Cell to Cell

CELL-CELL INTERACTIONS

Ca-Signaling in Neurons

Dictyostelium discoideum

Life Cycle of Dictyostelium discoideum

  • Spores: The reproductive form of the organism.

  • Aggregation: Notable behavior of D. discoideum where individual amoebae come together when food (bacteria) is scarce.

    • Free-living Amoebae: Under favorable conditions, amoebae reproduce and thrive.

  • Cyclic adenosine monophosphate (cAMP): A molecule involved in signaling that promotes aggregation in D. discoideum.

CELL-CELL INTERACTIONS

Epithelial Tissue Structure

  • Basement Membrane: A thin, fibrous layer that provides support and anchorage to epithelial cells.

  • Intercellular Space: The space between adjacent cells in a tissue.

Loose Connective Tissue Structure

  • Components of Loose Connective Tissue:

    • Reticular Fibers: Provide structural support.

    • Melanocyte: Pigment cells located in the skin.

    • Fixed Macrophage: Immune cells that remain in tissue, playing a role in defense.

    • White Blood Cell: Mobile immune cells.

    • Capillary: Small blood vessel, essential for nutrient transport.

    • Red Blood Cell: Carry oxygen throughout the body.

    • Fat Cell (Adipocyte): Store energy and insulate the body.

    • Mast Cell: Release chemicals during allergic reactions and inflammation.

    • Fibroblasts: Cells that secrete matrix proteins essential for tissue structure.

    • Ground Substance: Gel-like matrix that supports cells and fibers within loose connective tissue.

THE CELL AND ITS ENVIRONMENT

Structure of the Cell Membrane

  • Fibers of Extracellular Matrix (ECM): Structural components outside the cell membrane.

    • Glycoprotein and Glycolipid: Molecules made of protein and sugar, crucial for cell interactions.

    • Microfilaments of Cytoskeleton: Provide internal support to the cell.

    • Cholesterol: Stabilizes the membrane structure and fluidity.

    • Peripheral Proteins (Extrinsic): Loosely attached membrane proteins.

    • Integral Proteins (Intrinsic): Firmly embedded membrane proteins.

Functionality of the Plasma Membrane

  • Phospholipid Bilayer: A double-layer of phospholipids with hydrophilic heads and hydrophobic tails.

  • Selective Permeability: Regulates what enters and exits the cell, allowing certain substances while preventing others.

  • Dynamic Function: The membrane can change and accommodate various cellular activities.

EXTRACELLULAR MATRIX IN ANIMALS

General Structure

  • Most animal cells secrete ECM, a fiber composite primarily made of collagen:

    • Provides structural integrity and shape.

    • Ground Substance: Composed of proteoglycans; creates a gel that attracts water.

Composition Variability

  • The ECM composition varies significantly between different tissues (e.g., bone vs. lungs, which contain elastin).

CONNECTION BETWEEN CYTOSKELETON AND ECM

Integrins

  • Cross-linking Glycoproteins: Such as laminins, connect the ECM to the plasma membrane and anchor the cytoskeleton.

EXTRACELLULAR MATRIX IN PLANTS

Primary Cell Wall Structure

  • Primary Cell Wall: Composed of long polysaccharide cellulose strands that form a crisscrossed network and includes gelatinous pectins.

  • Turgor Pressure: Generated by water inflow, important for cell shape and rigidity, serves as a growth mechanism through expansins.

Secondary Cell Wall Structure

  • Secondary Cell Walls: Found in mature plant cells; vary structurally depending on cell function (e.g., wood cells contain lignin).

ADJACENT CELL CONNECTION AND COMMUNICATION

Physical Connections in Multicellularity

  • Adjacency of cells through junctions is crucial for maintaining tissue structure and function.

Types of Cell-Cell Attachments:

  1. Middle Lamella (Plants): A gelatinous layer that glues adjacent plant cells together.

  2. Tight Junctions (Animals): Membrane proteins form a seal between cells to prevent leakage.

  3. Desmosomes (Animals): Operate like molecular "Velcro" to reinforce cell adhesion and resist stress.

  4. Gap Junctions (Animals): Special channels that allow ion and small molecule passage for communication.

Cell Communication Mechanisms

  • Cells that are not physically connected may communicate through signaling molecules (e.g., neurotransmitters and hormones).

Hormones

  • Information-carrying molecules derived from cells, traveling through bodily fluids to act on target cells.

  • Lipid-soluble: Cross plasma membranes easily and bind to intracellular receptors.

  • Lipid-insoluble: Bind to receptors located on the cell surface, triggering signal transduction.

SIGNAL TRANSDUCTION PATHWAYS

General Process

  1. Binding of signaling molecules to receptors triggers a cascade of intracellular signals.

  2. Secondary Messengers: Small molecules that rapidly diffuse and amplify signal response.

  3. Enzyme-Linked Receptors: Transmit signals through phosphorylation cascades, primarily involving receptor tyrosine kinases (RTKs).

Key Second Messengers

  • Calcium Ion (Ca2+): Interacts with calmodulin.

  • Cyclic adenosine monophosphate (cAMP): Activates protein kinases.

  • Cyclic guanosine monophosphate (cGMP): Involved in opening ion channels.

  • Diacylglycerol (DAG) and Inositol trisphosphate (IP3): Both play roles in calcium channel regulation and signal amplification.

CELLS RESPOND TO SIGNALS

Changes in Gene Expression

  1. Modifying gene expression in target cells.

  2. Altering activation status of existing proteins.

Signal Deactivation

  • Critical for maintaining cellular sensitivity to signals.

  • Mechanisms include the activity of phosphatases that remove phosphate groups from proteins in signaling cascades.

Crosstalk

  • Signal pathways can intersect, providing integration of cellular responses to multiple signals. - May inhibit or stimulate different pathways and responses.

SIGNALLING BETWEEN UNICELLULAR ORGANISMS

Quorum Sensing

  • Signaling pathways in unicellular organisms that respond to population density by releasing species-specific signaling molecules.

  • Responses to such cues might involve biofilm formation, e.g., plaque on teeth.

Example of Quorum Sensing in Bacteria

  • Allows bacteria to coordinate activities as community organisms, influencing collective behaviors.

SIGNALING EXAMPLES

Unicellular Coordination

  • Example: Slime molds aggregate in response to signaling molecules, showcasing a collective response to environmental changes.