Topic 6: Cell Exteriors

Question 1

What is the simple epithelium?

Answer 1

cell layer that separates us from our environment

basal: side that faces the body and ECM

side that faces the environment is covered in microvilli

basal lamina: thick connective tissue layer that acts as a scaffold for epithelial

Question 2

What are the different forms ECM can come in depending on the tissue type?

Answer 2

cells in the fibroblast family synthesize various types of ECM

different types and amounts of all 3 categories (collagens, proteoglycans and elastins)

hard: eg. bone
flexible: eg. basal lamina
clear: eg. eye, cornea

Question 3

What is collagen?

Answer 3

main structural component of ECM

Question 4

What is fibronectin?

Answer 4

attaches to collagen and connects to the cell via integrins

Question 5

What are integrins?

Answer 5

integral membrane proteins that link ECM to cytoskeleton

Question 6

Why do cells have an ECM?

Answer 6

cells and tissues must resist mechanical forces such as tension and compression

ECM distributes force across a tissue

ECM polarizes the cell, attachment to ECM dictates basal character of cell, provides orientation signal

ECM allows communication of tensile forces

Question 7

What are components of the ECM?

Answer 7

collagen
procollagen
quaternary structure of collagen and complexities of forming collagen fibers
proteoglycan
elastin

Question 8

What is the structure of collagen?

Answer 8

25 different types of collagen that assemble into 15-20 different fiber types

25-30% of a body’s protein by weight is collagen

forms rope-like fibers (triple helical)

highly tension resistant (resists stretch on the cells)

secreted by fibroblasts as well as other cells

Question 9

What are characteristics of deficiencies in collagen?

Answer 9

VitC is an essential cofactor in collagen synthesis (helps makes collagen)

deficiencies cause scurvy as collagen in blood vessels and tooth sockets are lost (first symptom is loosening of teeth)

Question 10

What is procollagen?

Answer 10

procollagen is a precursor to collagen and has non-helical ends that prevent formation of collagen fibers inside the cell

procollagen is shorter than mature collagen fiber (has extra a.a. at N and C termini that prevent spontaneous assemble)

upon secretion proteases cleave procollagen so that it can self-assemble into collagen

Question 11

What is proteoglycan?

Answer 11

polysacc and polypep molecules become hydrated and form a gel-like material

consists of GAG chains (glycosaminoglycans) attached to a core protein

very negatively charged that attracts Na+ ions and brings water with it to form a gel

negative charge on sugars attract H2O

lubricate and hydrate a tissue

Question 12

What is elastin?

Answer 12

network of elastin proteins creates flexibility in ECM

complex/relaxed state forms with weak hydrophobic interactions

crosslinks hold polymer together but rest can be stretched

Question 13

What are cell junctions?

Answer 13

direct contacts between cells

desmosomes: resist mechanical forces (anchoring/adhesive junctions)

tight: seals the gaps between cells (occluding junctions)

gap: links the cytoplasm of adjacent cells (channel-forming junctions)

Question 14

What are desmosomes?

Answer 14

cell-cell adhesive junctions

allow distribution of stretching force across a tissue

Question 15

What is Hailey-Hailey disease?

Answer 15

deficiencies in desmosome structure and function can lead to the disease

skin disorder

genetic defect in desmosomal component

symptom: skin blistering

Question 16

What are hemi-desmosomes?

Answer 16

cell-ECM adhesive junctions

connects the CSK with ECM

tight linkage between basal lamina and hemidesmosome

Question 17

What are tight junctions?

Answer 17

occluding junctions seals the edges around intestinal epithelial cells

prevents transport of nutrients between cells in an epithelium

located apically

prevent movement between cells, seal adjacent cells together

Question 18

What is experimental evidence of tight junctions?

Answer 18

add electron opaque tracer to basal side of an epithelial sheet

perform TEM

results: staining corresponded to bond tight junctions

significance: tight junctions are key restrictive barrier to non-selective transport

Question 19

What are gap junctions?

Answer 19

channel-forming junctions allow small molecules to pass between intestinal epithelial cells

very close contact between the membranes of adjacent cells

connects the cytoplasm of adjacent cell directly

vertebrates have connexions, invertebrates have innexins

composed of integral membrane proteins (connexins) that form channel between cells

allows for free diffusion of ions and small molecules

function: coordinate cells in a tissue

Question 20

What are gap junctions in heart muscle?

Answer 20

heart muscle cells are mononucleated

gap junction allow rapid sharing of contractile signal (Ca+)

coordinates the muscle cells the contract at same time