The document discusses tissue engineering, focusing on the combination of scaffolds, cells, and biochemical factors to create functional tissues for the repair and regeneration of damaged tissues or organs. It highlights the significance of stem cells, particularly embryonic stem cells and mesenchymal stem cells, in developing tissues and various scaffolding approaches utilized in tissue engineering. The document concludes that ongoing advancements in tissue engineering and regenerative medicine may lead to enhanced clinical applications for tissue replacement therapies.

1. TISSUE DEVELOPMENT WITH TISSUE
ENGINEERING APPROACH
PRESENTED BY
FELIX CHIBUZO OBI (20144610) MSc.
SUPERVISOR: PROFESSOR S. ISMET DELILOGLU GURHAN

2. INTRODUCTION
 Tissue Engineering is the development and
practice of combining scaffolds, cells, and
suitable biochemical factors (regulatory
factors or Signals) into functional tissues. The
goal of tissue engineering is to assemble
functional constructs that restore, maintain,
or improve damaged tissues or whole organs.

3. Cells are the building blocks of tissue, and tissues are
the basic unit of function in the body. Generally, groups
of cells make and secrete their own support structures,
called extracellular matrix. This matrix, or scaffold, does
more than just support the cells; it also acts as a relay
station for various signaling molecules. Thus, cells
receive messages from many sources that become
available from the local environment. Each signal can
start a chain of responses that determine what happens
to the cell. By understanding how individual cells
respond to signals, interact with their environment, and
organize into tissues and organisms, Tissue Engineers
are now able to manipulate these processes to amend
damaged tissues or even create new ones.

4. STEM CELLS TECHONOLOGY Stem cells are undifferentiated biological cells that are capable of
differentiating into specialized cells and can divide through Mitosis to produce
more stem cells. Most current strategies for tissue engineering depend upon a
sample of autologous cells from the diseased organ of the host.
 However, for many patients with extensive end-stage organ failure, a tissue
biopsy may not yield enough normal cells for expansion and transplantation.
In other instances, primary autologous human cells cannot be expanded from a
particular organ, such as the pancreas. In these situations, pluripotent human
embryonic stem cells are envisioned as a viable source of cells because they can
serve as an alternative source of cells from which the desired tissue can be
derived.
 Embryonic stem cells exhibit two remarkable properties: the ability to
proliferate in an undifferentiated but pluripotent state (self-renew), and the
ability to differentiate into many specialized cell types. They can be isolated by
immunosurgery from the inner cell mass of the embryo during the blastocyst
stage (4-5 days after fertilization), and are usually grown on feeder layers
consisting of mouse embryonic fibroblasts or human feeder cells. More recent
reports have shown that these cells can be grown without the use of a feeder
layer, and thus avoid the exposure of these human cells to mouse viruses and
proteins. These cells have demonstrated longevity in culture by maintaining
their undifferentiated state for at least 80 passages when grown using current
published protocols.

5. ORGAN DEVELOPMENT FROM EMBRYONIC STEM CELLS

6. Human embryonic stem cells have been shown to differentiate
into cells from all three embryonic germ layers (Endoderm,
Mesoderm and Ectodrem) in vitro.
In addition, as further evidence of their pluripotency, embryonic
stem cells can form embryoid bodies, which are cell aggregations
that contain all three embryonic germ layers, while in culture,
and can form teratomas in vivo.
Due to the Ethical concern of the Human Embryonic stem cell,
some Tissue Engineers are working with Mesenchymal Stem cell
(MSCs) to develop Tissues, such Tissue have no risk of rejection
by the body.
STEN CELLS TECHNOLOGY CONT.

7. NEW DEVELOPMENT IN EMBRYONIC STEM CELL

9. SCAFFOLDS IN TISSUE ENGINEERING
 A scaffold is a material that can be formed in the shape of tissue that
needs to be replaced. The scaffold can be biologically derived or a
synthesized material. The scaffold material must be biologically
compatible for human implantation. The scaffold is typically
impregnated (seeded) with a patient’s cells before implantation.
 Typically, Scaffolds are synthesized from Biomaterials. These
Biomaterials replicate the biologic and mechanical function of the
native Extracellular Matrix (ECM) found in tissues by serving as an
artificial ECM. As a result, biomaterials provide a three-dimensional
space for the cells to form into new tissues with appropriate structure
and function, and also can allow for the delivery of cells and
appropriate bioactive factors (e.g., cell adhesion peptides, growth
factors), to desired sites in the body. Because the majority of
mammalian cell types are anchorage dependent and will die if no cell-
adhesion substrate is available, biomaterials provide a cell-adhesion
substrate that can deliver cells to specific sites in the body with high
loading efficiency. Biomaterials can also provide mechanical support
against in vivo forces such that the predefined three-dimensional
structure is maintained during tissue development. Furthermore,
bioactive signals, such as cell-adhesion peptides and growth factors,
can be loaded along with cells to help regulate cellular function.

