + Activin/Nodal (TGF-beta superfamily).
III. Induced Pluripotent Stem Cells (iPSCs)
Yamanaka factors (2006, Nobel Prize 2012): somatic cells reprogrammed to pluripotency by forced expression of four transcription factors: Oct4, Sox2, Klf4, c-Myc (OSKM) Originally demonstrated by retroviral delivery into mouse fibroblasts; iPSCs are functionally equivalent to ESCs: Express pluripotency markers, form teratomas, contribute to chimeras (in mouse) Epigenome is reset to a pluripotent state. Reprogramming process: Inefficient (~0.1-1% of cells); takes 2-4 weeks. Involves: silencing of somatic genes, mesenchymal-to-epithelial transition (MET), activation of endogenous pluripotency network, epigenetic remodeling. Barriers: p53/p21 pathway, senescence, epigenetic memory of somatic cell origin. Applications: Disease modeling: patient-specific iPSCs differentiated into affected cell type (e.g., motor neurons for ALS, cardiomyocytes for long QT syndrome) Drug screening: test drugs on patient-derived cells in vitro. Regenerative medicine: iPSC-derived cells for transplantation (avoids immune rejection if autologous) iPSC-derived retinal pigment epithelium for macular degeneration (first clinical trial 2014, Japan) Avoids ethical concerns associated with embryo destruction (ESCs) Safety concerns: tumorigenic potential (c-Myc oncogene, insertional mutagenesis from viral vectors) Newer methods: non-integrating approaches (mRNA, Sendai virus, episomal plasmids, small molecules).
<image>Induced pluripotent stem cell generation and applications. Panel A: Reprogramming — somatic cells (fibroblasts) transduced with Oct4, Sox2, Klf4, c-Myc transcription factors; gradual morphological change over 2-4 weeks; iPSC colony with ESC-like morphology emerging. Panel B: iPSC pluripotency demonstrated by differentiation into all three germ layers — ectoderm (neurons, skin), mesoderm (cardiomyocytes, blood, muscle), endoderm (hepatocytes, lung, pancreas). Panel C: Applications pipeline — patient with genetic disease -> skin biopsy -> fibroblasts -> iPSC generation -> gene correction (optional, via CRISPR) -> differentiation into disease-relevant cell type -> drug screening in vitro or cell therapy back to patient.</image>
IV. The Stem Cell Niche
Niche: specialized microenvironment that maintains stem cell identity and regulates behavior. Components of the niche: Supporting cells (stromal cells, nurse cells) that provide signals. ECM (laminin, fibronectin, proteoglycans) providing adhesion and sequestering growth factors. Soluble factors: Wnt, BMP, Notch ligands, Hedgehog, cytokines. Physical properties: stiffness, oxygen tension (many niches are hypoxic) Spatial constraints: orient the axis of stem cell division relative to the niche. Key signaling pathways in stem cell regulation: Wnt/beta-catenin: promotes self-renewal in intestinal stem cells, HSCs, hair follicle stem cells. Wnt ligand binds Frizzled/LRP -> inhibits the destruction complex (APC, Axin, GSK3) -> beta-catenin stabilized -> enters nucleus -> activates TCF/LEF target genes (including Lgr5, c-Myc, cyclin D1) Notch: lateral inhibition and cell fate decisions. Delta/Jagged on one cell binds Notch receptor on neighbor -> gamma-secretase cleaves NICD -> NICD enters nucleus -> activates Hes/Hey transcription factors. Maintains undifferentiated state in many contexts (intestine, neural, hematopoietic) BMP: often promotes differentiation; gradient of BMP vs. BMP antagonists (Noggin) helps define niche boundaries. Hedgehog: regulates stem cells in the hair follicle, brain, and many other tissues.
V. Tissue Renewal: Key Examples
Intestinal epithelium (fastest renewing tissue; 3-5 day turnover): Lgr5+ crypt base columnar (CBC) stem cells: reside at the base of intestinal crypts, intercalated between Paneth cells. Paneth cells form the niche: secrete Wnt3, EGF, Notch ligands, and express Delta-like ligands. Stem cells divide to produce transit-amplifying (TA) cells that rapidly divide and migrate upward along the crypt-villus axis. TA cells differentiate into: absorptive enterocytes, goblet cells, enteroendocrine cells, tuft cells. Cells reach the villus tip and undergo anoikis (detachment-induced apoptosis) and are shed into the lumen. Intestinal organoids (mini-guts): single Lgr5+ stem cells can form self-organizing 3D structures in culture (Matrigel + Wnt + R-spondin + Noggin + EGF) Skin/epidermis: Basal layer contains stem cells and transit-amplifying cells. Asymmetric divisions: one daughter stays basal (stem), one commits to differentiation and moves upward. Differentiation program: basal -> spinous -> granular -> cornified (stratum corneum, dead squames shed) Hair follicle bulge: contains quiescent stem cells that are activated cyclically for hair growth. Hematopoietic system: HSCs reside in the bone marrow niche (perivascular, endosteal) Long-term HSC (LT-HSC) -> short-term HSC -> multipotent progenitor -> lineage-committed progenitors -> mature blood cells. Quiescent LT-HSCs rarely divide; niche signals (SCF, CXCL12, thrombopoietin) maintain quiescence. HSC transplantation (bone marrow transplant): clinical cornerstone for leukemia, aplastic anemia, immunodeficiencies.
<image>Stem cell niches in the intestine and bone marrow. Panel A: Intestinal crypt — Lgr5+ stem cells (green) at the crypt base, intercalated with Paneth cells (red, providing Wnt, Notch, EGF signals). Transit-amplifying (TA) zone above, with upward migration and differentiation into absorptive enterocytes, goblet cells, and enteroendocrine cells along the villus. Cells are shed from the villus tip. Panel B: Bone marrow HSC niche — long-term HSC (blue) in close contact with perivascular niche cells (endothelial cells, CXCL12-abundant reticular cells, mesenchymal stromal cells) providing SCF, CXCL12, and thrombopoietin. Osteoblasts at the endosteal surface contribute additional niche signals. Differentiation hierarchy from LT-HSC to mature blood cell lineages (red blood cells, platelets, neutrophils, lymphocytes, monocytes) shown as a branching tree. Panel C: Intestinal organoid — 3D mini-gut grown from a single Lgr5+ stem cell in Matrigel, showing crypt-like budding structures with all differentiated cell types.</image>
VI. Therapeutic Frontiers and Ethical Considerations
Current clinical applications: HSC transplantation (bone marrow/cord blood): well-established. Skin grafts from cultured epidermal stem cells (severe burns) Limbal stem cell transplant for corneal regeneration. Emerging therapies: iPSC-derived cell therapies (retinal, cardiac, neural, pancreatic beta cells) Organoid-based approaches for disease modeling and drug testing. Gene-corrected iPSCs for genetic diseases (combine iPSC + CRISPR) Cancer stem cells: subpopulation of tumor cells with stem-like properties. Self-renewal, tumor-initiating capacity, therapy resistance. Identified in leukemia, breast, brain, colon cancers. Therapeutic implication: eradicating cancer stem cells may be necessary to prevent relapse. Ethical considerations: ESC research involves destruction of human embryos (blastocysts) iPSC technology partially circumvents this concern. Chimera research, synthetic embryo models, germline editing raise new ethical questions. Regulatory frameworks vary by country.
