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Transcript
Human
Intervention in
Evolution
Selective Breeding
Selective breeding is an example of artificial
selection.
In this procedure, only those animals that display
a particular trait in their phenotype or are known
carriers of the trait are chosen to reproduce.
The deliberate selection by a breeder of specific
animals to provide the genetic material for the
next generation is a process known as selective
breeding.
This is in contrast to the random mating that
occurs when any male animal in a population has
an equal chance of mating with any female.
Examples of Selective Breeding
From early times, artificial selection was carried out to
improve herd quality.
Farmers selected the best males for mating with their
breeding females. In the case of beef cattle, bulls were
chosen for their genetic superiority in terms of desirable
market characteristics, such as meat yield and non-fatty
carcass.
For dairy cattle, desirable market characteristics included
milk yield and butterfat content.
Merino rams are chosen for the fineness (thinness) of their
wool fibre and the yield of greasy fleece. Males were also
selected based on other inherited features, including good
conformation (form, outline or shape), high fertility based
on sperm counts and the absence of any known genetic
defects.
Through artificial selection, farmers could improve the
quality of their herds.
Problems with artificial selection
Artificial selection is in contrast to natural selection that favours
only those inherited features that enhance survival and
reproduction in the wild.
Features that are economically important or aesthetically
appealing but that do not contribute to survival and reproduction
are not favoured by natural selection and hence are not seen in
populations in the wild.
Examples of features maintained only by
artificial selection can be seen in domesticated
animals, such as:
– Jacobin pigeons, whose distinctive
arrangement of neck feathers forms a ruff
that masks their faces, except from
immediately in front
– hairless cats and dogs
– English bulldogs, whose greatly shortened
muzzles result in breathing problems
Features such as these are only maintained in
the gene pool of the populations through
human intervention using selective breeding.
Reproductive Technologies
In commercial herds and flocks, new
reproductive technologies associated
with selective breeding include:
– artificial insemination
– sex selection through sperm sorting
– multiple ovulation and embryo transfer
– oestrus synchronisation
Artificial insemination (AI)
This technique brought about changes in herd management
by altering the ‘how’, ‘when’ and ‘where’ of breeding.
AI involves collecting semen from a selected stud animal
and then introducing this semen by artificial means into the
reproductive tract of females of the same species.
When first developed, the technique of AI involved the use
of fresh semen only.
In 1949, a successful technique was developed for freezing
semen..
The freezing technique involves adding semen to a special
solution with a controlled pH and which comprises a
mixture of various chemicals, including glycerol.
Samples of this diluted semen (0.25 mL volume) are taken
up in ‘straws’, frozen rapidly and stored in liquid nitrogen at
–196°C.
Under these circumstances, semen samples can be stored
for many years and still retain their ability to fertilise an
egg after thawing.
Artificial insemination (AI)
The use of AI increases the number of offspring that one stud animal
could produce.
One ejaculate from a male bull contains sufficient sperm to fertilise ten
eggs.
This volume of semen can be divided into ten portions and used to
artificially inseminate ten cows and so can produce ten offspring.
In contrast, in a natural one-to-one
mating, that same volume of ejaculate
would normally produce just one
offspring.
Through AI technology using frozen
semen, physical and temporal barriers
to mating are removed.
This technology means that one prize
stud animal can:
– fertilise many more females than
under natural conditions
– fertilise females located hundreds or
thousands of kilometres distant from
that stud animal because its frozen
sperm can be easily transported
over great distances
– fertilise female animals and produce
offspring long after its death.
Genetic impact of AI
in animal breeding
Through the use of AI technology and the transport of frozen semen, the
genetic influence of a small number of stud animals has been greatly
extended over time and space.
The use of a small number of stud males in a breeding program means
that genetic variation is reduced compared with the situation that would
exist if random mating occurred.
Because stud animals are chosen for their superiority in a limited number
of inherited traits, several consequences from the widespread use of a few
stud animals may result, for example:
– specific alleles of a few selected genes will become predominant in the herd and
alternative alleles of the genes concerned will be lost
– potentially valuable alleles may be unwittingly lost from the genetic composition
of the herd because other inherited features are ignored.
Overall, the widespread use of AI using a limited number of stud animals
can result in a loss of genetic variation from the genetic composition from
that species.
