Key Terms

• Tissues are a collection of cells that work together toward a common function

• Extracellular Matrix (ECM): secreted by cells, it's what holds all of the cells together

  • complex network of polysaccharides & proteins

  • acts as a structural component of tissues

• Cell junction: links cells together, directly touching and linking

1. connective

   1. lots of ecm present

2. epithelial

3. nervous

4. muscular

​Animal cells (Tissues)

• cell wall

• cellulose microfibers give plant tensile strength

  • synthesized on the surface by cellulose synthase complexes

Plant cells (ECM)

Plants vs Animal Cells

Connective Tissue

• Found in bone, cartilage, tendons, and skin

• There is LOTS of ECM in this tissue, which makes it super tough and strong

• All the strength comes from the protein collagen

  • Collagen is a huge family of proteins and is very good at providing strength 

  • The type of collagen, amount, and other molecules interwoven determine the type of connective tissue

Collagen fibrils

• Polypeptide bonds (made of amino acids) are formed

• The bundles are super thick and tough

• They are VERY large

• Ehlers danlos syndrome: lack of the enzyme that converts procollagen to collagen, causing stretchy skin 

  • Defects in collagen breakdown can lead to arthritis or cancer

Fibroblasts

• A fibroblast is a type of cell that contributes to the formation of connective tissue, a fibrous cellular material that supports and connects other tissues or organs in the body.

• Fibroblasts secrete collagen proteins that help maintain the structural framework of tissues.

Integrins

• Integrins are cell surface receptors

• “Grabs” fibronectin to pull and move the cell within the ECM

• The connections to the intracellular cytoskeleton stop it from being “ripped out”

• Fibronectin: reach out to grab the collagen

• Integrin dimers: connect to ECM and help with the movement

• Plasma membrane: does not take any weight

• Actin filament: this is the part of the cytoskeleton that all of this connects to

Integrins = signalling molecules

• relay messages between ECM and cell

• binding causes conformational changes

  • signals for survival, differentiation, etc.

GAGs

• Glycosaminoglycans form bottle brush-like structures that are hydrophilic and take up a lot of room

Epithelia

• Cell joined together side-to-side

• Stratified- sheet many cells thick(covering the skin)

• Simple epithelium- only one cell thick(gut)

• Keeps some molecules in and others out

• Takes up nutrients and exports waste

• Contains receptors for environmental signals, which makes them really good at signaling 

• Protects the interior of organisms and fluid loss

Epithelial cells are polar

• Basal lamina: a strong distinct layer of ECM that contains collagen

• Laminin binds to epithelial cell integrins allowing communication between the basal lamina and the epithelium

• Epithelial cells are polarized

• Basal layer: inside, near blood

• Apical side: near the air

  • EX: lungs, the basal side will be on the outside of the lungs near the blood side, and the apical side is on the inside where the air is

Junctions

1. Cell-cell junctions

• Make the epithelium leakproof and separate apical and basal surfaces

• The cells connect to one another

2. Adherens junctions

• Can form a strong continuous “belt” around the epithelium

• Cadherin molecule: what allows actin to join 

• Actin allows contraction, and can form tubes

• “Hemi”- half

• We have half a desmosome (aka one cell)

• One cell is stapled to the basal lamina (ECM)

• Anchor epithelial to other tissues via integrins

Hemidesmosomes

• Connects intermediate filament in one with intermediate filaments in another

• Intermediate filaments: thick and solid, super tough

• STRONGEST

• Cadherin: connects everything

Desmosomes

Desmosomes vs Hemidesmosomes

Gap Junctions

Gap junctions

• Connexons: form channels across two plasma membranes 

• Pores through the cell that connects

• Linking insides of one cell to the inside of others

• Usually happens in the heart

• Connexon: what connects everything

• The holes don't stay open 24/7, they open and close depending on the cell type

Signaling can change the size and permeability of gap junctions to respond to the environment

• Open: cell signaling is happening

• Closed: the cells finished signaling 

Stem cells

• Many cells which need renewing can’t actually divide for themselves as they are “terminally differentiated”

• Replacements are generated from a circulating stock of “proliferating precursor cells”

• These are derived from small numbers of dividing stem cells

• Stem cells can either divide without limits or become differentiated

• Adult stem cells have undergone the first layer of differentiation to define progeny type

• Not all cells divide, stem cells and precursors are dividing cells (they then turn to specific cell types)

WNT pathway

Stem cell “status” is maintained by the WNT pathway

• In differentiated cells, B-catenin (protein) is degraded before entering the nucleus

• When WNT is expressed it triggers a signaling cascade that stops the degradation of B-Catenin which then activates the “stem cell-specific” gene transcription

   

Malignant growth

• invade surrounding tissue

• metastasis

  • formation of secondary tumors by entering bloodstream and invading tissues in other parts of the body

Cancer cells

1. reduced dependence on signals from other cells for growth, survival, and division

2. less likely to undergo apoptosis

   1. due to increased survival factor signaling

   2. p53 mutation

3. can divide indefinitely (reactivate telomerase to maintain telomere length)

Cancer cells cont.

1. genetically unstable

   1. increased mutation rate

   2. different types of mutations can lead to activation of an oncogene

2. invasive

3. angiogenesis

   1. ability to grow its own blood supply

Cancer treatments

1. surgery-> cut out tumor

2. radiotherapy

3. chemotherapy

   1. block cell growth

4. gene therapy

   1. program immune cells to attack cancer