Cellular Junctions and Signaling
Intercellular Junctions
Desmosomes
- Intercellular junctions that provide strong adhesions between cells.
- Linked intercellularly to the intermediate filament cytoskeleton (visually indicated by blue lines).
- Form adhesive bonds and networks that impart mechanical strength to tissues.
Tight Junctions
- Perform two vital functions:
- Limit Passage of Molecules and Ions: Restrict the movement of molecules and ions through the space between cells.
- Most materials must enter the cell either by diffusion or active transport, which is now more tightly regulated through the tissues themselves.
- Provides tighter control over what substances are allowed through and when.
- Block Movement of Membrane Proteins: Prevent membrane proteins from moving between the apical and basolateral surfaces of the cells.
- This preserves the special functions of each cell (e.g., endocytosis or exocytosis).
- Limit Passage of Molecules and Ions: Restrict the movement of molecules and ions through the space between cells.
- Characterized by having no space between them, as observed visually.
- Perform two vital functions:
Gap Junctions
- Specialized intracellular connections that directly connect the cytoplasm of two different cells.
- Allow molecules, ions, and various electrical impulses to directly pass through a regulated gate between the cells.
- Distinguishing features:
- Desmosomes are attached to the cytoskeleton.
- Gap junctions facilitate cytoplasm-to-cytoplasm connections, allowing easier transport of materials.
- Tight junctions have no space between cells, unlike gap junctions which allow direct passage.
Cellular Communication and Signaling
Importance of Cellular Communication
- Essential for cells to maintain homeostasis.
- Regulates growth and division within tissues.
- Coordinates various cellular functions.
Types of Contact-Dependent Signals (Cell Signaling Mechanisms)
- Paracrine Signaling:
- A cell produces a signal.
- This signal induces changes in nearby cells.
- Thereby altering the behavior of those nearby cells.
- Autocrine Signaling:
- A cell secretes a hormone or a chemical messenger (called an autocrine agent).
- This agent binds to an autocrine receptor located on that same cell.
- Leads to changes within the cell of itself.
- Hormonal Signaling:
- Involves the recognition of a hormone by an associated cell membrane or intracellular protein.
- This recognition then acts on the cell to elicit a response.
- Neurohormonal Signaling:
- A neurohormone is any hormone produced and released by neuroendocrine cells into the bloodstream.
- It is then uptaken by the target cell, leading to cellular changes.
- Neurotransmitters and Various Chemicals:
- These substances cross barriers and communicate within each cell, facilitating signaling.
- Paracrine Signaling:
Signal Transduction
- Definition: The transmission of a molecular signal from the outside to the inside environment of the cell.
- Process: A signal is conveyed from a signal cell to a target cell via receptor proteins.
- Purpose: Ensures that the cell produces an appropriate response to the external signal.
- Initiation: Typically initiated by surface proteins on the target cell.
- Extracellular First Messenger:
- Initially conveys instructions to the cell's interior.
- Example: Ion Channel:
- An ion channel can either open or close.
- This action allows for an influx or deflux of ions into or out of that cell.
- Secondary Messenger:
- The first extracellular messenger initiates a secondary intracellular messenger.
- This secondary messenger is a molecule that relays the signal received at the cell's surface.
- It then relays this signal to the internal machinery or constituents of the cell.
- Examples include signals from arriving protein hormones or growth factors.
- Provides information that instructs the cell to perform a specific function (e.g., proliferation, differentiation, migration, basic survival, or apoptosis).
- Illustrative Pathway Example:
- A ligand binds to a specific protein in the cell membrane.
- This binding initiates a step-by-step process within the cell.
- This internal process (e.g., creating a specific biochemical pathway for energy) then acts directly on the nucleus itself.
- A channel receptor can be involved in this initiation or subsequent steps.