This loss of genetic variation may not be immediately noticed. If, however,
conditions change (for example, the outbreak of a new disease), the loss
of alleles such as those for disease resistance may have significant
consequences.
Sex selection through sperm sorting
Under normal circumstances, a sex ratio of about one male to one female
is expected in live born mammals.
In the beef industry, however, male calves are preferred because they
have more beef (muscle) on their carcasses at a given age than females.
In contrast, in the dairy industry, female calves are necessary for milk
production.
Sex selection is now possible.
After semen has been collected from a stud bull, for example, it is possible
to treat the semen and separate the sperm with X chromosomes from
those with Y chromosomes.
Sperm cells are first labelled with a harmless fluorescent dye that binds to
DNA. The X chromosome in mammals is larger and contains more DNA
than the Y chromosome.
As a result, the sperm with X chromosomes fluoresce more brightly than
those with Y chromosomes.
After labelling, the sperm are then separated into two groups depending
on their fluorescence. One group comprises the more brightly fluorescing
sperm with X chromosomes and the other group comprises the less
brightly fluorescing sperm with Y chromosomes.
The use of this sperm separation technique has allowed sex selection to
occur on a large scale.
Multiple Embryo Transfer in Livestock
Multiple ovulation and embryo transfer (MOET) allows highquality cows and ewes to make a much greater than normal
contribution to the future generations.
Multiple ovulation refers to a process whereby a female receives
injections of the follicle-stimulating hormone (FSH) that stimulate
her to super-ovulate, or produce multiple eggs.
An injection of gonadotrophin-releasing hormone (GnRH) is also
given to make all the eggs mature at the same time.
Embryo transfer
refers to the process
through which embryos
at day 6 to 7 of
development are
removed from the
reproductive tract of a
female and
transplanted into the
tracts of other females
of the same species.
These females act as
surrogate mothers and
carry the embryos to
term and give birth.
MOET is another example of human intervention in the evolutionary
process.
Multiple Embryo Transfer in Sheep
Process is as follows:
– A high-quality donor ewe is treated so that she super-ovulates.
– When her eggs are released, they are fertilised, typically through AI
with sperm from a selected ram.
– Fertilised eggs develop within the ewe’s uterus for about 6 days.
– At the end of that time, the embryos are flushed from the ewe’s
uterus.
– On average, about seven embryos can be collected from a single flush.
– These embryos are immediately transferred directly into the uterus of
young recipient ewes that will be the surrogate mothers of these
embryos.
– Embryos not transferred to recipient ewes are frozen in liquid nitrogen
and stored for later use. (Frozen embryos can be stored indefinitely.)
– The same donor ewe can be used for MOET procedures several times
during a breeding season.
– Over a normal reproductive lifetime, one ewe might produce 30 eggs.
– Multiple ovulation, however, greatly increases this egg output.
The advantages of embryo transfer are that genetically important
female lines can be multiplied at much faster rates than can occur
through normal reproduction and that valuable embryos can be
stored.
Under conditions of natural selection, this would not be possible.
One consequence is that embryo transfer reduces the genetic
variability in a flock by restricting the number of breeding ewes.
Manipulating breeding cycles
It is now possible to synchronise the time of oestrus or sexual
receptivity of female farm animals, such as cattle and sheep.
Oestrus synchronisation results in all sexually mature females
being in oestrus within a predictable and narrow time frame, with
the result that the time of fertilisation in a herd or flock, either by
AI or natural mating, can be more efficiently managed.
Synchronisation is also necessary for MOET procedures so that the
intended embryo donor and the recipient surrogate mothers will
come into oestrus at the same time.
Advantages of synchronisation include:
– less time (and hence lower labour costs) needed to test animals to see
if they are in oestrus or not
– higher fertilisation rates and birth rates
– more uniform and manageable crops of calves or lambs, since all
young are born within a short period
– lower mortality rates because greater oversight of all newborns is
possible.
Oestrus Synchronisation
Oestrus synchronisation can be achieved in a number of ways.
One method depends on the fact that the hormone, progesterone,
inhibits ovulation by stopping production of another hormone,
oestrogen, that is needed to bring female animals into oestrus.
By adding an external source of progesterone to female livestock,
oestrus production and the associated ovulation are suppressed.
When the source of progesterone is simultaneously removed from
a group, mature females go into oestrus and ovulate within a
short time period. How is this external progesterone delivered?
