For people who actually want to read the paper, I'll put it after the break.
Meat derived from stem cells:
How,
what and why
By
Jack
Williams
RESARCH PAPER
BASED ON
PATHOLOGY LECTURES
AT MEDLINK 2011
Abstract
The world's meat demand is increasing at
such a rate that farming animals conventionally will not always be possible to
satisfy it. If meat can be made in laboratories or factories on an industrial
scale without the need for the whole animal, many of the problems associated
with its production and consumption will be solved. Currently, economical and technological
limitations prevent this from being a reality.
Introduction
Problems
relating to meat in the world today can be broadly grouped into three main
areas: ethical, ecological, and health.
Ethical
concerns about meat are embodied in organisations such as PETA, and revolve
around the treatment of animals1. There is evidence that some
livestock, especially pigs, may be more conscious than had been previously
thought. In the UK ,
many animals are killed unnecessarily inhumanely2, and ethical
concerns do not stop at the animals: workers are reported to be mistreated and
underpaid3.
Ecological
worries over rearing and consumption of meat are more complex, but perhaps more
concrete. The world's population is larger than at any point in history, and more
people are consuming more meat.4 Increased demand for meat results
in more land being used for livestock, with consequent deforestation and
destruction of other habitat.5 As such, the meat industry is
contributing significantly towards climate change and desertification.5
Eating
meat is inherently inefficient. Energy from the original food (e.g. wheat) is
lost over the course of the animals' lifetime from their metabolic processes so
we lose much of the original energy stored in the crop grown as animal feed. It
is thought that more vegan diets could hold the key to combating world hunger7,
as we could produce significantly more energy from the same amount of land.
This argument is debatable: it implies that the world shares food across
nations and continents. The idea however does have its merits, and the world
cannot afford meat production to continue to rise at its current rate without a
serious revolution of the entire process.
In an age
when we are dealing with world hunger and climate change, meat is a hindrance
on more than one front. Not only are we being energy inefficient, but animals
(particularly cows, pigs and chicken) contribute to the greenhouse effect
themselves through the release of methane and other greenhouse gases.8 Some
estimates place livestock as the origin of 50% of the world's greenhouse gas
emissions (including land use, etc.)8a
All
this is to say nothing of the processing and transportation that the meat
products in our supermarkets go through before being bought and eaten. This
involves fossil-fuels to transport them from farm to factory, to extract the
meat and then to transport the meat to the shops.
Several
widespread health issues in the world can be attributed to meat, particularly
in the developed world. The most notable and infamous of these is heart
disease. It is thought that the current level of cholesterol intake,
artificially high due to the huge supply of meat, is influential in the high
rate of circulatory disease related deaths (158,084 in the UK in 2010 - 32% of all deaths9)10.
Meat may
also be linked to cancer11, a growing concern especially in the
developed world. In 2010, there were 141,446 deaths from all cancers in the UK
(29% of total deaths)9. While this figure is for all cancers, and
there are many risk factors for different cancers, it was shown that even after
controlling for other lifestyle risks, such as smoking, eating meat still
increases the incidence of cancer in general.11
It is for
these reasons that there needs to be an alternative to meat. Vegetarian and
vegan diets would be the most cost efficient and effective methods to adapt to
but most people have grown too fond of meat to give it up. In vitro meat offers an alternative,
which could possibly address all these issues. This meat would be grown in
factory-style settings from stem cells.
Stem cells
can differentiate into many different types of cell thus they are mainly found
in the development stages of organisms. There just one type of cell - the
fertilized zygote - divides, matures and differentiates into a full organism
with many different types of cell. These cells, found in the blastocyst stage
of a mammalian embryo's development, are called embryonic stem cells. Because
they can become any cell in the body, they are known as "pluripotent". They can survive and divide outside the body
in a tissue culture, and are the most flexible type of stem cell.12
There
is one other type of stem cell found naturally: adult stem cells. These can be
classified into smaller groups, but all are distinct from embryonic stem cells.
Unlike the latter, adult stem cells cannot differentiate into any cell type but
are more prone to develop into one line of cells, for example a blood cell.They
can still differentiate into more than one type of blood cell, but not
something entirely different, for example a neuron.12 Adult stem
cells are grouped into epithelial,
haematopoietic, and neural. Epithelial stem cells are the most likely to be
useful in the production of in vitro
meat, because the main component of meat is muscle. 13 Muscle
however, is not the only component of meat, raising issues which will be dealt
with in the discussion section.
