Cell Interactions with
Biomaterials
Topics:
•Cell Structure and
Components
•Properties of Cell
Components
•Interaction of Cells with
Extracellular Material (ECM)
•Cell Adhesion and Migration
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A successful biomaterial implant must support all the
required functions of the attached (or neighboring) cells,
including
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Viability
Communication
Protein synthesis
Proliferation
Migration
Activation/differentiation
Programmed cell death
All cell types
All cell types
All cell types
Some cell types
Some cell types
Some cell types
Some cell types
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Cell Structure
2 types of cells:
•Differentiated: perform specific
tissue functions
•Undifferentiated: progenitors for
many different cell types
Cell tasks are compartmentalized
in various organelles. Organelles
in all mammal cells include the
plasma cell membrane,
mitochondria, Golgi apparatus,
cytoplasm, lysosome,
cytoskeleton, nucleus, and smooth
and rough endoplasmic reticulum
The Cell
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The cell membrane separates the
cytoplasm, or cell interior, from
the aqueous external environment.
It is a bilayered structure made up
of phospholipids, or fatty acids
with a polar (hydrophilic) head and
nonpolar tail.
The mitochondria produce the
energy for cell functions via a
process called oxidative
phosphorization. Surrounded by
a phospholipid membrane, the
mitochondria contains enzymes
that help break down molecules
Transmembrane proteins span the
cell membrane to channel ions
into and out of cell and maintain
proper cell chemistry
In oxidative phosphorization ATP
(adenosine triphosphate) is
converted to ADP (adenosine
diphosphate, an exothermic
reaction that releases energy to
drive the cellular process.
Other proteins target specific
extracellular molecules
Phospholipid
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The cytoskeleton is made up of three
filaments:
•Actin microfibrils,~6-8 nm diameter
•Intermediate filaments ~10 nm diameter
The Golgi apparatus
modifies, sorts and packages
proteins for their final
destination
•Microtubules ~25 nm diameter
Made up of proteins, the cytoskeleton
•Gives the cell shape
•Can provide cell locomotion
Lysosomes are
specialized vesicles
with digestive
enzymes
•Aids in separation/duplication of DNA
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The nucleus is the control
center for the cell. It
contains:
DNA – (deoxyribonucleic acid) that is
condensed into chromatin, a
complex combination of DNA and
protein that makes up chromosomes
The nuclear envelope, a bilayer of
phospholipid membranes that
surrounds the nucleus
The outer membrane of the nuclear
envelope is contiguous with the
endoplasmic reticulum and is
connected with the inner membrane at
specific locations called nuclear
pores. The pores are composed of
proteins that form gates to allow only
specific molecules in and out of the
nucleus
Ribosomes, minute round particles
composed of RNA and protein that are
found in the cytoplasm of living cells and
catalyze reactions in which mRNA is used
to synthesize proteins. Ribosomes are
located in the nucleolus
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DNA
DNA (deoxyribonucleic acid) is a
polymer of nucleic acid subunits.
Each nucleic acid has a phosphate
group, a sugar, and a base.
DNA contains genes, and forms the
template for all proteins synthesized
by the cell
When a gene is expressed, the cell
is actively producing the protein
encoded by the gene
Genes contain codons which
determine the structure of the
protein
DNA
The bases are directed toward the
interior, where they form hydrogen
bonds with other bases.
Bases can be either double ring
structures (e.g., purines Adenine
(A) or guanine (G)), or single ring
(pyrimidines, e.g., thymine (T) or
cytosine (C))
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RNA
RNA participates in DNA synthesis and
protein production.
Three types of RNA:
Messenger or mRNA
Transfer, or tRNA
Ribosomal, or rRNA
RNA is similar to DNA, but the sugar in the
RNA is single stranded , and
backbone contains an additional O2, and
does not form the helical
thymine (T) in DNA has been replaced
structure of DNA
with uracil (U).
mRNA: messenger RNA; DNA is unzipped, and mRNA strands are synthesized that
are complementary to DNA
tRNA: serves as an adaptor to combine mRNA strands in the rough ER
rRNA: the central component of the ribosome, the function of the rRNA is to provide a
mechanism for decoding mRNA into amino acids and to interact with the tRNAs during
translation
DNA-RNA Protein Synthesis
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The outer membrane of the
nucleus is connected to the
endoplasmic reticulum (ER).
The ER is the site of protein
synthesis.
The ER is made of long, flattened
sheets of phospholipids, and may
be either rough or smooth.
The rough ER has ribosomes
attached to the surface that act
as catalysts for protein synthesis.
Vesicles transport proteins from the ER
to the Golgi, or from the Golgi to the
target destination (exocytosis).
Specialized vesicles (lyosomes) may
also take in and digest particles from
the ECM
The smooth ER is more tubular
and does not contain ribosomes.
It packages the proteins
produced in the nucleolus and
rough ER in phospholipids for
delivery to the Golgi apparatus.
