Topic 6: Cell Exteriors Flashcards
What is the simple epithelium?
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
What are the different forms ECM can come in depending on the tissue type?
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
What is collagen?
main structural component of ECM
What is fibronectin?
attaches to collagen and connects to the cell via integrins
What are integrins?
integral membrane proteins that link ECM to cytoskeleton
Why do cells have an ECM?
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
What are components of the ECM?
collagen
procollagen
quaternary structure of collagen and complexities of forming collagen fibers
proteoglycan
elastin
What is the structure of collagen?
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
What are characteristics of deficiencies in collagen?
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)
What is procollagen?
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
What is proteoglycan?
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
What is elastin?
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
What are cell junctions?
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)
What are desmosomes?
cell-cell adhesive junctions
allow distribution of stretching force across a tissue
What is Hailey-Hailey disease?
deficiencies in desmosome structure and function can lead to the disease
skin disorder
genetic defect in desmosomal component
symptom: skin blistering
What are hemi-desmosomes?
cell-ECM adhesive junctions
connects the CSK with ECM
tight linkage between basal lamina and hemidesmosome
What are tight junctions?
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
What is experimental evidence of tight junctions?
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
What are gap junctions?
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
What are gap junctions in heart muscle?
heart muscle cells are mononucleated
gap junction allow rapid sharing of contractile signal (Ca+)
coordinates the muscle cells the contract at same time