Discussion
Health
concerns surrounding the consumption of what is essentially cloned meat were
allayed in April 2010 with a risk assessment by the FDA into animal cloning, in
which they state: "no … hazards were identified that could pose food
consumption risks from ... clones."14
The
remaining issues facing in vitro meat production are threefold: practical, economical
and ethical. I address them in that order, as economics must follow the
practicality, and a system needs to have been formulated before it can be judged on moral grounds.
Practical
In
order to look at the difficulties of mass production of stem cell meat, it is
important to first understand the process by which it is made on a small scale.
Muscle has been kept alive ex vivo since 1912, when Alexis Carrel successfully
grew tissues in his lab.15 Most current work is done in Holland , whose government
has provided US $4 million for the venture., 4 times the prize that PETA was
previously offering
There
are two potential types of stem cells to use: the pluripotent embryonic stem
cells, and satellite cells, specific types of adult stem cell that can fuse
together and mature to form skeletal muscle. Here,
I will focus on the embryonic stem cells because there may be need for more
than one type of cell - meat is composed not just of myocytes, but adipocytes
and blood vessels too.
Muscle is
generally formed before birth, where myoblasts, a more specific embryonic stem
cell type, first differentiate into muscle cells, which then fuse together to
form long, multi-nucleated muscle cells.16 The long myocytes arrange
themselves first to be parallel in a 2-D structure, then in the 3-D shape of
the muscle they will grow into. Thus the simplest way to produce meat from stem
cells would be to use myoblasts. While it is an attractive concept use thigh
myoblasts to produce thigh meat, and wing myoblasts to produce wing meat, in
reality this will not happen. Meat gets its characteristic texture and taste
not from the type of myoblast that makes it, but from use and structure.
Proliferation &
differentiation:
The texture
of muscle has always been the biggest hindrance in imitation meats, which
heretofore have been largely limited to textured vegetable proteins. Much
texture is due to the fibrous nature of muscle. Muscles can be split up into 5
basic units: protein filaments (grouped together in sarcomeres), myofibrils,
myofibres, and fascicules. The protein filaments are either thick or thin, and
can slide across each other, giving the motor action that is typical of
muscles. The myofibrils are long chains of these filaments, contained within
the cylindrical myofibres, the multi-nucleated cells formed from myoblasts.17
Fascicules are bundles of myofibres, and the biggest component of muscle.
Because all of this comes from the progenitor myoblasts, to produce muscle
containing those 5 basic units should be possible with the right culture and
stimulation. This type of stem cell has already been grown in the lab and
considered for production on a large scale (albeit for other purposes). The
first step in the production involves getting as many stem cells from as few
stem cells as possible. This can be achieved by stimulating proliferation,
proved to be possible through the use of leukaemia inhibitory factor (LIF)18,
a cytokine. However, as one of the drivers of this proposal is ecological and
to reduce waste, as few chemicals as possible should be used. Alternatives to
sera and stimulants are preferable to than using anything that has to be
manufactured. An alternative to LIF presents itself in the form of the gene
nanog, which is present in pluripotent embryonic stem cells, but only before
differentiation. According to a recent study, this gene is at least partially
responsible for maintaining pluripotency while cells continue to proliferate.
Cells without nanog proliferate, but differentiate at the same time, thus the
number of pluripotent cells decreases.19 By overexpressing this gene
in stem cells, it is technically possible to produce as many cells as you want,
provided that the stem cells are given the correct nutrients to survive.
Because only one ‘batch’ of cells would need to be genetically modified to do
this, the price could be regarded as a start-up cost. As the procedure is
routinely performed within research studies, it is not cripplingly expensive.
Once they have
proliferated it is then important
to make myoblasts differentiate into myofibres. It has been shown that this can
be accelerated by stretching and then relaxing the cells; the area of myofibres
increased by 40% over 8 days.21 With this in mind, the ideal system
for their growth would include a way to stretch the cells as well as providing
them with nutrients and encouraging them to divide.
