Epithelial Sheets and Cell Junctions

Epithelial Sheets

Layers of epithelial cells joined side-by-side that form protective barriers and line body surfaces

Functions of Epithelial Sheets

Protection, absorption, secretion, transport, and barrier formation

Simple Epithelium

Single layer of cells specialized for absorption, diffusion, and secretion

Stratified Epithelium

Multiple layers of cells specialized for protection

Cell Polarity

Structural and functional differences between apical and basal surfaces of epithelial cells

Apical Surface

Free surface exposed to external environment or lumen

Basal Surface

Surface attached to underlying basal lamina

Basal Lamina

Thin extracellular matrix layer anchoring epithelial cells

Extracellular Matrix (ECM)

Network of extracellular proteins providing support and communication

Major ECM Components

Collagen, proteoglycans, fibronectin, and integrins

Collagen

Main structural protein providing tensile strength

Fibronectin

ECM glycoprotein involved in cell adhesion and migration

Integrins

Transmembrane receptors connecting cells to extracellular matrix

Functions of ECM

Support, adhesion, movement, and regulation of cell behavior

Cell Junctions

Specialized structures connecting neighboring cells or cells to extracellular matrix

Three Major Cell Junction Categories

Tight junctions, anchoring junctions, and communicating junctions

Tight Junction

Occluding junction that seals neighboring cells and prevents leakage between them

Main Proteins in Tight Junctions

Claudins and occludins

Functions of Tight Junctions

Prevent diffusion, maintain cell polarity, and create selective barriers

Example of Tight Junction Function

Blood-brain barrier

Anchoring Junction

Junction providing mechanical attachment and tissue strength

Adherens Junction

Anchoring junction where cadherins connect neighboring cells and link to actin filaments

Cadherin

Cell adhesion protein that binds neighboring cells together

Function of Adherens Junction

Allows coordinated shape changes through actin contraction

Desmosome

Anchoring junction connecting neighboring cells through intermediate filaments

Function of Desmosomes

Provide strong tensile strength and resist mechanical stress

Hemidesmosome

Anchoring junction attaching cells to the basal lamina

Main Protein in Hemidesmosomes

Integrin

Difference Between Desmosomes and Hemidesmosomes

Desmosomes connect cell-to-cell; hemidesmosomes connect cell-to-ECM

Gap Junction

Communicating junction allowing direct exchange between adjacent cells

Main Protein in Gap Junctions

Connexins

Function of Gap Junctions

Allow ions and small molecules to pass directly between cells

Importance of Gap Junctions

Rapid cell communication and electrical signaling

Gap Junction Size Limit

Allows passage of molecules generally less than 1000 Da

Cell Cortex

Meshwork of proteins beneath plasma membrane that supports cell shape

Functions of Cell Cortex

Mechanical strength, movement, shape maintenance, and membrane organization

Spectrin

Cell cortex protein important in red blood cells

Dystrophin

Cell cortex protein important in muscle cells

Cell Wall

Extracellular structure providing protection and support in plants

Main Component of Plant Cell Wall

Cellulose

Functions of Plant Cell Wall

Protection, shape maintenance, and prevention of excess water uptake

Primary Cell Wall

Thin flexible outer wall formed first

Middle Lamella

Layer connecting neighboring plant cells

Secondary Cell Wall

Additional rigid layer deposited inside primary wall

Plasmodesmata

Channels connecting adjacent plant cells

Function of Plasmodesmata

Allow transport of water, small molecules, proteins, and signals between cells

Most Important Junction Comparison

Tight junction = seal, Anchoring junction = strength, Gap junction = communication