Lecture 16 - Tissue Organization

Question 1

What are tissues?

Answer 1

Cells organized into cooperative assemblies, which permits complex processes, compartmentalization and communication

Question 2

What are the two principal types of tissues?

Answer 2

1. Cellular tissues - composed mostly of cells, integrity/properties derived from cell-cell and cell-ECM interactions (cell junctions)
2. Connective tissues - tissue primarily composed of ECM with few cells, cells synthesize/remodel and bind the ECM

The properties of the tissue is derived from the composition of the ECM

Question 3

Describe the 3 basic components of adhering/anchoring junctions and their function

Answer 3

1. Transmembrane glycoprotein, interacts w/ proteins on adjacent cells or ECM
2. Complex of linker proteins designed to stabilize link to 3rd compoentn
3. Cytoskeleton

Fxn - maintain tissue integrity when subjected to mechanical stress, achieved by interactions with actin and intermediate filaments

Question 4

Describe cell-cell adherens junctions (zonula adherens)

Answer 4

Cadherins (Ca2+) = transmembrane protein (homophilic)

Connected to actin microfilaments

Help provide strength, and contractions of epithelial sheets (ex. neural tube)

Question 5

Describe Cell-ECM Focal adhesions/contacts

Answer 5

Connect to the ECM (fibronectin) via integrins (Ca2+ sensitive), which are heterodimers.

Connects to actin cytoskeleton

Found in vascular endothelium and near tendons

Question 6

Describe the structure and function of desmosomes

Answer 6

Cell-cell anchoring junction.

Use cadherins (Ca2+ sensitive) as transmembrane component

Connect to intermediate filaments in cell

Help to maintain cellular tissue integrity, link intermediate filaments across epithelial sheets

Question 7

Describe the structure and function of hemi-desmosomes.

Answer 7

Cell-matrix anchoring junctions

Integrins (heterophilic) are the transmembrane component (only integrin to associate w/ int fil network)

Connects to intermediate filaments in the cell

Link epithelial cells to the underlying basal lamina

Question 8

Describe the structure and function of tight junctions (zonula occludens)

Answer 8

Tight junctions provide a permeability barrier across epithelial sheets (small intestine)

Also maintain the cells “polarity,” partitioning membrane proteins into the correct regions

Composed of claudin and occludin - strings of these proteins interact w/ each other on adjacent cells (homophilic interactions)

Question 9

Describe the structure and function of Gap junctions

Answer 9

Provide communication between neighboring cells (electrical conduction in heart, and passage of small molecules)

Individuals components are called connexins - each junction is comprised of 2 connexons (group of 6 connexins). Permability regulated by pH and Ca2+

Question 10

Describe the structure and function of the extracellular matrix

Answer 10

Synthesized by a small # of cells

ECM is major component of connective tissue (cartilage, bone, tendons)

Provides tensile strength, elasticity, permeability

Produced mostly by fibroblasts

Some tissues made from specialized cells (chondroblasts > cartilage, osteoblasts > bone)

Question 11

What are the 3 major protein families of the ECM?

Answer 11

1. Fibrous proteins (collagens and elastin)
2. Bulky-space filling proteins (proteoglycans)
3. Cross-linking proteins (fibronectin/laminin)

Question 12

Describe the structure and function of fibrous proteins of the ECM

Answer 12

Collagen I, II, and III - most abundant protein, provides tensile strength to tissues

(can be absent, ex. Ehlers-Danlos syndrome)

Question 13

Briefly describe the biosynthesis of fibrillar collagen (and note the exception)

Answer 13

Synthesized as pro-alpha chain in the ER

Post-translational modifications in the Golgi and 3 pro-alpha chains combine to make a triple helix (pro-collagen molecule)

SECRETED FROM CELL

Various enzymes clip the protein, and then it self-associates into fibrils and fibers

Exception: Non-fibrillar collagen (type IV), cannot form fibrils, forms meshwork of basal lamina

Question 14

Describe the structure and function of elastin

Answer 14

Provides elastic component to the skin, lungs, and blood vessels

Extensive cross-linking between molecules @ lysine residues

Elasticity due to molecule containing large # of random coil and turns due to high proline content

Question 15

Describe the structure and function of firbillin

Answer 15

Glycoprotein sheath around elastin that aids in its stability (resists stretching)

Mutations in fibrillin are seen in Marfans syndrome (aorta ruptures)

Question 16

Describe the structure and function of proteoglycans (bulky filler proteins/molecules)

Answer 16

Space filling, resist compression, permit rapid diffusion and cell migration

Proteoglycans - protein core with long chains of glycosaminoglycans (made of repeating disaccharides)

Complex further to make hyaluronic acid

There are numerous sulfate and carboxyl groups (negative charges) on them, attracts water

Question 17

Describe the structure and function of fibronectin (cross-linker protein)

Answer 17

Contains multiple binding sites for ECM and other cells (integrins, hyparins, collagens, and fibrin)

Important in loose connective tissue/blood clotting/wound repair, etc

Question 18

Describe the structure and function of laminins (cross-linker proteins) and the basal lamina

Answer 18

Binds integrins on cell surface, type IV collagen, component of the basal lamina

Basal lamina = specialized ECM forming a dense meshwork

Composed of type IV collagen, laminin, and proteoglycans

Surrounds and supports muscle cells, epithelial sheets, molecular filter in glomerulus of kidney