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Schematic of the endoplasmic reticulum (ER), which is responsible for
protein synthesis. The rough ER, which contains a large number of
ribosomes, is attached to the nuclear envelope. The rough ER
transforms into the smooth ER away from the nucleus. Pieces of the
ER will then split off and transfer to the Golgi apparatus, where the
proteins created in the ER are further modified and transported ot their
final destinations.
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Interactions between a cell and its
environment can result in cell
spreading, migration,
communication, differentiation
and activation. This is called
“outside-in” signaling.
Conversely, a cell may secrete
molecules or rearrange contacts to
alter the ECM. This is called
“Inside-out” signaling
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Types of cell contacts
Tight junctions: cells adhere
fast to each other – no
molecular transport
Gap junctions: small
hydrophilic channels between
cells and membranes
Desmosomes: mechanical
attachment between two cells.
Cells can attach to ECM via
hemidesmosomes or focal
adhesion. These form strong
adhesion to the ECM
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Cell membrane receptors & ligands
Cellular interactions are facilitated
by cell membrane receptors, each
of which is specific for a small range
of target molecules, or ligands.
Common receptor molecules are:
Cadherins: responsible for demosomes; a
cadherin molecule on one cell binds to a
cadherin on another cell. This is homophilic
binding
Selectins: selectins are like cadherins, but bind
to other types of receptors, or heterophilic
binding
Location of cadherins in epithelial cells.. They link to
each other to bind cells, and their cytoplasmic regions
attach to intermediate filaments linking the ECM to
the intracellular environment
Mucins: participate in heterophilic binding to
selectins.
Integrins: transmembrane proteins involved in
both cell-cell and cell-matrix contacts.
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Cell membrane receptors & ligands
Integrins have two distinct a and b
subunits, and are called heterodimers.
Variations in the composition of the a
and b chains results in selective
adhesion to different ligands
Other cell adhesion molecules (CAMs)
comprise a large group of membrane
proteins that mediate cell-cell
interactions via both homophilic and
heterophilic binding.
An example of these other CAMs is the
immunoglobulin (Ig) family in the picture
at right.
The receptors described consist mainly
of protein with a small attached
carbohydrate (sugar). Similar molecules
with small protein and large sugar
content are called proteoglycans.
Various types of cell membrane receptors:
mucins, integrins, selectins, and Ig-cell adhesion
molecules (Ig-CAMs).
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Extracellular Matrix – ECM
Environment
Understanding the cell-ECM interaction and response is
key to designing new biomaterials
The ECM may be thought of as a fiber reinforced
composite with fibers made of collagen or elastin, and a
matrix made of glycoproteins and proteoglycans
ECM video
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Collagen
Collagen is the most abundant protein
in mammals. It is responsible for tissue
tensile strength.
Upon secretion of procollagen from
the cell into the ECM, small peptide
sequences are cleaved to from the
molecule to allow for more efficient
packing of collagen molecules into
fibrils (10-300 nm). Individual fibers
are then assembled into larger
fibers (0.5 – 3 mm diameter)
Collagen is made up of a-polypeptides,
or amino acids in a gly-x-y pattern
(above). Gly is a small molecule
resulting in tight packing
X and Y molecules are often proline
and hydroxyproline
Cell excretes procollagen molecules
that self-assemble into fibers (right)
Properties of collagen may be manipulated
by controlling crosslinking of molecules
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Elastins
Elastin is responsible for the
resiliency and elasticity of the
ECM.
Elastin is made up of 85%
hydrophobic amino acids
When relaxed, elastin molecules coil
up. When a tensile load is applied
they unfold into long chains. The
chains are crosslinked to adjacent
elastin molecules
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Additional ECM molecules include
proteoglycans and glycoproteins. These
proteins are mainly carbohydrate, with some
protein side chains.
These molecules attract and interact strongly
with water.
Carbohydrates in proteoglycans form long
chains of polysaccharides called
glycosaminoglycans (GAGs)
Proteoglycans have several GAGs attached to
a protein core.
An aqueous environment is favorable to
transport and store bioactive molecules.
Example of the bottle-brush
structure of a large proteoglycan
including areas of keratan sulfate
and chrondroitin sulfate that are
attached to a protein core
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Two molecules that represent
glycoproteins are fibronectin and
laminin.
Each consists of peptide subunits held
together by disulfide bonds and contain
many sites for binding to various ECM
molecules.
Laminin (right) consists of three
disulfide linked peptide chains in a
loosely woven structure with multiple
binding sites
Glycoproteins are important in
blood clotting (coagulation) since
they posses binding domains for
heparin and fibrin
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Cell-Environment Interactions that Affect
Cellular Functions
Interactions between cells and cell/ECM can alter cells
function and affect gene expression in the nucleus
Alterations in gene expression
affect four major functions:
1. Cell viability
2. Proliferation
3. Differentiation
4. Protein synthesis and
communication
Necrosis: cell death from membrane
permeability and enzyme leakage,
leading to disintegration
Changes in ECM can cause cell
death via necrosis or
apoptosis due to changes in
the local chemistry (e.g. pH),
factors that attach to cell
membrane leading to cell
death.