Nutrition:
The cells
must also stay alive. Many stem cells can survive in fetal calf serum (FCS),
which is the standard culture. Unfortunately, using this would be contrary to
the original agenda in this particular situation, because FCS requires the
slaughter of pregnant cows. The cows need to be alive to be impregnated, and
reduction in the use of live animals is one of the aims of this research.
Breeding cows only to kill them when they are pregnant would raise new ethical
issues and would not solve the maltreatment of the animals and workers in the
meat industry. Thus, in order for in vitro meat to be successful, an alternative to FCS
would have to be developed. There is already pressure to replace it in cell
culture for these reasons and also because of the risk of contamination if the
foetus contained microbes or pathogens, or if the abattoir was not sterile.20
Each type of stem cell requires specific sera to enable proliferation and
differentiation. Media for turkey breast, goldfish, and porcine cells have been
found, but these also contain animal derived substances such as chicken serum. Development
of non-animal related substances that stimulate growth and differentiation while
providing stem cells with necessary nutrients is a prerequisite to the use of in vitro meat as described.
Structure
& texture:
The tissue culture,
once it begins differentiating, needs a way of organising itself into the 2D
and 3D structure. A scaffold needs to fulfil several criteria: it should be
flexible enough that the muscles can contract (essential to muscle growth,
particularly for the texture of the meat), have a sufficiently large surface
area to promote attachment (muscle cells are require anchorage in order to grow)
and aid diffusion. It should also be able to be separated from the finished
product easily, or alternatively be edible. In vivo, muscles grow according to
the structure they are on and the blood supply to them. It was found that in
order to produce muscle in myofibres and fascicules, the myoblasts needed to be
cultured on something with a stiffness that simulated healthy, non-damaged muscle.22
The two main proposals currently are either small edible beads or a porous
sponge, both made from collagen, and the sponge derived from cows.23 As with FCS, the second method is inappropriate if
it cannot be replicated without the use of animals. It is also much more
difficult to retrieve the cells from once they have diffused into the sponge.
Therefore, of these two options, the beads are the better.. Even with the
beads, it would be hard to stretch the cells, particularly on an industrial
scale. One solution suggested is to synthesise spheres of the same size and
porosity, but which expand and contract as a response to environmental factors,
like pH.25 So far, all in vitro meat research has been geared
towards unstructured meat, useful for mince and patty-based foods such as
burgers or meatballs. The collagen bead structure is useable for anything needing
only muscle cells, but if cultured meat is ever to be considered a real
alternative to meat, there needs to be more variety in the form of more
‘natural’ meat cuts. Examples are chicken breasts, or even a steak. These both
have other types of cells (e.g. adipocytes and blood vessels). The second of
these is worth focusing on; and highlights another hindrance to the production
of meat from stem cells. The importance of blood vessels may seem trivial, but
in fact they serve a very specific purpose in the body during myogenesis and
later throughout the life of the organism and its muscles. The purpose of the
blood itself will be discussed later, but first it is important to understand what
needs to change to the scaffold before it is possible to grow complex meat
structures. Muscle cells need to be within 0.5nm of a nutrient supply in order
to survive, which means that in a 3D structure there needs to be an oxygen
carrier that can diffuse between the cells, or a vascular system. In 2D sheets,
this is not a problem - the whole sheet can be submerged in the medium, and all
cells will be exposed to it. Therefore, without significant advances in the blood
vessel synthesis, in vitro meat will be limited to flat sheets of muscle that
have to be pressed together.
Adipocytes (fat cells)
also contribute to the texture. In the patty meat method, it would not matter
where the fat was grown as long as it was grown in the right proportions to
give the meat the desired texture, because all the material is simply pressed
together. Because of this, the fat could be grown in a separate culture, then
added at the end.
Blood:
Blood is important for muscles in several
ways. Firstly, blood supplies muscles with the glucose and oxygen needed for
respiration and takes away the waste products. Tissue culture can supply
nutrition, and waste products can be removed through letting them run off or
diffuse into the air. However, oxygen delivery cannot be performed by the
medium itself. This can either be done through an artificially produced
haemoglobin or by using perfluorochemicals (PFCs). PFCs, while easy to produce,
are not broken down by any known process (the carbon-fluorine bond is very
strong), so remain in the environment as a persistent pollutant, and reports
suggest that they do serious damage to several organs in animals.24
With this in mind, artificial haemoglobins (or similar) are the more
appropriate substances to use. These have already been produced from
genetically modified plants and microbes,23 but no studies have been
done on an industrial scale.