10. The ideal biomaterial should be biocompatible in that it is biodegradable
and bioresorbable to support the replacement of normal tissue without
inflammation. Furthermore, the biomaterial should provide an
environment in which appropriate regulation of cell behavior (e.g.,
adhesion, proliferation, migration, and differentiation) can occur such that
functional tissue can form. Cell behavior in the newly formed tissue has
been shown to be regulated by multiple interactions of the cells with their
microenvironment, including interactions with cell-adhesion ligands and
with soluble growth factors.
Generally, three classes of biomaterials have been used for engineering
tissues: naturally derived materials (e.g., collagen and alginate), Acellular
tissue matrices (e.g., bladder submucosa and small intestinal submucosa),
and synthetic polymers (e.g., polyglycolic acid (PGA), polylactic acid (PLA),
and poly(lactic-co-glycolic acid) (PLGA). These classes of biomaterials have
been tested in respect to their biocompatibility. Naturally derived materials
and acellular tissue matrices have the potential advantage of biologic
recognition. However, synthetic polymers can be produced reproducibly on
a large scale with controlled properties of their strength, degradation rate,
and microstructure.

11. Synthetic Breast Scaffolds
Figure a & b: Synthetıc Scaffolds
Fıgure c & d: Acellular Scaffolds

12. SCAFFOLDING APPROCHES IN TISSUES ENGINEERING
There are basically four major scaffolding approaches for tissue
engineering.(Fig. 1). Table 1 (on next slide) highlights some of the
working principles and the characteristics of these approaches.

13. Scaffolding
approach
(1) Pre-made porous
scaffolds for cell
seeding
(2)Decellularized
extracellular
matrix for cell
seeding
(3) Confluent cells
with secreted
extracellular
matrix
(4) Cell
encapsulated in
self-assembled
hydrogel
Raw materials
Processing or
fabricating
technology
Strategy to
combine with
cells
Strategy to
transfer to host
tissues
Synthetic or natural
Biomaterials
Incorporation of
porogens in solid
materials; solid free-
form fabrication
technologies;
techniques using
woven or non-woven
fibers
Seeding
Implantation
Allogenic or
Xenogenic Tissue
Decellularization
technologies
Seeding
Implantation
Cells
Secretion of
extracellular
matrix by
confluent cells
Cells present before
extracellular matrix
secretion
Implantation
Synthetic or natural
biomaterials able to
self-assemble into
hydrogels
Initiation of self-
assembly process
by parameters
such as pH and
temperature
Cells present before
self-assembly
Injection
Table 1: Characteristics of different scaffolding approaches in tissue engineering

14. Scaffolding
approach
(1) Pre-made porous
scaffolds for cell
seeding
(2)Decellularized
extracellular
matrix for cell
seeding
(3) Confluent cells
with secreted
extracellular
matrix
(4) Cell
encapsulated in
self-assembled
hydrogel
Advantages
Disadvantages
Preferred
applications
Most diversified
choices for materials;
precise design for
microstructure and
architecture
Time consuming
cell seeding
procedure;
inhomogeneous
distribution of
cells
Both soft and hard
tissues; load-
bearing tissues
Most nature-
simulating scaffolds
in terms of
composition and
mechanical
properties
Inhomogeneous
distribution of cells,
difficulty in retaining
all extracellular
matrix,
immunogenicity
upon incomplete
decellularization
Tissues with high
ECM content; load-
bearing tissues
Cell-secreted
extracellular
matrix is
biocompatible
Need multiple
laminations
Tissues with high
cellularity, epithelial
tissues, endothelial
tissues, thin layer
tissues
Injectable, fast and
simple one-step
procedure; intimate
cell and material
interactions
Soft structures
Table 1: Characteristics of different scaffolding approaches in tissue engineering (Cont)

15. Pre-made porous scaffolds for cell seeding
Scaffolds are made of degradable biomaterials and
these has become the most commonly used and well-
established scaffolding approach. This approach
represents the bulk of biomaterial research in tissue
engineering, leading to enormous efforts in
development of different types of biomaterials and
fabrication technologies.
Many types of biomaterials can be used to make porous
scaffolds for tissue engineering provided that a
fabrication technology compatible with the biomaterial
properties is available

16. Decellularized ECM from Allogenic or Xenogenic Tissues for cell seeding
Acellular ECM processed from allogenic or xenogenic tissues are the most nature-
simulating scaffolds, which have been used in tissue engineering of many tissues
including heart valves, vessels, nerves, tendon and ligament. This scaffolding
approach removes the allogenic or xenogenic cellular antigens from the tissues as they
are the sources for immunogenicity upon implantation but preserves the ECM
components, which are conserved among species and therefore well tolerated
immunologically. Specialized decellularization techniques are developed to remove
cellular components and this is usually achieved by a combination of physical,
chemical and enzymatic methods. In brief, cell membranes are lysed by physical
treatments such as freeze-thaw cycles or ionic solutions such as hypo or hypertonic
solutions before separating the cellular components from the ECM by enzymatic
methods.

