Tissue Engineering and Liver Regeneration: An Overview

In this assignment we will discuss about tissue engineering and below are the summaries point:- Tissue engineering aims to reconstruct cells into desired organs or tissues to improve functioning. Liver is a regenerative organ, but its cells deteriorate faster in vitro. Liver tissue engineering primarily uses biologically compatible scaffolds for growth and stem cells for proliferation.

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06 Feb 2019
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Introduction
Tissue engineering is defined as the reconstruction of cells into the differential cell of the
desired organ or tissue, with an intention to improve the functioning of that organ (Lanza,
2011). This involves the combination of cells, engineering materials, factors suitable for
biochemical and physiological conditioning. Regenerative medicine is a part of the tissue
engineering which uses the bio-engineering principle for treatment (Atala, 2018). To solve
the health related issues, it uses innovative methods, ideas and synthesizes various
biomaterials constructs (Ratner, 2013).
Cell and tissue structure and function
Liver is the largest gland present in human body that comprises of mainly hepatic cells (60%
of tissue). Other cell line include kupffer cells, and vascular system. It secretes bile, digestive
fluids, stores glycogen, metabolizes fat, protein, and carbohydrate, and synthesize blood
clotting factors. The hepatocytes present in liver consists of high regenerative capacity which
makes the liver one of the regenerative organ. However, when these hepatocytes are
maintained in vitro they tend to deteriorate faster functionally (Vildhede, 2018).
Tissue dynamics
Liver secretes more than 900 ml of bile every day, which is required for digestion.
Additionally, it is also involved in secretion of enzymes, blood-clotting factors and is
involved in storage. The cells proliferated with the help of matrix based transplantation
system leads to dysfunction and loss of cells. This might be caused because of the change in
nutrients and the absence of blood flow, which is important to be considered while focusing
on liver tissue engineering.
Natural rate of renewal/maintenance
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Liver tissue have the maximum capacity of regeneration in human body system, which is
commonly known as pluripotency (Arumugaswami, 2016). Despite its inherent tendency of
regeneration in vivo, the hepatocytes proliferate weakly in vitro. This eventually makes the
culturing process and effort more difficult for the researcher. The functional characteristics of
the cell line demands immediate access to the vascular pool in the search of nutrient and
removal of metabolic waste. This in turn creates an obstruction on the engineering histio-
substitutive constructs (Burra, 2016, pp.433).
Wound healing/regenerative processes
Tissue engineering of liver uses biologically compatible scaffolds that facilitates smooth
conditions for the growth. This form engineering is primarily concerned with i) designing of
external device for liver support, ii) in vivo liver regeneration with the help of cell
transplantation, and iii) implantable devices (Mazza, 2018, pp.131).
Cells
Phenotypes (identification)
Human hepatocytes are recognized cell source to be used in implantable devices and BAL.
Due to isolation from the discarded cadaveric liver samples, there exists only limited primary
human hepatocytes.
Sources
For the purpose of liver tissue engineering adult, induced and embryonic important stem cells
are being used. The primary benefit of using stem cells is its proliferation characteristics that
makes is possible to grow scalable amount of cells (Sakiyama, 2017, p.1974). Apart from
this, the stem cells are adept of expanding into any cell type within human body. This feature
of stem cell represents an inexhaustible source of cell. The various types of stem cells used in
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liver regeneration includes adult stem cells, Oval cells, induced and embryonic pluripotent
stem cells, hepatoblast, hepatocyte-like cells, and human foetal hepatocytes.
In-vitro culture, bioreactors, cell and gene therapy, immune rejection
In this section, it is important to discuss about porcine hepatocytes, which is considered as
one of the most efficient and cheap source of cell for liver tissue engineering. Due to limited
availability of human hepatocytes, the cells of pig are used (because of similar physiology) in
implantable devices and BAL. Several of efforts are being made to maintain functional
performance of hepatocytes through extra cellular matrix (ECM) (Swinehart, 2016, pp.351).
