Cell junctions

📌 Lateral Cell Surface Specializations

🟣 i. Cellular Interdigitations
· Description: Interlocking folds between adjacent cells.
· Function: Increase surface area for transport.
· Site: Intestinal & kidney tubule cells. (Exam Point)

🟠 ii. Intercellular Junctions

Link neighboring epithelial cells.

🖇 1. Tight Junction (Zonula Occludens)
· Site: Apical part of cells.
· Function: Prevents molecular passage between cells (seal). (Exam Point)
· Structure: Fused outer membranes via occludin & claudin proteins. (Exam Point)

🖇 2. Anchoring Junctions
· Site: Areas of mechanical stress.
· Examples: Cardiac muscle, epidermis. (Exam Point)
· Function: Strong attachment between cytoskeletons of adjacent cells.
· Types:

• Zonula Adherens:
• • Belt-like junction near apical region.
  • Intercellular space 20 nm.
  • Cadherins (Ca²⁺ dependent) link cells.
  • Cytoplasmic ends attach to actin filaments. (Exam Point)
• Macula Adherens (Desmosome):
• • Spot-like junction.
  • Dense plaques on cytoplasmic sides anchor tonofilaments (cytokeratin).
  • Cadherins connect cells.
  • Intercellular space 30 nm with dense vertical material. (Exam Point)

🖇 Junctional Complex (Exam Point)
· Combination of Zonula Occludens, Zonula Adherens, and Macula Adherens near apical region.

🖇 3. Communicating Junction (Gap Junction / Nexus)
· Sites: Epithelial, cardiac muscle, smooth muscle cells.
· Function: Exchange of small molecules and electrical/chemical signals. (Exam Point)
· Structure:

• Spot-like protein channels called connexons.
• Each connexon = 6 connexins.
• Two opposing connexons align to form cytoplasmic channel.
• Intercellular space ~3 nm. (Exam Point)