Apoptosis: cell shrinkage followed
by fragmentation into vesicles
containing small groups of
organelles. No inflammatory
response
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Cell classifications
Labile: replicate continuously
Terminal: terminally differentiated
The cell cycle is divied into two phases:
Mitosis (the M phase) and interphase
(G1,S,G2)
Stable: don’t change once
differentiated but can be induced to
proliferate
Mitosis: cell division
Interphase: cellular DNA and
organelles replicated in
preparation for mitosis
Go: quiescent phase of stable
cells
G1: general cell growth
S: DNA replicated
G2: proteins and structures
enabling cell division ar
assembled
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Mitosis is divided into several
characteristic periods:
Prophase
Metaphase
Anaphase
Telophase
Prophase: dissipation of the nucleolus
and formation of the mitotic spindles
Metaphase: chromosomes are
aligned between two mitotic spindles
Anaphase: chromosomes are pulled
apart by the spindle microtubules and
arrange themselves at the spindle
poles
Telophase: nuclear envelope begins
to reform and the cell starts to undergo
cytokinesis
Cytokinesis: division of cell
cytoplasm
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Cell Differentiation
Progenitor or stem cells can form
more than one type of cell
The cells produced may be committed
or differentiated and can be labile,
stable or permanent
Or
Create additional pluripotent or
totipotent cells (produce other or all
cell types)
Red blood cells generated from hematopoietic
stem cell can either replicate itself of
differentiate into various cell types. These cells
are pluripotent
Embryonic stem cells are an
example of a totipotent cell
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The commitment and
progression of an MSC to
and through a specific
lineage involves the action
of bioactive molecules such
as growth factors and
cytokines.
In the process of
differentiation and
maturation the cell
increases its production of
tissue-specific molecules.
Mesenchymal stem cells (MSCs) can
differentiate into bone, cartilage, muscle, tendon,
ligament, and adipose tissue.
Differentiation stages can be initiated and
controlled by soluble and insoluble elements in
their environment and is directly applicable to
tissue engineering
Terminally differentiated
cells may alter their levels
of synthesis of matrix
molecules to play an
increased role in tissue
maintenance and
homeostasis
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Pixie Dust – Truth or Fiction?
Video 1
Video 2
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Models for Adhesion, Spreading and Migration
The DLVO theory: (Derjaguin, Landau, Verway and Overbeek)
Based on thermodynamics
Particles potential energy the sum
of attractive and repulsive forces:
U = UA + UR
Particles approaching a surface
reduce their potential energy, and
are loosely attached at the
secondary minimum – long range
electrostatic/van der Waals forces
Particles overcoming primary
minimum become firmly attached
through short range electrostatic
forces
Model shortcomings: does not include
steric repulsion, surface
topography/roughness or ligandreceptor interactions
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Spreading & Migration
Cell spreading: After attachment, cells extend fingerlike pseudopodia along surface. The integrin receptors
in the cell membrane interact with ligands on the
material surface to firmly anchor the cell in place.
Cell spreading includes cytoskeleton rearrangement and
production/adsorption of adhesive proteins on surface
Cell migration: extension of the cell membrane in long
pseudopodia is directed by polymerization of actin
microfibrils near the leading edge of the cell. (b) the
membrane then attaches to the substrate via integrin
receptors. (c,d) After the pseudopodia are firmly
adhered, there is generation of a contractile force along
with a release of rear receptors, leading to forward
motion. (e) the integrin receptors are recycled to the
leading edge so they may be used again as the process
continues.
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Tracking cell movements
Plots of cell trajectory, such as the one
at right, provide information on cell
movement in the form of translocation
speed (s) and persistence time (t).
Translocation speed is the speed of
cell movement over any straight-line
portion of the graph, between changes
in direction
Persistence time is the length of time
that the cell moves along the substrate
without a drastic change in direction.
Measurements of this type are used in
mathematical models developed to
model cell migration
Trajectories of bovine pulmonary artery
endothelial cells migrating in a uniform
environment. Symbols represent the location
of the centroid of each cell at 30 minute
intervals. Arrows indicate starting points.
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The End
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Protein Synthesis
Receptor-ligand binding can change the
function of committed cells which is associated
with alterations in the amount of protein
synthesized
Transcription: Chromatin becomes less
compact; RNA “unzips” DNA and synthesizes
mRNA strands that are complimentary to DNA
then moves into the endoplasmic reticulum
(ER)
Translation: the process of converting codons
from the mRNA to a polypeptide. This takes
place in the ER through complex interaction
with tRNA.
Post Translation: fully formed proteins are
combined into various molecules
Block diagram of the
steps for the creation
and modification of
proteins.
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Summary of the steps
of collagen synthesis.
Transcription,
translation, synthesis
to create the a-chain
and the joining of three
of thee chains (post
translational
modification) to create
the collagen triple helix
occur within the cell.
The procollagen
molecule is then
secreted and
assembled into fibrils
and finally fibers.
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