NOTES

Cell Junction and Membrane Junction

Cell junction or intercellular junction → connection between two neighboring cells.

Tight junction, gap junction, adherence junction and desmosome are intercellular junctions.

Membrane junction → contact between cell and extracellular matrix.

Membrane junctions are:

• Focal adherence
• Hemidesmosome

Classification of Cell Junctions

Cell junctions are classified into 3 types:

1. Occluding junction
2. Communicating junction
3. Anchoring junction

I. OCCLUDING JUNCTIONS

Cell junctions which prevent the movement of ions and molecules from one cell to another cell → occluding junctions.

Tight junctions belong to this category.

Tight Junction

• Tight junction is formed by tight fusion of cell membranes from adjacent cells.
• Area of fusion is very tight and forms a ridge.
• Present in the apical margins of epithelial cells in:
  • Intestinal mucosa
  • Wall of renal tubule
  • Capillary wall
  • Choroid plexus

Proteins Involved in Tight Junction

1. Occludin
2. Claudin
3. JAMs
4. Cingulin
5. Symplekin
6. ZO-1, 2, 3

Functions of Tight Junction

1. Holds neighboring cells of tissues firmly → provides strength and stability to tissues.
2. Provides barrier/gate function by regulating interchange of:
   • Ions
   • Water
   • Macromolecules
   between the cells.
3. Acts like a fence by preventing lateral movement of integral membrane proteins and lipids from the cell membrane.
4. By fencing function, maintains cell polarity by keeping proteins in the apical region of cell membrane.
5. Tight junctions in brain capillaries form the blood-brain barrier (BBB), which prevents entrance of many harmful substances from blood into brain tissues.

II. COMMUNICATING JUNCTIONS

Cell junctions which permit movement of ions and molecules from one cell to another cell → communicating junctions.

Types of Communicating Junctions

1. Gap junction or nexus
2. Chemical synapse

1. Gap Junction or Nexus

Gap junction or nexus is present in:

• Heart
• Basal part of epithelial cells of intestinal mucosa
• Etc.

Structure of Gap Junction

• Covering membranes of two adjacent cells lie very close to each other.
• Intercellular space becomes a narrow channel.
• Cytoplasm of both cells is interconnected.
• Molecules move from one cell to another through these channels without having contact with extracellular fluid (ECF).
• The channel is surrounded by 6 subunits of proteins, called connexins or connexons.

Functions of Gap Junction

1. Diameter of channel → about 1.5–3 nm.
   • Substances having molecular weight less than 1,000, such as glucose, can pass through this junction easily.
2. Helps in exchange of chemical messengers between cells.
3. Helps in rapid propagation of action potential from one cell to another cell.

2. Chemical Synapse

Chemical synapse → junction between:

• A nerve fiber and a muscle fiber, or
• Two nerve fibers.

Signals are transmitted through the release of chemical transmitter.

III. ANCHORING JUNCTIONS

Anchoring junctions → junctions which provide firm structural attachment:

• Between two cells, or
• Between a cell and extracellular matrix.

Anchoring junctions are responsible for the structural integrity of tissues.

Present in tissues like:

• Heart muscle
• Epidermis of skin

These tissues are subjected to severe mechanical stress.

Firm attachment between two cells or between a cell and extracellular matrix is provided by either:

• Actin filaments, or
• Intermediate filaments.

Types of Anchoring Junctions

A. Actin filament attachment

1. Adherens junction → cell to cell
2. Focal adhesion → cell to matrix

B. Intermediate filament attachment

3. Desmosome → cell to cell
4. Hemidesmosome → cell to matrix

1. ADHERENS JUNCTIONS

• Adherens junction → cell-to-cell junction.
• Connection occurs through actin filaments.
• Present in:

• Intercalated disk of cardiac muscles
• Epidermis of skin

Protein Involved

Cadherins.

Function

Cell-to-cell attachment.

2. FOCAL ADHESIONS

• Focal adhesion → cell-to-matrix junction.
• It is the junction between:
  • Cell
  • Extracellular matrix
• Connection occurs through actin filaments.
• Seen in epithelia of various organs.

Protein Involved

Integrins.

Function

Cell attachment to:

1. Basal lamina
2. Extracellular matrix

3. DESMOSOME

• Desmosome → cell-to-cell junction.
• Membranes of cells are thickened and connected by intermediate filaments.
• Desmosome functions like tight junction.
• This type of junction is found in areas subjected to stretching, such as skin.

Protein Involved

Cadherins.

Function

Cell-to-cell attachment.

4. HEMIDESMOSOME

• Hemidesmosome → cell-to-matrix junction.
• Connection occurs through intermediate filaments.
• It is like half desmosome because membrane of only one cell thickens.
• Therefore, it is known as:

• Hemidesmosome
• Half desmosome

Mostly, hemidesmosome connects cells with their basal lamina.

Protein Involved

Integrins.

Function

Cell attachment to:

1. Basal lamina
2. Extracellular matrix

CELL ADHESION MOLECULES

Cell adhesion molecules (CAMs) or cell adhesion proteins → protein molecules situated on the surface of cell.

Functions of Cell Adhesion Molecules

1. CAMs are responsible for binding cells:
   • To their neighbors, or
   • To basal lamina (or basal membrane).
2. CAMs form important structures of intercellular connections and are responsible for structural organization of tissues.

Types of Cell Adhesion Molecules

1. Cadherins

• Form molecular limbs between neighboring cells.
• Form:
• Adherens junction
• Desmosome

2. Integrins

• Form:
• Focal adhesion
• Hemidesmosome

3. IgG Superfamily Molecules

• Form cell adhesion molecules in the nervous system.

4. Selectins

• Act as receptors for carbohydrates:
• Ligand
• Mucin
• Found in:
• Platelets
• Endothelial cells

CELL JUNCTIONS — SOURCE TABLE