However, there is risk of transfer of the protein related incompatibility, zoonotic diseases, and
immune responses (Morini, 2018, pp.301).
ECM
Composition/identification & Man-made substitutes
Various materials (including natural components, biomaterials, and nano-materials) used in
the tissue engineered scaffolds includes the followings, as discussed below:
 Poly-Terephthalate (PET) film - The 3-D culture on the film was started for surface
modification like of ECM proteins coated on collagen, fibro-nectin, and gelatine.
These strategies are helpful for BAL-based devices and drug screening platforms
(Saldin, 2017, pp.1).
 Matrigel - When the non-transformed cell line of human hepatocyte is cultured on the
3D matrigel, which then forms structure similar to tissue, from the cells which further
reacquire the liver specific functions.
 Hydrogels – These are the injectable scaffolds used for the purpose of cell
transplantation for their proliferation and biodegradability. This technique is
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considered as useful for the improvement of applications related to engineered liver
tissue.
 Polyvinyl Fluoride (PVF) - It is used mostly in the construction of BAL in the form of
3D porous scaffold which can immobilize the pig and mouse foetal liver cells. Collagen based scaffold - In the present day, collagen being a natural extra cellular
matrix (ECM) polymer is implemented as a scaffold for the hepatocyte
transplantation. This form of unique 3D cell provides opportunity for maintaining the
cell within a differentiated state (Ribeiro, 2015, pp.46).
Signaling: Tissue specific signaling and signaling mechanisms
The concepts in modern tissue engineering includes: scaffolds, cells, growth factors and
signalling molecules. The prime role of molecular signalling includes multifunctional protein
like, cytokines and growth factors, as well as the neurotransmitters. It helps in modulating
and transmitting the cell to cell signalling and other relative cellular activities. The
knowledge of tissue specific signalling molecules contributes important role in advancement
of liver tissue engineering (Kaur, 2016, pp.1).
A link to the signalling mechanism with the aspects of tissue engineering, can be correlated
with associated limitations. The basic problem for the use of hepatocytes while differentiating
it from either human-induced pluripotent stem (iPS) cells or embryonic stem (ES) cells
includes, the functional deterioration of matured cells in the vitro conditions for a short span
of time. Another major limitation in liver support as well as regeneration therapy is the
availability of proper and suitable cell types. Differentiation of stem cell into hepatocytes is
not mature enough and is still in its infancy stage. Thus, there is need for developing a robust
and efficient protocol which can effectively differentiate the stem cell in to a mature
hepatocyte.
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Tissue damage/dysfunction/pathologies/failure/lose
According to a survey report of World Health Organization, there are almost 500 million
people being affected by chronic liver disease worldwide and out of them 2% are reported
with mortality (Lanza, 2005). In such a situation, liver transplant is the only solution for the
patients, however the availability of donor organs is limited. In order to fulfil the lack of
donor organ for the patients, tissue engineering is introduced in the medical field. The tissue
engineering focuses on creating a fully functional organ (liver) which can be either
transplanted in the body or used as extracorporeal object. The technical method used in liver
tissue engineering is based on allocation of adult hepatocytes or the stem cell derived
hepatocytes in the three-dimensional structure which is able to survive or maintain a
functional phenotype.
Regenerative solutions under research, and clinical successes
The findings of medical science have founded the base of liver tissue engineering, as because,
hepatocytes which are present in liver cell have maximum capacity of regeneration among all
the tissues present in human body. However, it is important to note that in-vitro condition is
not favourable for the functioning of hepatocytes, as a result of which, it tends to deteriorate
faster. Therefore, it is important to discuss the research findings and success in medical
science related to tissue engineering.
