Cell-Cell Junctions and the Extracellular Matrix: Structure and Function in Animal Tissues

Cell-Cell Junctions and the Extracellular Matrix

Introduction

This study guide covers the structure and function of cell-cell junctions and the extracellular matrix (ECM) in animal tissues, focusing on epithelial cells. These components are essential for maintaining tissue integrity, enabling communication, and providing mechanical support.

Columnar Epithelial Cells

Structure and Function

• Columnar epithelial cells are elongated cells found lining organs such as the intestine.

• They are shaped as columns to maximize surface area for absorption and secretion.

• Their shape is maintained by interactions with neighboring cells and the underlying extracellular matrix.

Example: Intestinal epithelial cells are columnar to facilitate nutrient absorption.

Cell Adhesion: Historical Experiments

Sponge Cell Adhesion Experiment

• In 1907, H.Y. Wilson demonstrated that sponge cells, when dissociated and mixed, could reaggregate and sort themselves by type.

• This experiment showed that cells possess specific adhesion molecules that allow them to recognize and bind to similar cells.

Amphibian Embryo Experiment

• Johannes Holtfreter extended these findings to amphibian embryos, showing that dissociated neural and epidermal cells reaggregate according to tissue type.

• This sorting is mediated by cell adhesion molecules.

Cell Adhesion Molecules

Cadherins

• Cadherins are transmembrane proteins that mediate cell-cell adhesion.

• Different cell types express different cadherins, ensuring selective attachment between similar cells.

• Cadherins connect to the cytoskeleton, providing structural integrity.

Integrins

• Integrins are transmembrane proteins that connect cells to the extracellular matrix.

• They link the cytoskeleton (microfilaments and intermediate filaments) to ECM components, reinforcing tissues under stress.

• Integrins are crucial for anchoring cells and transmitting signals between the ECM and the cell interior.

Cell Junctions

Overview

Cell junctions are protein complexes in the plasma membrane that allow cells to adhere to one another and communicate. They are especially important in epithelial tissues.

Types of Cell Junctions

• Anchoring Junctions: Provide mechanical stability by connecting the cytoskeletons of adjacent cells or to the ECM.

• Barrier Junctions: Prevent the passage of substances between cells.

• Communication Junctions: Allow the transfer of ions and small molecules between cells.

Anchoring Junctions

Adherens Junctions

• Connect the actin cytoskeleton of one cell to another via cadherins.

• Form a "belt" around the cell, providing structural support.

Desmosomes

• Connect intermediate filaments of adjacent cells via cadherins.

• Form "snap" junctions, providing strong adhesion, especially in tissues under mechanical stress (e.g., skin, heart).

Hemidesmosomes

• Connect intermediate filaments inside the cell to the extracellular matrix via integrins.

• Anchor epithelial cell layers to underlying connective tissue.

Barrier Junctions

Tight Junctions

• Form a seal between adjacent cells using proteins such as claudins and occludins.

• Prevent the movement of water and solutes between cells, maintaining distinct apical and basal sides.

• Essential for creating tissue barriers (e.g., intestinal lining).

Communication Junctions

Gap Junctions

• Formed by connexin proteins, creating channels between adjacent cells.

• Allow ions and small signaling molecules to pass directly from one cell to another.

• Enable coordinated activity, such as synchronized contraction in cardiac muscle.

Extracellular Matrix (ECM)

Structure and Components

• The ECM is a complex meshwork of proteins and polysaccharides outside cells, especially abundant in connective tissue.

• Major components include collagen, elastin, and fibronectin.

• Produced mainly by fibroblast cells.

Functions of the ECM

• Provides structural support to tissues.

• Influences cell shape and behavior.

• Facilitates cell signaling and communication.

Major ECM Proteins

Collagen

• Most abundant protein in animals; provides tensile strength to tissues.

• Type I collagen is predominant in skin and connective tissue.

• Structure: Triple helix of three polypeptide chains, forming fibrils and fibers.

Example: Type I collagen in human skin supports tissue integrity and resists stretching.

Elastin

• Provides elasticity and flexibility to tissues.

• Composed of cross-linked, unstructured polypeptide chains with hydrophobic regions.

• Allows tissues such as lungs and blood vessels to stretch and recoil.

Summary Table: Cell Junctions

Junction Type	Main Proteins	Function	Connection
Adherens Junction	Cadherins	Anchoring	Actin cytoskeleton to actin cytoskeleton
Desmosome	Cadherins	Anchoring	Intermediate filaments to intermediate filaments
Hemidesmosome	Integrins	Anchoring	Intermediate filaments to ECM
Tight Junction	Claudins, Occludins	Barrier	Seal between cells
Gap Junction	Connexins	Communication	Channel between cells

Key Equations

• Collagen Triple Helix: $\text{Collagen} = 3 \times \text{polypeptide chains}$

• Cell Adhesion Strength: $\text{Force} = \text{Tensile strength} \times \text{Cross-sectional area}$

• ECM Elasticity (Hooke's Law): $F = kx$

Additional info: The notes have been expanded to include definitions, examples, and a summary table for clarity and completeness.