cellular interactions

chapter 20

tissues• cellular interaction

• in some organisms, cells interact to form defined tissues

• extracellular matrix• allows for cellular

interaction• extremely important in

certain tissues• connective tissue• epithelial tissue

epid

erm

isde

rmis

epid

erm

isde

rmis

keratinized cells

actively-dividing cells

basement membrane

cell to cell contact

cell/substrate contact

basement membrane

reticular fiber

proteoglycan

collagen fiber

integrin

fibroblast

elastic fiber

extracellular space• glycocalyx

• carbohydrate projections from the plasma membrane

extracellular matrix• ECM -

• present in pretty much all animals• evolved in some ancestor of the animals

• sponges• mesohyl • basic ECM elements

• type IV collagen• spongin

• laminins• what about plants?

• special ECM• cell wall matrix

basement membrane• basal lamina

• flattened layer of ECM• forms substrate for

epithelium/endothelium• sits above loose connective

tissue• ECM secreted by

fibroblasts

basement membrane• basal lamina

• 4 main functions• structural foundation for

epithelium• selective membrane• facilitate access to proteins• serve as guide for developing

neurons

ECM fibers• collagens

• fibrous glycoproteins• most abundant protein

in humans• very strong• structure

• triple helix• arranged in staggered

array• overlap 25%

ECM fibers• collagens

• types I-XIX• I-III most common in humans• IV - not fibrillar

• spongin matrix in sponges• cuticle of nemata• bone• cartilage• type III - reticular fibers

• branching network

collagen problems• collagen-related diseases

• type I (osteogenesis imperfecta)• type II (dwarfism)• type IV

• Alport syndrome• kidney disease (glomerular nephritis)• blindness (ocular lesions)• hearing loss

• Ehlers-Danlos syndrome• hyperflexibility / extensible skin

ECM fibers• elastic fibers

• elastin• cross-linked into network

• provide elastic strength• important for organs that stretch

elastic fiber

proteoglycan complexes• proteoglycans

• core protein• GAGs (glycosaminoglycans)

• repeating disaccharide structure• cushion cells, help resist crushing

fibronectin• fibronectin structure

• glycoproteins• found in nearly all animal connective tissue

• two nearly identical polypeptides

fibronectin• role in development

• migration of cells • guided by fibronectin • provides a substrate for cells to migrate over• cells of the neural crest adhere to fibronectin

• connections• can bind to cells and ECM• allow cells to attach to ECM

laminin• laminins

• glycoproteins - triple helix (like collagens)• many diverse forms (little homology)• form web-like networks

• resistant to tensile force

proteolytic venoms• action of venoms

• Loxosceles sp.• often result in edema,

inflammation, necrosis • interferes with basement

membrane fibers• break down entactin

• ophidian hemorrhagic toxins• enzymes hydrolyze various

ECM fibers

interaction with cells• integrins

• receptors with α and β subunits• each with several domains• many possible configurations

of 26 known subunits• contain relatively short

transmembrane domain• conformations

• inactive - bent• active - unbent / legs apart• can adopt many different

active configurations

α subunitβ subunit

β-propellerβ-I domain

αI domain

interaction with cells• integrins

• cell-binding sites in extracellular proteins• RGD - (arginine-glycine-aspartic acid) recognized• integrin binds with RGD sequence

• two main functions• adhesion of cells to substrate• transmission of signals from external environment

• β subunits• three main classes: β1, β2, β3

• clotting• fibrinogen (RGD sequence) binds to integrin• clot busters - competitive inhibitors

cell-ECM junctions• hemidesmosomes

• different class - β4• link to intermediate filaments• dense plectin plaque

• linked to ECM by integrins • can be disassembled and

reassembled

cell-cell junctions• junction complex

• found on lateral aspect of plasma membranes• comprise several types of junctions

• tight junctions• regions of lateral membrane joined by transmembrane proteins• complex collections of proteins

• transmembrane proteins• polarity proteins• cytoskeletal proteins• signaling proteins

cell-cell junctions• junction complex

• found on lateral aspect of plasma membranes• comprise several types of junctions

• tight junctions• transmembrane proteins

• claudins• occludins• junctional adhesion molecule (JAM)

• function• paracellular transport• creates membrane polarity: apical, basal

• membrane domains• prevent movement of proteins between domains

cell-cell junctions• adherens junctions

• link adjacent cells• located just basal to tight

junctions• transmembrane receptor

proteins• cadherin dimers

• bind to identical cadherins on neighboring cells

• anchor proteins (catenins)• link to actin bundles

cell-cell junctions• septate junctions

• only in invertebrates• similar in function to tight junctions• located basal to adherens junctions

cell-cell junctions• desmosomes

• cytoplasmic plaques• inner dense plaque• outer dense plaque• connected to

intermediate filaments• main function:

• link cells together• important for cells

exposed to physical stress

cell-cell junctions• junction complexes

A. tight junctionsB. adherens junctionsC. desmosomeD. hemidesmosomeE. (gap junction)

cell-cell communication

hydrophilic channel

cell membrane

cell membrane

cytoplasm

cytoplasm

connexon• gap junctions• analagous to

plasmodesmata• vertebrates

• connexon (2)• connexins (6)

• invertebrates• innexon (2)

• innexins (6)

cell-cell communication• membrane nanotubes

• long tubes between cells• allow for exchange

cell-cell communication• plasmodesmata

• intercellular cytoplasmic channels • desmotubule

• trapped portions of ER• passage of materials limited

• size constraints• interconnect protoplasts

• symplast cell wall

plasma membrane

Cell wall

Microfibril

Cellulose
microfibrils in a
plant cell wall

Cellulose
molecules

β Glucose 
monomer

10 μm

0.5 μm

plant cell walls • comparative cell wall composition

• for bacteria - mainly peptidoglycan• for fungi - chitin• for plants cellulose

• cell wall matrix• microfibrils• proteins• pectin• hemicelluloses

cell wall matrix• microfibrils

• arranged in radial arrangement• restrict cell expansion

• cell elongation• influx of water

• increases cell volume

Cellulose
microfibrils

Nucleus

Elongation

Vacuoles

5 µm

cell wall matrix• composition of cell wall

matrix• hemicellulose

• highly branched• crosslinked to microfibrils

• pectin• negatively charged

galacturonic acids• form a gel

• microfibrils• glycoproteins

pectin

cellulose microfibril

hemicellulose

soluble protein

cell wall matrix• layers

• middle lamella (outer layer of pectin)• primary cell wall

• more flexible, laid down first • secondary cell wall

middle lamella

plasma membrane

primary cell wall

cell wall matrix• lignin

• used to stiffen and strengthen cell walls• 2nd most abundant organic polymer• hydrophobic polymer

1. p-coumaryl alcohol 2. coniferyl alcohol3. sinapyl alcohol

summary• major ECM fibers

• collagens• elastic fibers• proteoglycans• fibronectin• laminin

• cell-ECM connection• integrins• hemidesmosomes

summary• cell-cell junctions

• junction complex• tight junctions• adherens junctions• septate junctions• desmosomes

• gap junctions• membrane nanotubes• plasmodesmata