Molecular Cell Biology: Integrating Cells into Tissues Study Notes

Chapter Overview

  • Chapter Title: Integrating Cells into Tissues

  • Key Topics:

    • Epithelia

    • Cell-Cell and Cell-Extracellular Matrix Adhesion

    • Types of Extracellular Matrix (ECM)

Lecture and Chapter Objectives

  • Outcomes:

    • Describe main types of epithelia.

    • Understand structural-function relationships and implications of structural defects.

    • Identify molecular structures linking epithelial cells.

    • Recognize main molecules associated with these structures.

1. Tissues and Epithelia

  • Overview: Cells aggregate into tissues to perform specific functions.

  • Types of Tissues:

    • Epithelia

    • Connective Tissue

    • Loose and Dense: Structurally support organs; low cellularity, high collagen.

    • Specialized Types: Adipose tissue, cartilage, lymphoid tissue.

    • Muscular

    • Neuronal

    • Blood

2. Principal Types of Epithelia

  • General Features:

    • Cells exhibit apical, lateral, basal surfaces.

    • Layers: Simple, stratified, pseudostratified.

    • Cell forms: Squamous (flat), cuboidal (cube-like), columnar (tall).

  • Specific Types:

    • Simple Columnar Epithelia: Elongated cells, e.g., mucus-secreting gastric lining, absorptive lining of intestine; microvilli increase surface area.

    • Simple Squamous Epithelia: Very thin, e.g., lining blood vessels (endothelium) and body cavities.

    • Transitional Epithelia: Multiple layers with varying shapes; found in organs subject to expansion (e.g., urinary bladder).

    • Stratified Squamous Epithelia: Line surfaces exposed to abrasion (e.g., skin, oral cavity).

    • Pseudostratified Columnar Epithelia: Respiratory tract, ciliated and mucus-secreting cells.

3. Integrating Cells into Tissues

  • Functions of Cell-ECM Interactions: Critical for assembling cells into tissues and regulating cell shape and function.

  • Extracellular Matrix (ECM): Complex network of proteins and polysaccharides supporting tissue structure.

4. Overview of Cell Junctions

  • Basic Types of Cell Junctions:

    • Tight Junctions: Establish epithelial cell polarity; control paracellular flow.

    • Anchoring Junctions: Connect cells to each other (adherens junctions, desmosomes) and to the ECM (hemidesmosomes).

    • Gap Junctions: Facilitate electrical and metabolic coupling between adjacent cells.

5. Molecular Composition of Cell Junctions

  • Anchoring Junctions:

    • Adherens Junctions: Use cadherins for cell-cell adhesion, associated with actin filaments. Calcium-dependent homophilic interactions (e.g., E-cadherin).

    • Desmosomes: Composed of desmosomal cadherins, linking to intermediate filaments, providing strength.

    • Hemidesmosomes: Connect integrins with ECM; important for structural stability.

    • Focal Contacts: Connect cells to ECM, utilizing integrins that bind actin filaments.

  • Tight Junctions: Composed of occludins and claudins, critical for solute control and cell polarity.

  • Gap Junctions: Formed by connexins, allowing small molecules to pass between cells; important for intercellular communication.

6. Cell Junctions: Integrated Structures

  • Molecules Involved:

    • Cadherins: Single-chain glycoproteins crucial for cell adhesion via homophilic interactions.

    • Integrins: Heterodimeric glycoproteins that mediate adhesion to the ECM, varying in binding specificities.

    • Selectins: Involved in heterophilic interactions, mediating cell rolling and tethering during immune responses.

7. Clinical Relevance of Cell Adhesion Molecules

  • Cadherins: Loss of E-cadherin is linked to tumor invasion and metastasis due to loss of adhesion.

  • Integrins: Defects may lead to leukocyte adhesion deficiency, epidermolysis bullosa, and impaired wound healing.

8. Desmosomes and Their Components

  • Construction: Composed of two specialized cadherins (desmoglein, desmocollin) attaching to adapter proteins.

  • Pathology: Pemphigus vulgaris is an autoimmune disease caused by antibodies targeting desmoglein, leading to blistering.

9. Tight Junctions: Structure and Function

  • Characteristics: Formed by a network of proteins; regulate permeability across epithelial linings. Critical for maintaining epithelial barrier integrity.

10. Gap Junctions: Molecular Structure and Diseases

  • Functionality: Composed of connexons, allowing direct communication between adjacent cells.

  • Genetic mutations associated with connexins can lead to neurosensory deafness, cataracts, and heart malformations.

11. Transepithelial Transport Pathways

  • Transport Mechanisms: Paracellular and transcellular transport are essential processes regulated by tight junction integrity. Barrier properties are tissue-dependent.

12. The Extracellular Matrix (ECM)

  • Structure and Components:

    • Fibrous proteins (e.g., collagen, elastin)

    • Hydrated gel (e.g., hyaluronate derivatives)

    • Adhesion glycoproteins

  • Functions:

    1. Support tissue architecture and define boundaries.

    2. Influence mechanical properties of tissues.

    3. Regulate cell survival, migration, and response to external stimuli.

    4. Serve as a reservoir for growth factors, influencing cellular function.

13. Major Components of the Basal Lamina

  • Interaction with Cells: Intermediate filaments connect to the basal lamina via hemidesmosomes (integrins to laminin).

  • Contains Type IV collagen and laminin, forming structural networks essential for tissue integrity.

14. Fibroblasts and Tissue Remodeling

  • Role of Fibroblasts: Principal source of the ECM, crucial in healing and remodeling processes. Their role in cancer progression is also recognized.