What are Cell Junctions?
- Cell junctions are specialized structures that connect one cell to another or to the surrounding extracellular matrix.
- They help in communication, support, and transport between cells.
Types of Cell Junctions
There are 3 major types of cell junctions:
- Tight Junctions (Occluding junctions)
- Adherens Junctions & Desmosomes (Anchoring junctions)
- Gap Junctions (Communicating junctions)
Mnemonic to remember:
"TAG your Junctions"
T – Tight
A – Anchoring (Adherens & Desmosomes)
G – Gap
1. Tight Junctions (Zonula Occludens)
Location:
· Found in intestinal epithelium, renal tubules, blood-brain barrier.
Function:
- Seal adjacent cells tightly, so that no substance leaks between them (paracellular pathway is blocked).
- Maintain polarity of the cell by separating apical and basolateral surfaces.
Structure:
- Made of claudins and occludins.
Remember:
"Tight junctions = Tape between cells"
2. Anchoring Junctions (Zonula Adherens & Desmosomes)
A. Adherens Junctions (Zonula Adherens)
- Connect actin filaments of one cell to another.
- Provide mechanical strength.
- Found in epithelial tissues.
Remember:
"Adherens = Actin = Adherence"
B. Desmosomes (Macula Adherens)
- Connect intermediate filaments (like keratin) of neighboring cells.
- Function like “spot welds” – give tissues tensile strength.
- Found in skin, heart muscle.
Remember:
"Desmosome = Durable Spot weld"
Proteins: Cadherins (desmoglein, desmocollin)
3. Gap Junctions
- Allow direct communication between adjacent cells.
- Permit passage of ions, small molecules (e.g., Ca²⁺, cyclic AMP).
- Found in heart, smooth muscles, nervous tissue – helps in synchronous contraction.
Structure:
- Made of connexons (formed by connexin proteins).
Remember:
"Gap = Google Meet of cells" (cell communication)
Comparison Table of Cell Junctions
|
Junction Type |
Function |
Structure |
Location |
|
Tight |
Seal/leak-proof |
Claudins, Occludins |
Intestine, Brain |
|
Adherens |
Attach via actin |
Cadherins |
Epithelium |
|
Desmosomes |
Strong adhesion via keratin |
Desmoglein, Desmocollin |
Skin, Heart |
|
Gap |
Communication |
Connexins |
Heart, Smooth muscles |
Quick Mnemonics Recap
Mnemonic: TAG-CAD-CON
- T – Tight: Claudins, Occludins
- A – Adherens: Cadherins, Actin
- G – Gap: Connexins, Communication
- D – Desmosomes: Desmoglein, Desmocollin, Keratin
FAQs for Exams
Q1. What is the main function of tight junctions?
Ans: They seal the space between cells to prevent leakage of substances (seen in intestine, BBB).
Q2. Which cell junctions help in mechanical strength?
Ans: Adherens junctions and desmosomes provide mechanical stability by anchoring cytoskeletal filaments.
Q3. What are gap junctions made of?
Ans: Gap junctions are made of connexons, which are composed of connexin proteins.
Q4. Where are desmosomes found?
Ans: Desmosomes are found in skin and cardiac muscle, providing tensile strength.
Q5. Which junction helps in synchronized contraction of heart?
Ans: Gap junctions help in rapid conduction of ions → synchronized contraction of cardiac muscle.