The second function of blood vessels in meat
is to provide texture and to encourage the muscles to grow in a particular way.
More advanced types of meat (non-patty based meat) would require vessels, and
that is one reason why it is so difficult to make anything more advanced than a
sheet of muscle. An area of research that could lead to more complex structures
is vasculogenesis, the production of blood vessels without the presence of
other vessels already there. This focuses around angioblasts, another type of
stem cell. Technology for the use of vasculogenesis in tissue culture is not
yet being researched, so this cannot be performed in the short term.
Health
issues:
Two health
issues that present themselves are the content of the meat and the sterility of
the factories. If the meat is to help combat the health issues related to our
diets, changing what the meat is made of could prove vital. By altering the DNA
of the progenitor cells, meat could be made that contains less omega 6 and more
omega 3 fatty acids. In the western world, the ratio of these two is in favour
of omega 6, rather than omega-3, which we cannot manufacture but improves our
health.26
The second health issue
is the risk of infection. Factories would need to be kept free from pathogens
in order to keep the meat safe for human consumption. This could be achieved by
using antibiotics, but that could select for super resistant strains, as
happened with MRSA. A better system would be to vet the cells that came in, and
keep them in an argon atmosphere so that any aerobic microbes would be killed. The
muscle would continue to draw its oxygen from the oxygen carrier, not from the
air, so would survive. This suggestion does not account for any pathogens that
live in the culture, so the meat would have to be tested after production as
well. To minimise risks, hygiene in the factories would need to be kept to a
very high standard and any contact with the culture, scaffold and cells kept to
a minimum.
Economical:
From an economical
perspective, the implementation of a scheme like this would require massive
investment. Harvesting the original cells can be done from a simple biopsy, and
bears negligible cost. Genetic modification may be more expensive, but if successful it should be a cost paid very
infrequently. The real expense rests in the medium and the scaffold. Both are
consumed in the process, so need replenishing. As mentioned above, there is not
yet a suitable serum for the growth of the cells, and the scaffold is a very
specific structure that cannot be substituted by anything cheaper Thus it is
incredibly unlikely for meat to be grown in vitro on anything larger than a
laboratory scale in the foreseeable future. Even if both serum and scaffold
were inexpensive and available, a vast start-up cost would be necessary to
build the facilities needed: nothing could be retrofitted for such a particular
purpose.
Another issue to
consider is whether consumers would want to eat something cultured in a lab. In
February of this year, 68% of participants in a poll in the Guardian said that
they would eat stem cell meat;27 it has more opposition than in vivo
meat, but a significant proportion of the public appear to be accepting of it.
Ethical:
Ethically, there are
not many problems with in vitro meat. PETA backs the concept, describing, it as
the morally acceptable alternative to eating meat grown on an animal. The
issues arise from the harvesting of cells and the use of animal sera. In order
to obtain embryonic stem cells, embryos have to be killed. This is questionable
when the embryos are human, but there are fewer objections for animals. Each
animal embryo would be used to feed millions. Harvesting cells from adult
animals would have to be done under anaesthetic and with sterile tools, but is
essentially no different to surgery, so again there is no issue. The one moral
problem is with animal sera, which has been discussed above. Collection of FCS
and other media from animals is often done unethically in abattoirs, which is
one of the problems in vitro meat could tackle, if the cells were less
dependent on substances such as FCS. In order for in vitro meat to be
considered truly ethical, it must use synthetic alternatives to these sera.
Conclusion
In vitro
meat from stem cells, while physically possible to produce in the small scale,
cannot be economically scaled up to make a large enough amount of meat for it
to be economically viable. Before that is possible, several advances need to be
made. Firstly, there needs to be an alternative to using animal sera in the
cell culture. Without this, in vitro meat will be both unethical and
impractical. Secondly, a scaffolding material must be created that is porous,
edible, and stretches in response to a stimulus that does not affect the meat.
Thirdly, without a way to create functioning blood vessels, including
circulation of a natural or synthetic blood, in vitro meat will not be able to
progress past 2D sheets of meat that need to be pressed together to create
patty-based meat products. On this basis, in vitro meat is unlikely to be
either an alternative or a supplement to in vivo meat.
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