Tissue regeneration has a major impact and advantage for treating a wound, by providing the
patients with regenerated tissue for filling the large wounds. This treatment procedure
reduces scarred possibility and heals the wound easily, providing a greater quality of life to
the patients. In the present scenario, the liver tissue can be collected from the patients and
grown within animals in lab. This procedure helps in finding the most efficacious treatment
method and design the medication accordingly. This method is much more efficient and
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effective than the usual error and trial method on the patient itself. Researchers are planning
to grow whole organs in future which can be used as the transplant replacement for liver and
also for kidney and heart. This technique would help in eliminating the requirement of
injecting the anti-rejection drugs. Moreover, most of the researchers have proved the benefit
of using scaffold material, ECM, cells, and protein factors in tissue engineering. Most of the
scientists have claimed that using ES cell differentiation with in vitro hepatocytes results in
better functioning as well as mature expression (Sol, 2017, pp.157).
Researchers have implemented another new approach on hepatocyte transplantation. In this
technique, an engineered functional hepatic tissue graft is implanted effectively in skeletal
muscle of mouse having acute hepatic failure conditions. The experiment revealed that after
transplanting, the rate of survival is increased by 60 percent. Following are the practical
considerations that are available as an effort of medical science, in the field of liver tissue
engineering.
 Extracorporeal liver support devices- These devices are used for the patients as the
life-supporting instrument for a limited time period, while waiting for the donor organ
to come. In this technique bio-artificial liver (BAL) containing hepatocytes, cellular
component and a bioreactor component is used for the patient. The bioreactor is
particularly designed so as to provide an optimal condition for better performance of
cell. It is reported that for an actual clinical therapy with minimum of 10% of the total
liver function should be provided by the bio-artificial liver and for this it requests 10 ^
10 hepatocytes.
 Cell transplantation- Cell transplantation is considered to be more effective than any
other solid liver transplantation. From a single donor organ, this technique can cure
the livers of several patients and the remaining cells can be cryopreserved for use in
the time of emergency. Experimental studies are being done to access the
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transplantation of foetal hepatocytes, adult hepatocytes and stem cells. For cell
infusion into the organ both splenic and intrahepatic sites are considered to be the best
sites. This technique of tissue engineering is found to be successfully for curing the
inherited metabolic disease in most of the young patients. However, it is also to be
noted that the rate of engraftment of the hepatocytes (transplanted) is less than 30%. Implantable devices- In order to increase the life-span and functionality of the
transplanted hepatocytes, the implantable devices have been developed. Implantable
devices in the form of encapsulation vehicles are considered to be the best as it can
prevent immunological problem (Lee, 2015, pp.36).
Practical hurdles
Ethics, Regulation, and Translation
Ethical and regulative measures are first important for the animal usage for tissue
regeneration, as the purpose should be linked with human benefits at a balance to
environment. Secondly, for clinical trial, the evaluation of harm and risk should be explicitly
described in the proposals (Raschzok, 2015, pp.67). Other ethical consideration include
confidentiality and privacy, such that patient record must not be shared with unauthorized
entity. Other than this, ethical debates are still continuing in the field of using cloning, gene
therapy, and nanotechnology in the tissue engineering field.
Manufacture and storage of tissue engineered products
For the improved storage purpose of primary human hepatocytes cryopreservation technique
is used to preserve the cells for further use. For example, the vitro models like spheroid
culture and sandwich have been used to preserve the specified function of hepatocytes (Vries,
2015, pp.771).
Validation/models for evaluation of efficacy
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One of the significant challenge in tissue engineering is the requirement of more complex
functionality, which provides extra factors for organ, tissue, and scaffolds development.
Sometimes there is seen certain limitations in tissue transplantation such as; scarcity of
matched organ for the patients waiting (Kashte, 2018). Moreover, constant and regular
immunotherapy on the patients during the treatment procedure can deteriorate.
Summary
In the present report, the tissue engineering of liver has been described. Tissue engineering
has become significantly important for the restoration or replacement of the hepatic tissue
which has been damaged by injury or disease. For this purpose, the tissue engineering
requires three basic components, viz; biocompatible materials, cells, as well as growth
factors. Liver transplantation is considered as one of the most effective liver treatment for
end-stage diseases (Wobma, 2016, pp.101). This research for restoring the functions of a
normal liver is under investigation. In this purpose, particular material or matrix acts as a
scaffold by providing a 3D environment towards cell interaction and proliferation.
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References:
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