Cell Communities: Tissues, Stem Cells, and Cancer in Molecular Biology

Overview

• Examines the organization of cells into tissues and organs within a multicellular organism.
• Explores mechanisms governing tissue assembly and the role of the extracellular environment.
• Considers various cells and their organization into diverse tissues, emphasizing cytoskeletal networks.
• Investigates the extracellular matrix (ECM) and cell junctions in preserving tissue integrity.
• Discusses the significance of stem cells in tissue renewal and disorders like cancer.

Tissue and Organ Organization

• How Tissues Are Constructed
  • Cells combine to form tissues, which further organize to create organs.
  • Emphasis on the extracellular environment's contributions to structural organization.
• Example of Kidney Cells
  • Cells stained with H & E stain highlighted in the extracellular matrix (ECM).
  • Demonstrates tissue types such as epithelial, connective, and muscle tissues.

Strengthening Mechanisms in Plant and Animal Tissues

• Plant Tissues Strengthened by Cell Walls

  • Cell walls surround plant cells, composed of cellulose (blue) and pectin (green).
  • Provides mechanical support to withstand stress.

• Animal Tissues Strengthened by ECM and Cytoskeleton

  • Connective tissue primarily consists of ECM, with fewer cells than other tissue types.
  • Examples include:
  • Rigid bone
  • Flexible tendons
  • Shock-absorbing cartilage
  • Jelly-like vitreous in the eye.
  • Connective tissues bear mechanical stress using cytoskeletal linkages.

Collagen and ECM in Animal Tissues

• Role of Collagen in Connective Tissues

  • Major ECM proteins; collagens are fibrous proteins providing tensile strength.
  • Secreted by fibroblasts and osteoblasts, collagens account for 25% of total protein mass in mammals.
  • Characterized by a triple-helix structure rich in glycine and proline, with a Gly-X-Y amino acid repeat.

• Collagen Synthesis and Assembly

  • Fibrillar collagens are synthesized in the rough endoplasmic reticulum (rER).
  • Procollagen is secreted and cleaved by proteases to form collagen fibrils (trimers).

• Clinical Relevance

  • Defects in procollagen processing can lead to disease, characterized by reduced tensile strength and overstretchable connective tissues.

• Diversity in Collagen Fiber Structures

  • Different tissues form various collagen networks (e.g., skin contains oriented bundles at 90-degree angles).
  • Tendons have parallel collagen fiber bundles, while other tissues may have a meshwork structure.

Cell Interaction with ECM

• Common Properties of Cell Interaction with ECM

  • Cells can either immobilize (attach) or migrate on the ECM.
  • Surface receptors are required for attachment, primarily interacting with fibronectin.

• Fibronectin and Integrins

  • Fibronectin is a protein dimer that binds cells to ECM proteins, including collagens.
  • Integrins act as transmembrane receptors connecting Fibronection with the actin cytoskeleton. They can be activated by binding to cytoskeletal proteins or ECM ligands.

• Basal Lamina (Basement Membrane)

  • An essential ECM type for cell attachment in epithelial cells, using integrins to bind laminin proteins.

Clinical Relevance: Integrin Mutations

• Mutations Impacting Integrins
  • Various integrins and associated diseases, including lethal impacts due to mutations.
  • Examples:
  • a5β1 (fibronectin): Ubiquitous, mutations lead to severe developmental defects.
  • α1β1 (laminin): Present in muscles, critical for muscle development.

Proteoglycans and GAGs in ECM

• Importance of Proteoglycans
  • GAGs such as hyaluronan contribute to ECM structure and functionality, resisting compression.
  • Dense connective tissue has less GAG, while jelly-like ECM is enriched with proteoglycans.

Cancer and Stem Cells

• Cancer as a Disease of Tissue Renewal

  • Disruption of tissue renewal processes leads to uncontrolled cell proliferation and cancer.
  • Characterization of tumors (benign vs malignant based on localized growth vs metastasis).

• Stem Cells in Renewal

  • Adult stem cells are responsible for tissue repair and renewal, committing to specific cell types.
  • Example: Gut epithelium contains stem cells at the base of crypts, which proliferate and differentiate into specialized cells.

Clinical Applications of Stem Cells

• Embryonic Stem Cells (ES Cells)

  • Pluripotent stem cells derived from embryos can differentiate into any cell type, offering potential organ regeneration therapies.
  • Issues with immune rejection must be addressed.

• Somatic Cell Nuclear Transfer

  • A technique for producing ES cells using somatic cell nuclei implanted into enucleated eggs for therapeutic cloning.

• Induced Pluripotent Stem (iPS) Cells

  • Generated from adult cells without destroying embryos, iPS cells hold promise for diverse clinical applications.

Cancer Development and Mechanisms

• Accumulation of Mutations

  • Cancer arises through genetic mutations that may be spontaneous or induced by mutagens.
  • Genetic instability in cancer cells can lead to chromosomal anomalies.

• Natural Selection in Tumor Evolution

  • Cancer cells undergo evolution, acquiring mutations that enhance survival and proliferation, resulting in selective advantages.

• Key Genes in Cancer

  • Oncogenes: Activate mutations leading to increased cell growth and division (e.g., Ras).
  • Tumor Suppressor Genes: Loss-of-function mutations result in uncontrolled cell proliferation (e.g., p53, APC).

• Pathway Impact on Cancer

  • Mutations affect critical pathways influencing cell proliferation, DNA repair, and survival.
  • Example of colorectal cancer illustrates mutation-driven tumor development.

Therapeutic Approaches in Cancer Treatment

• Targeting Oncogenes and Gene Therapy
  • Immunotherapy and specific gene targeting (e.g., Gleevec for chronic myeloid leukemia) are effective treatments.
  • Genetic therapies, like engineered T cells targeting cancer cells, exemplify the integration of gene therapy with immunotherapy strategies.