4. Cells and Tissues 2

what is epithelium

Epithelia are coherent sheets of cells that cover body surfaces and line cavities. They are supported by a basement membrane that separates them from underlying connective tissue

functions of epithelium

• protection

• absorption

• secretion

• selective diffusion/transport

how are epithelia classified

• number of layers

• cell shape

• surface specialisations

how are epithelia cells classified by number of layers

Type	Description
Simple	Single cell layer
Stratified	Multiple cell layers

how are epithelia cells classified by cell shape

Shape	Appearance
Squamous	Flat
Cuboidal	Cube-shaped
Columnar	Tall cells

how are epithelia classified by their surface specialisations

• cilia

• microvilli

general features of simple epithelia

• Single layer of cells.

• Found where exchange occurs between different environments.

• Involved in diffusion, absorption and secretion

features of simple squamous epithelial cells

• flat, thin cells

• minimal diffusion distance

function of simple squamous epithelia cells

rapid diffusion and exchange

example of simple squamous epithelial cells

• blood vessel lining (endothelium)

• alveoli of lungs

features of simple cuboidal epithelium

• cube-shaped cells

• central round nuclei

• more cytoplasm than squamous cells

functions of simple cuboidal epithelium

secretion and absorption

example of simple cuboidal epithelium

kidney tubules, small glad ducts

features of simple columnar epithelium

• tall cells

• nuclei near basement membrane

• may possess microvilli or cilia

functions of simple columnar epithelium

• absorption

• secretion

• transport

example of simple columnar epithelium

• intestinal epithelium

• parts of respiratory tract

features of stratified epithelia

• more than one cell layer

• primarily protective

• resistant to physical and chemical damage

features of stratified squamous epithelium

• multiple layers of cells

• surface cells are squamous

function of stratified squamous epithelia

protection against abrasion

examples of stratified squamous epithelia

• oral cavity

• vagina

• anal canal

• skin (keratinised form)

three key epithelial cell specialisations

1. junctions

2. polarity

3. glands

how are cell-extracellular matrix junctions enabled

by integrins

which are transmembrane adhesion proteins that link the cell's cytoskeleton to the components of the extracellular matrix (such as collagen, laminin, and fibronectin)

cell-extracellular matrix junctions: how do cells attach to the ECM

through integrins

intern-mediated adhesion

cells attach to the ECM through integrins

functions of integrin-mediated adhesion

• anchor cells to ECM

• connect ECM to intracellular cytoskeleton

• allow cells to sense and respond to their environment

example of integrin-mediated adhesions

• focal adhesions (actin)

• hemidesmosomes (intermediate filaments)

focal adhesions (actin)

sites where a cell attaches to the ECM and links the ECM to the actin cytoskeleton

structure of focal adhesions

Outside cell:

• ECM proteins (e.g. fibronectin, collagen, laminin)

Cell membrane:

• Integrins (transmembrane receptors)

Inside cell:

• Adaptor proteins such as talin, vinculin, paxillin, and α-actinin

• Connected to actin filaments (microfilaments)

functions of focal adhesion sites

• Cell adhesion to the ECM

• Cell migration (formation and breakdown of focal adhesions during movement)

• Mechanotransduction (sensing and responding to mechanical forces)

• Signal transduction affecting growth, survival, and differentiation

hemidesmosomes

cell-ECM junctions that anchor epithelial cells to the basement membrane

structure of hemidesmosomes

Extracellular side:

• Laminin in the basement membrane

Transmembrane proteins:

• Integrin α6β4 (main receptor)

• BP180 (collagen XVII)

Intracellular side:

• Plaque proteins (e.g. plectin, BP230)

• Connected to keratin intermediate filaments

function of hemidesmosomes

• Provide strong adhesion between epithelial cells and the basement membrane.

• Resist mechanical stress (important in skin and other epithelia).

• Prevent epithelial layers from detaching from underlying tissues.

3 major classes of cell-cell junctions and their functions

Junction Type	Function
Occluding	Seal spaces between cells
Communicating	Allow communication
Anchoring	Provide mechanical strength

functions of occluding/tight junctions

• Seal adjacent cell membranes.

