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Cell and Tissue Architecture (The Cytoskeleton (Cytoskeleton functions…
Cell and Tissue Architecture
Cell Junctions
Cell adhesion molecules (transmembrane proteins)
cadherins: adhesion of cells to other cells
E-cadherin
(epidermal cadherin)
N-cadherin
(neural cadherin)
integrins: adhere cell to ECM
linked to microfilaments or intermediate filaments
act as receptors (communication from ECM)
Anchoring junctions
Adhere cell to cell
adherens
Attached to band of actin and epithelial cells
most universal
desmosomes
button-like
cadherins attach to intermediate filaments
Adhere cell to ECM
hesidesmosome
integrins
intermediate filament to ECM proteins (basolamina)
Tight junctions
band of interconnected strands of integral membrane proteins
claudins and occludins
actin microfilaments
water-tight, molecule tight
Creates two sides
apical membrane: faces lumen
Basolateral membrane "bottom"
Communicating junctions
Gap junctions
connexins arranged in ring
ions and signaling molecules pass through (cell communication)
diffusion
Plasmodesmata
Size of opening larger
transfer of RNA molecules and proteins
The Cytoskeleton
Polymers of protein subunits
microtubules
structure
tubulin proteins --> dimers (a and B) --> profilament
filaments cross-reacting with each other; cross-filament forms hollow tube
Function: help maintain cell shape and cell's internal structure; withstand compression, guide and arrangement of organelles
Radiate out of centrosome
Microfilaments
Structure: Polymers of actin monomers (form helix); short, branched in cell cell cortex; cross-linked into mesh (
different actin fiber rearrangements
)
Function: maintain shape and size of cell
Example: cortical microfilaments maintain shape and size of epithelial cells
bundles of microfilaments in microvilli
bundles of microfilaments form band around circumference of epithelial cells
Dynamic structures
High concentrations of subunits --> assembled onto both ends of polymers
faster assembling end = plus end (ends of microtubules)
slower-assembling end = minus end (centrosome emanating from minus end)
Polymerization/depolymerization important for movement
microtubules make up spindles for cell division
dynamic instability
can grow fast and fall apart fast (catastrophy)
microtubules can change/adjust as cell changes
plus ends (depolymerization) followed by smaller polymerization --> find chromosome
Movement with motor proteins
microtubules
tracks associate with motor proteins
melanophores
pigment granules move around cell in response to hormones or neuronal signals
pigment granules aggregate at center of cell around centrosome -> white (dynein)
pigment dispersed outward (kinesin) --> dark color
kinesin
transports towards plus end (cell periphery)
dynein
carries load away from plasma membrane (toward centrosome, interior of cell)
conformational changes in protein (ATP) to move along track
Found in cilia (short) and flagella (long)
dynein --> forms bundles
microfilaments
Example: contraction of muscle cells with myosin microfilaments (powered by ATP)
motor isn't moving, fiber is moving
sarcomeres
(repeated structures); actin fibers agree with myosin motors attached to actin, move fibers --> fibers come together --> contracts bigger muscle cells --> contracts muscles to move
actin rearrangements
cable-like structures are changing
stress fibers pull back of cells
making mesh --> lamellipodia (pushes front end) and filopeia (antennae sensing the environment)
Intermediate filaments
filament proteins differ in cells
Structure
polymers differ in cells
strong, cable-like structures in cell (provide mechanical strength)
attached to cell junctions at cytoplasmic side --> structural continuity (strengthens)
Example: keratin
keratin: coils --> more coils --> one big coils
entire cell meshed together (attached to
nuclear lamin
)
epidermolysis bullosa
: defective keratin genes: intermediate genes do not polymerize activity
Cytoskeleton functions
cell movement (actin)
Cell morphology (microtubules)
cell division (microtubules)
intracellular mobility: moving things around
mechanical stability: intermediate filaments
Organs and Tissues
Communities of cells
tissue
collection of cells that work together
Example
animal
epithelium, connective tissue (
mesechyme
), muscle, nerves
plant
epidermal, ground tissue, vascular (xylem and phloem)
Different cell shape per different organ; determined by cytoskeleton, cell junctions, and ECM
Form determines function
Example: structure of skin
epidermal layer
epithelial layer
keratinocytes
have cytoskeletal filaments; connected to gap junctions --> keratinocytes connected to basal lamina
melanocytes (contain pigment)
basal lamina
underlies and supports all epithelial tissues
dermis
made up mostly of connective tissue
few cells (fibroblasts--> synthesizes ECM), lots of ECM
strong, many blood vessels, and nerve endings
The ECM
Plants = cell wall
Cellulose
polymer assembled outside of cell (extracellular surface of plasma membrane)
Outermost middle lamella
synthesized first during late stages of cell division
gluelike complex carb (helps plant cells adhere to one another
Primary cell wall
cellulose and other molecules (pectin)
assembled by enzymes on surface of cell
thin and flexible
secondary cell wall
cellulose and lignin (hardens cell wall and makes water-resistant--> xylem and phloem)
allows large growth of trees
Connective tissue in animal cells
Collagen
most abundant protein in ECM
Structure
Every third amino acid = glycine (small, tight turns)
3 polypeptides in triple helix --> form fibril, fibrils assembled into fibers
covalent crosslinks (intermolecular and intramolecular)
synthesized in osteoblasts and fibroblasts and
chondroytes
no cross links --> scurvy
Glycoaminosglycan (GAGS) and proteoglycans
makes gel (absorbs lots of water)
negatively charged
Elastic fibers
elastin fibers
Martan's syndrome: lack of fibrillin 1 in ECM (glycoprotein essential for elastin fibers)
Basal lamina
Made of special type of collagen
fibronectin
: attaches integrin to ECM components
laminin
: lets interaction with proteins of proteins of basal lamina with integrins
Function: cell attachment and cues for growth, differentiation
composed of large fibrous proteins
attracts charged ions and water molecules
dermis cell = fibroblast: synthesize most of ECM proteins
Spread of cancer cells
Tumor development
Invasion: cross basal lamina (developing carcinoma)
Metastasis: moving through some system of transport to spread through body and find another site
cells pass basal lamina twice
malignant tumors: cells pass through basal lamina twice (integrins potential studied)
Cancer: high E-cadherin expression
ECM proteins influence cell shape and gene expression
Composition of ECM
neurons (absence/presence of laminin)
Putting cell in fibronectin induces cell division
ECM proteins and hepatocyte --> albumin
In laboratory: cells can begin to grow more complex structures --> organoids