cell adhesion

Tissue

cooperative assembly of cells and extracellular matrix woven together to form a multicellular fabric with a distinctive function

what gives supportive tissues their strength

extracellular matrix

2 ways that cells adhere to each other to transmit physical stresses

extracellular matrix (ECM)

cytoskeleton and cell-cell adhesions that connect the cytoskeletons of neighbouring cells

which method of stress transmittingness do plants vs animals use

plants- ECM

animals- both in diff. tissue types

cell wall

supportive matrix in plants

can be hard (wood) or flexible (leaf)

newly formed vs cell growth stops cell walls

new= primary cell wall (flexible for additional growth)

old= secondary cell wall (hard for structure)

what gives a cell wall its tensile strength

cellulose microfibrils

cellulose

long unbranched chains of linked glucose subunits

how many cellulose molecules assemble to form a microfibril

16

primary vs secondary cell walls ingredients

primary= pectin

secondary= cellulose

matrix=

cellulose + pectin

pectin

long complex polysaccharides- rich in galacturonic acid

highly hydrated, binds cations

middle lamella function

cements cell walls together (is rich in pectin)

wood structure

cells thick walls remain when cell dies

form long channels vertically in tree

lignin in cell wall rigid when wet

cell wall structure

polysaccharides (CO2 and H2O)

some proteins

connective tissues vs epithelial tissues

connective have more ECM

collagen fibril organisation

Monomer- trimer- fibril- fibre

fibroblast secretes

collagen and other ECM

why are collagen fibrils arranged in layers in skin

allows skin to resist tensile stress in multiple directions

collagen fibril arrangement in tendon

tendons attach muscle to bone

collagen fibres align in parallel, along the axis of extension

integrins coordination of cell movements

new attachment points are established at the front of a moving cell

old adhesions are released at the back

enables the cell to crawl forwards

why are GAGs highly negatively charged

keeps high cation conc. in ECM

draws in water

creates swelling pressure

tissue specific ECM composition

dense connective tissue (bone, tendon)- less GAG + more collagen

vitreous humour- lots of GAG, little collagen

proteoglycan functions

binding growth factors

controlling migration of cells through the ECM

epithelium

sheets of cells joined together side by side

epithelia functions

cover external surfaces and line all internal cavities

create barriers

controls movement of molecules in and out

has receptors for external signals

protects against invading microbes and fluid loss

columnar epithelia

secretion/ absorption in the lining of the intestine

cuboidal epithelia

secretion/ absorption in the lining of the kidney tubules

squamous epithelia

filtration in the lining of the lungs

stratified epithelia

protection in the epidermis of the skin

basal lamina

thin, tough sheet of ECM (collagen, laminin)

seperates the epithelial cells from the network of collagen fibres in the underlying connective tissue

supports epithelial sheets

composition of basal lamina

ECM composed mainly of type IV collagen and laminin

where is the collagen and laminin supplied from (basal lamina structure)

colagen supplied by the underlying stromal cells (connective tissue)

laminin supplied by overlying epithelial cells

polarity of the epithelium

epithelial sheet has 2 faces

apical surface is free and exposed to the air or bodily fluids

basal surface is attached to a sheet of connective tissue called the basal lamina

why is polarity essential for epithelial cell function

the gut is lined by absorptive epithelial cells

also contains secretory goblet cells

polarity is critical for the function of both cell types

tight junctions

stitch cells together

adherens junctions

join actin filaments of neighbouring cells

desmosomes

join intermediate filaments of neighbouring cells

hemidesmosomes

join intermediate filaments to the basal lamina

gap junctions

form tunnels of aqueous connectivity between cells

Spina bifda

occurs when neural tube fails to close or form

abnormalities of the spinal chord

often leads to neurological deficits, typically affecting motor/ sensory function

Epidermolysis bullosa (EB)

genetic skin disorder resulting in blisters and lesions

gene therapy is used to correct defect in mutant gene (eg encoding laminin)

gap junction

small region of plasma membrane where two cells are very closely apposed in parallel

gap junction function

to allow small, intracellular, watersoluble molecules (eg metabolites) to move from cell to cell

gap junction physiology

molecular flow creates an electrical and a mechanical coupling between cells

gap junction permeability

gap junctions are gated

GJ permeability can be regulated by extracellular signals

gap junctions in retinal neurons

increased light triggers dopamine release, which switches retina from using rod (good in low light) to cone photorecetors (good in bright light)

plants and gap junctions

do not have gap junctions

plasmodesmata in cell walls carry out the function of gap junctions

different types of cadherins

E-cadherins

N-cadherins

P-cadherins

E-cadherins

mainly expressed on epithelial cells

N-cadherins

expressed on nerve, muscle and lens cells

P-cadherins

expressed in placenta and epidermis

cadherins mediate homophilic adhesion

provides a mechanism by which cells can recognise and stick to other similar cells

cadherin- cadherin interaction strength

weak but with multiple it is very strong

cadherin expression and affinity of cell-cell interaction

cells with higher cadherin levels form a stronger interaction, more tightly compacted in centre of assembly

selectins

cell surface proteins that bind carbohydrates

What causes leukocyte adhesion deficiency (LAD) disorders

molecular defects in selectin ligands, integrin expression or integrin activation on leukocytes

why must the ECM be degraded

to allow cells to pass between endothelial cells

ECM degrading enzymes

matrix metalloproteinases (MMPs)

Serine proteases

activity of ECM degrading proteases

some secreted in inactive form

some are confined to cell surface receptors

some are inhibited by the actions of locally secreted inhibitors