17. Cell sheets with self-secreted ECM
Cell sheet engineering represents an approach where
cells secrete their own ECM upon confluence and are
harvested without the use of enzymatic methods. This
is achieved by culturing cells on thermo-responsive
polymer, such as poly(N-isopropylacrylamide) coated
culture dish until confluence. The confluent cell sheet
is then detached by thermally regulating the
hydrophobicity of the polymer coatings without
enzymatic treatment. Such approach can be repeated
to laminate multiple single cell layers to form thicker
matrix.

18. Cell encapsulation in self-assembled hydrogel matrix
Encapsulation is a process of entrapping living cells within the confines of a
semi-permeable membrane or within a homogenous solid mass. The
biomaterials used for encapsulation are usually hydrogels, which are
formed by covalent or ionic crosslinking of water-soluble polymers. Many
types of biomaterials including natural and synthetic hydrogels can be
used for encapsulation provided that the conditions inducing the hydrogel
formation or the polymerization are compatible with living cells.
Encapsulation has been developed over several decades and the
predominating use is for immunoisolation during allogenic or xenogenic
cell transplantation.
A Synthetic Hydrogel

19. GROWTH FACTORS IN TISSUE DEVELOPMENT
 A growth factor is a naturally occurring substance capable
of stimulating cellular growth, cellular growth,
proliferation, healing, and cellular differentiation. Usually
it is a protein or a steroid hormone. Growth factors are
important for regulating a variety of cellular processes.
 Growth factors typically act as signaling molecules between
cells. Examples are cytokines and hormones that bind to
specific receptors on the surface of their target cells.
 They often promote cell differentiation and maturation,
which varies between growth factors. For example, bone
morphogenetic proteins stimulate bone cell differentiation,
while fibroblast growth factors and vascular endothelial
growth factors stimulate blood vessel differentiation
(angiogenesis).

20. TISSUE DEVELOPMENT
 Basically Tissue development begins with building a
scaffold from a wide set of possible sources, from
proteins to plastics. Once scaffolds are created, cells
with or without a “cocktail” of growth factors can be
introduced. If the environment is right, a tissue
develops. In some cases, the cells, scaffolds, and
growth factors are all mixed together at once, allowing
the tissue to “self-assemble.”

21. Another method to create new tissue uses an existing
scaffold. The cells of a donor organ are stripped and the
remaining collagen scaffold is used to grow new tissue.
This process has been used to bioengineer heart, liver,
lung, and kidney tissue. This approach holds great
promise for using scaffolding from human tissue
discarded during surgery and combining it with a
patient’s own cells to make customized organs that
would not be rejected by the immune system.

22. CONCLUSION
Tissue engineering efforts are currently underway for
virtually every type of tissue and organ within the human
body. Because tissue engineering incorporates the fields of
cell transplantation, materials science, and engineering,
personnel who have mastered the techniques of cell
harvest, culture, expansion, transplantation, and polymer
design are essential for the Successful application of this
technology. Various engineered tissues are at different
stages of development, with some already being used
clinically, a few in preclinical trials, and some in the
discovery stage. Recent progress suggests that engineered
tissues may have an expanded clinical applicability in the
future because they represent a viable therapeutic option
for those who require tissue replacement. More recently,
major advances in the areas of stem cell biology, tissue
engineering, and nuclear transfer techniques have made it
possible to combine these technologies to create the
comprehensive scientific field of regenerative medicine.

23. REFERENCES
 http://www.nibib.nih.gov/science-education/science-
topics/tissue-engineering-and-regenerative-medicine
 http://www.regenerativemedicine.net/Tissue.html
 http://rsif.royalsocietypublishing.org/content/8/55/153
 http://jasn.asnjournals.org/content/15/5/1113.full.pdf+html
 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2587658/
 http://en.wikipedia.org/wiki/Growth_factor
 John P, Fisher A, Mikos J, Tissue Engineering. CRC Press,
Taylor & Francis Group
 Professor S. Ismet Deliloglu Gurhan Tissue Engineering
Lecture Note, Department of Biomedical Engineering,
Near East University.

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