Plasma Membranes 3

Cellular Biology & Homeostasis

  • Cell Membrane

  • Part 3 - Cell Junctions

  • VP 2025

  • Instructor: Clara Camargo, DVM, Cert AqV

Objectives

  1. Identify junctions along the lateral and basal areas of the cell.

  2. Describe the structure of the main types of cell-cell junctions and cell-matrix junctions.

  3. Describe the components of the junctional complex.

  4. Understand the physiological relevance (functions) of each junction to cells and tissues.

Tissue Formation

  • Some cell types move freely (e.g., blood cells, immune cells).

  • Many cells are densely packed into tissues, which are assemblies of similar cells and their extracellular matrix from the same origin that perform specific functions.

Types of Tissues

  • Muscular Tissue:

    • Cardiac Muscle

    • Skeletal Muscle

    • Smooth Muscle

    • Example: Brain, Spinal Cord

  • Nerve Tissue:

    • Nerve Cells

  • Epithelial Tissue:

    • Lining of gastrointestinal tract and other hollow organs

  • Connective Tissue:

    • Tendon, Bone, Soft Padding (Fat cells)

    • Example Structures: Sinusoids, Central vein, Bile duct

Cell Junctions

  • Definition:

    • Multiprotein complexes that provide contact between neighboring cells and between a cell and the extracellular matrix.

  • Functions of Cell Junctions:

    1. Bond cells together.

    2. Reduce mechanical stress on cells.

    3. Build the paracellular barrier of epithelia.

    4. Control the paracellular transport.

    5. Enable intercellular communication.

    6. Adhesion of cells to the extracellular matrix.

Structure of Tissues

  1. Epithelial Tissue:

    • Covers body surfaces and lines body cavities and hollow organs.

    • Functions: Protection, secretion, absorption, excretion, filtration, diffusion, sensory reception.

    • Cells are tightly bound into sheets (epithelia).

    • Attachments made by cell-cell junctions; anchored to basal lamina via cell-matrix junctions.

  2. Connective Tissue:

    • ECM produced by fibroblasts, sparsely distributed in interstitial matrix.

    • Direct attachments between cells are rare; epithelial cells link to ECM through cell-matrix junctions.

Types of Cell Junctions

Cell-Cell Junctions

  1. Tight Junctions

  2. Cell-Cell Anchoring Junctions

    • a. Adherens Junctions

    • b. Desmosomes

  3. Gap Junctions (Communicating Junctions)

Cell-Matrix Junctions

  1. Cell-Matrix Anchoring Junctions

    • a. Actin Linked Cell-Matrix Junctions

    • b. Hemidesmosomes

Junctional Complex

  • Located at the apex of the lateral membrane of polarized epithelial cells.

  • Involved in regulating cell-cell adhesion, paracellular permeability, and cell polarity.

Types of Specific Junctions

1. Tight Junctions

  • Seal intercellular spaces, preventing movement between epithelial cells.

  • Impermeable to macromolecules and selectively permeable to ions/small molecules.

  • Proteins: Claudin and Occludin.

  • Examples: Lining of intestinal mucosa, urinary bladder, respiratory tract.

2. Cell-Cell Anchoring Junctions

  • Form strong, membrane-spanning structures tethered to cytoskeleton.

  • Two forms: Adherens junctions and desmosomes.

  • Cadherins (calcium-dependent adhesion proteins) are crucial for binding.

  • Adherens Junctions are located basal to tight junctions.

3. Desmosomes

  • Connect intermediate filaments in adjacent cells, providing mechanical strength.

  • Key in tissues under stress (skin, muscles, intestines).

4. Gap Junctions

  • Allow passage of small molecules (e.g., ions, glucose) between cells.

  • Important in tissues like cardiac muscle for rapid action potential spread.

Cell-Matrix Anchoring Junctions

  • Depend on Integrins which sense mechanical forces across junctions.

  • Integrins maintained connections to the ECM.

  • Actin-Linked Cell-Matrix Junctions: Focus on anchoring actin filaments to ECM.

  • Hemidesmosomes: Anchor intermediate filaments to ECM, stabilizing epithelial cells to the basement membrane.

Conclusion

  • Cell Junctions play crucial roles in maintaining tissue integrity and communication in cellular processes.

  • Understanding these mechanisms is vital for comprehending physiology and pathology.

Contact Information

  • Instructor: Clara Camargo

  • Email: ccamargo@rossvet.edu.kn

Happy Studying!