• Prevent leakage between epithelial cells.

• Form selective barriers.

• Separate apical and basolateral membrane domains

• e.g. bladder epithelium preventing urine leakage

major proteins in occluding/tight junctions

claudins and occludins

structure of communicating/gap junctions

• channels connect neighbouring cells

• build from proteins called connexins

function of communicating/gap junctions

allow movement of:

• ions

• small metabolites

• signalling molecules (<1000 Da)

importance of communicating/gap junctions

• cell-cell communication

• electrical coupling in some tissues

function of anchoring junctions

link cytoskeletons of neighbouring cells into a mechanically strong sheet

2 major types of anchoring junctions

1. adherens junctions

2. desmosomes

components of adherens junctions

• cadherins

• catenins

• actin cytoskeleton

function of adherens junctions

maintain tissue architecture

how do adherens junctions structure support their function

Cadherins (transmembrane proteins)	Cadherins on adjacent cells bind to each other, creating direct cell-cell adhesion.
Calcium-dependent binding	Ca²⁺ stabilizes cadherins, allowing strong, specific adhesion between neighbouring cells.
Catenins and other adaptor proteins	Link cadherins to the actin cytoskeleton inside the cell.
Actin filament attachment	Forms a continuous actin network across many cells, allowing forces to be transmitted through a tissue.
Belt-like arrangement (zonula adherens)	Encircles the cell, distributing tension evenly and helping maintain tissue architecture.

example of adherens junctions in epithelia

In epithelial sheets, adherens junctions form an adhesion belt just below tight junctions. Because they are connected to contractile actin filaments, they can change cell shape during processes such as:

• Tissue folding

• Wound healing

• Embryonic morphogenesis

components of desmosomes

• cadherins

• desmoplakin

• intermediate filaments

function of desmosomes

• resist mechanical stress

• particularly important in:

  • skin

  • cardiac muscle

how does desmosomes structure support its function

Structural feature	How it supports function
Cadherins (desmoglein and desmocollin) span the membrane	Bind to cadherins on neighbouring cells, creating strong cell-cell adhesion.
Dense intracellular plaque (desmoplakin, plakoglobin, plakophilins)	Forms a robust anchoring structure beneath the membrane.
Attachment to intermediate filaments (keratin)	Distributes mechanical forces throughout the cell and tissue, preventing cells from being pulled apart.
Numerous desmosomes between cells	Creates a network of strong connections across the tissue.

why are intermediate filaments important

Unlike actin filaments, keratin intermediate filaments are very good at resisting tensile forces. When a tissue is stretched, the force is transmitted from:

1. Keratin in one cell

2. → Desmosome

3. → Keratin in the neighbouring cell

This spreads the stress across many cells, preventing damage.

where are desmosomes found

They are especially abundant in tissues exposed to mechanical stress:

• Epidermis (skin)

• Cardiac muscle (intercalated discs)

clinical relevance of desmosomes

autoantibodies against desmosomes cause pemphigus, a severe blistering disease

what polarity do epithelial cells exist

pico-basal polarity

3 domains in epithelial cells for epithelial polarity

• Apical surface → faces the lumen/exterior

• Lateral surface → contacts neighbouring cells (contains tight junctions, adherens junctions, desmosomes)

• Basal surface → contacts the basement membrane (contains hemidesmosomes)

apical surface

the free surface of an epithelial cell that faces the lumen or external environment

specialises in microvilli and cilia

how is the apical surface specialised to perform the cells specific role (structure → function)

Apical specialization	Function
Microvilli	Increase surface area for absorption (e.g. intestinal epithelial cells).
Stereocilia	Long microvilli that aid absorption or sensory functions (e.g. epididymis, inner ear).
Cilia	Move fluid or particles across the cell surface (e.g. respiratory tract).
Glycocalyx	Protective carbohydrate-rich coating involved in protection and cell recognition.

examples of apical surfaces

• Small intestine: apical microvilli increase nutrient absorption.

• Trachea: apical cilia move mucus and trapped pathogens towards the throat.

• Kidney tubules: apical membrane contains transport proteins for reabsorption.