Methods to supply progesterone to farm livestock include:
–
–
–
–
feeding using a dietary supplement
implants under the skin
sponges inserted into the vagina
CIDRs (controlled internal drug releasing devices) inserted into the
vagina.
In the case of CIDR (pronounced cee-dar) use in cattle, the insert
is left in place for seven days. When the insert with its supply of
progesterone is removed, the level of circulating progesterone
drops and oestrus begins within three days.
Artificial pollination in plants
A process similar to AI is used by plant breeders with populations of
cultivated plants. In plants, the process is termed artificial pollination.
Artificial pollination is another example of human intervention in the
evolutionary process.
Unlike AI, artificial pollination has been used for centuries (Mendel and his
peas)
The process of artificial pollination involves:
– removal of unripe stamens from the plant to be fertilised
– protection of the stigma of the selected female plant from stray pollen
– collection of pollen to be used in the artificial pollination
– transfer of the donor pollen onto the stigma of the female parent.
Artificial pollination using a limited number of plants as the source of
pollen may alter the genetic composition of a plant population under
cultivation and result in less genetic variation in the population compared
with a situation of random mating.
Artificial pollination is used in the creation of new plant species.
In this case, pollen is collected from one species and it is transferred to
the stigma of a second closely related species.
Creating new species using
artificial pollination in plants
One example of this was the creation of a wheat–rye hybrid plant. A wheat
species (Triticum turgidum) was artificially pollinated using rye (Secale
cereale).
The result of this artificial pollination was a new plant species with one set
of wheat chromosomes and one set of rye chromosomes.
Such a plant would be infertile because its chromosomes could not
undergo the normal pairing that occurs during meiosis.
By using a specific chemical treatment, a doubling of the chromosome
number in the plant cells occurred so that the cells then contained two
sets of wheat chromosomes and two sets of rye chromosomes. As a
result, the mature plant would be fertile because it could undergo normal
meiosis.
The new species is known as triticale (triti- from the wheat parent and –
cale from the rye parent). This new species combined the desirable
genetic qualities of wheat with the inherited hardiness of rye. Rye can
grow in cold climates and on low nutrient soils. In contrast, wheat is
grown mainly in temperate parts of the world.
Triticale is the first artificially created cereal crop to be developed and is
grown in a number of countries.
Artificial pollination combined with the use of chemical treatment to double
the chromosome number in cells accelerates evolution.
This technique allows genetic material from two species that would
naturally have remained reproductively isolated to be artificially combined
Cloning
Reproductive technologies, such as artificial insemination and artificial
pollination, involve modifications to the sexual reproduction that occurs
in animal and plant populations.
These technologies restrict the source of sperm or pollen to that from
selected animals and plants and use artificial means to transfer the
selected sperm and pollen to the reproductive structures of females.
In sexual reproduction, two parents contribute equally to the genotype of
the new organism.
In contrast, other reproductive technologies, such as cloning, involve
methods of asexual reproduction in which the genetic information of the
new organism comes from one ‘parent’ cell only.
Mammals are normally produced through a sexual route, that is, from the
fertilisation of an egg by a sperm, with the fertilised egg then developing
into a new embryo. However, other techniques exist in which a new
mammalian embryo does not arise from a single fertilised egg but from
other artificially created cell types.
These techniques are typically referred to as ‘cloning’ but it is important to
realise that there are different cloning techniques.
Cloning techniques are yet another example of how humans can intervene
in the evolutionary processes.
Embryo Splitting
Embryo splitting occurs when the cells of an early embryo
are artificially separated.
Typically, the embryo is produced through in-vitro
fertilisation (IVF) and, using a very fine glass needle, the
embryonic cells are separated in the laboratory.
Each single cell is then implanted into the uterus of a
surrogate female parent where embryonic development
continues. As a result, organisms produced through the
splitting of one embryo are identical.
Embryo splitting has been used for some years in the
livestock industry.
In cattle, for example, embryo splitting enables the genetic
output from one mating of a top bull and a prize cow to be
multiplied.
Instead of just one calf from such a mating, several calves
can be produced using surrogate mothers.
