8. Cells Connections in Different Tissues: Epithelium and Mesenchyme

Acknowledgment

  • Dr. Sonja McKeown, Department of Anatomy & Developmental Biology, Monash University
  • Acknowledges Dr. Julia Young and the Kulin Nations, the traditional custodians of the land.

Learning Outcomes

At the end of the session, you should be able to:

  • Define the extracellular matrix (ECM).
  • Describe key features of connective tissues, including their composition.
  • Explain composition and function of the six types of cellular junctions.
  • Compare and contrast features of epithelial and mesenchymal tissues.
  • Describe functions of epithelia and their major characteristics.
  • Classify epithelia by shape, layers, and specializations.
  • Address the role of cell adhesion molecules in early development and tissue formation.

Definition of Tissues

  • A tissue is a coherent group of cells bound by cell junctions or the extracellular matrix (ECM).
  • Tissue types include:
    • Epithelia
    • Connective tissue
    • Muscle
    • Nervous tissue
  • Each tissue type has a specialized function and organization.

Extracellular Matrix (ECM)

  • The ECM consists of proteins and other molecules outside cells.
  • Functions of ECM:
    • Forms barriers/connections (e.g., organ capsules, tendons).
    • Serves as a loose network for cell organization.
    • Acts as a substrate for cell attachment (e.g., epithelial cells on basal lamina).
    • Provides pathways for cell migration.
  • Major classes of ECM molecules:
    • Fibrous proteins (collagens)
    • Proteoglycans and glycosaminoglycans (e.g., hyaluronan)
    • Glycoproteins (e.g., fibronectin, laminin)
  • ECM varies by site and changes during development.

Connective Tissue Components

  • Cells of connective tissues include:
    • Fibroblasts, chondrocytes, osteocytes
    • Immune cells (e.g., macrophages)
  • ECM fibers consist of:
    • Collagen (various types)
    • Elastic fibers
    • Reticular fibers (delicate)
  • Ground substance contains:
    • Proteoglycans (e.g., HSPG, CSPG)
    • Glycosaminoglycans (GAGs)
    • Glycoproteins (e.g., laminin, fibronectin)

Classifications of Connective Tissues

  1. Loose connective tissue (e.g., lamina propria, adipose tissue)
  2. Dense connective tissue:
    • Dense regular (tendon, ligament)
    • Dense irregular (dermis, organ capsules)
  3. Blood (considered loose connective tissue)
  4. Cartilage: hyaline, elastic, fibrocartilage
  5. Bone: compact and spongy

Cell Junctions

  • Cell junctions connect cells to each other and to the ECM and are crucial for:
    • Stress transmission
    • Cell communication
    • Signal transduction
Epithelia vs. Mesenchyme
  • Epithelia:
    • Organized as tightly adherent sheets
    • Non-motile and polarized
    • Apical and basal surfaces distinct
  • Mesenchyme:
    • Cells are often motile
    • Less organized in ECM
    • Not polarized

Six Types of Cell Junctions

  1. Tight Junctions (Zonula Occludens):
    • Prevent passage of molecules; maintain integrity of epithelial layers.
    • Block movement of integral membrane proteins between apical and basolateral surfaces.
  2. Adherens Junctions (Zonula Adherens):
    • Provide adhesion via actin filaments; connect to similar bundles in neighboring cells.
  3. Desmosomes (Macula Adherens):
    • Strong junctions that connect the intermediate filaments between cells.
  4. Gap Junctions (Nexus):
    • Allow ion and small molecule passage (e.g., cyclic AMP, Ca2+).
  5. Focal Adhesions:
    • Connect actin filaments to ECM proteins (e.g., collagen, fibronectin).
  6. Hemidesmosomes:
    • Anchor epithelial cells to the basal lamina.

Characteristics of Epithelial Cells

  • Polarity: Apical and basal surfaces with distinct functions.
  • Specialized Contacts: Tight junctions and desmosomes aid in adhesion and barrier function.
  • Basement Membrane: Provides support and connects to underlying connective tissue.
  • Avascularity: Epithelia have no blood vessels but are innervated.
  • Renewal: Epithelial cells can regenerate efficiently.

Epithelial Classification

  • By Shape:
    • Squamous (flat)
    • Cuboidal (square)
    • Columnar (tall)
  • By Layers:
    • Simple (single layer)
    • Stratified (multiple layers)
    • Pseudostratified
    • Transitional
  • Specializations:
    • Microvilli (increased surface area)
    • Cilia (movement of substances)
    • Goblet cells (secretion of mucus)

Epithelial-Mesenchymal Interactions

  • Fate of epithelial tissues is influenced by mesenchymal signals.
  • Tissue recombination experiments demonstrate mesenchyme control over epithelial development.

Cell Adhesion Molecules (CAMs)

  • Critical in cell adhesion and communication during development.
  • Types of CAMs include:
    • Cadherins: homophilic attachments.
    • Integrins: heterophilic attachments; connect to ECM.
    • Ig superfamily proteins: varied binding capabilities.

Cell Adhesion and Morphogenesis

  • Adjustments in cell adhesion affect structure, migration, and organization in tissues.
  • Variations in CAM expression dictate adhesive properties and functionality during development.