Cloning by nuclear transfer
Some possibilities exist to manipulate
cells and their nuclei. It is possible, for
example, to:
– remove the nucleus from a cell (when this
occurs the cell is said to be enucleated)
– transfer the nucleus from one cell to an
enucleated cell to form a re-designed
nucleated cell
– fuse a somatic cell with an enucleated cell
The birth of two sheep, Megan and
Morag, in 1995 marked a significant
scientific milestone.
These two sheep were the first
mammals ever to be cloned using
nuclear transfer technology.
Each of these sheep developed from an
unfertilised enucleated egg cell that was
fused with an embryonic cell that
contained its nucleus.
In each case, the embryonic cell used
came from the culture of one embryonic
cell line; as a result, Megan and Morag
were identical twins.
What about Dolly?
Dolly the sheep is the first animal that comes to mind when we talk about
cloning?
Why was Dolly so special?
Dolly was created by nuclear transfer like Megan and Morag, but with one
key difference. In Dolly’s case an adult somatic cell was used as the donor
rather than embryonic or fetal cells.
The arrival of Dolly in 1996 represented the first time that cloning via
nuclear transfer using adult somatic cells was successful.
The use of adult somatic cells, such as skin cells, to construct new
organisms represents remarkable human intervention in the evolutionary
processes.
Through this means, cells from sterile animals or from animals past their
reproductive period, or even dead animals, can provide all the genetic
information of new organisms.
In nature, the normal evolutionary processes would not allow these events
to occur.
As a side note - Dolly was named in fun after Dolly Parton, because she
was derived from an udder (mammary gland) cell.
Other animals cloned by nuclear
transfer using adult somatic cells
Matilda, the sheep, was the first lamb to be
cloned in Australia and was born in April 2000
Suzi and Mayzi were Australia’s first calves to be
artificially cloned from the skin cells of a cow
fetus. Suzi and Mayzi are identical twins but
were born two weeks apart in April 2000.
cc (short for carbon copy) was the first cat to be
artificially cloned using a cumulus cell from an
adult female cat, Rainbow, as announced by a
group of American scientists in February 2002.
Snuppy, the Afghan hound, was the first dog to
be artificially cloned from an ear cell of a 3-yearold Afghan hound, as announced by a group of
South Korean scientists in August 2005.
Snuppy is short for Seoul National University
puppy.
Snuppy and his surrogate mother
Cloning: the downside
The success rate in initiating development of the egg cell after transfer of
the donor nucleus is low.
For example, in the case of an artificially cloned calf, known as Second
Chance, 189 implantations were made into surrogate cows before a
pregnancy was achieved.
This case, however, was remarkable because the adult cell that provided
the donor nucleus came from a 21-year-old Brahman bull called First
Chance.
This was an extremely old adult cell to use as the starting point for
cloning. Because of testicular disease, First Chance had been castrated so
that he was sterile when one of his body cells was successfully cloned.
The kitten cc, produced by somatic cell cloning, was the only one of 87
embryos implanted into surrogate mothers that survived to term.
To get Snuppy, 123 dog embryos were surgically implanted into surrogate
females and, of these, only three survived for a significant period, with
one dying before birth, one dying soon after birth, and the sole survivor
being Snuppy.
Dolly was the only live birth from a series of 277 cloned embryos.
Clearly, somatic cell cloning is presently far from routine, with less than
one per cent of the cloned embryos surviving beyond birth.
Cloning: the downside
There is evidence that each time a mammalian cell divides, the
specialised ‘ends’ of their chromosomes lose some DNA base pairs
and become shorter.
These ‘ends’, which are known as telomeres, do not carry
structural genes.
Some scientists suggest that the shortening of the chromosome
ends is associated with ageing.
One question that has been asked is: Will ageing be more rapid in
a cloned animal that originates from an adult cell which already
has shortened chromosome ‘ends’ than in a normal organism?
The death of Dolly in February 2003 suggested that this may be
the case. Six-year-old Dolly was put to sleep because of a
deteriorating lung disease and arthritis, unusual conditions for a
sheep of Dolly’s age and one that was housed indoors, since
sheep can live for about 12 years.
The question of possible premature ageing in cloned mammals
continues to be explored by scientists.
Interestingly, some animals cloned by nuclear transfer from adult
somatic cells have shown increased telomere length compared to
age-matched controls. How? We don’t know at this stage!
Other problems with cloning
Apart from our lack of knowledge and the low
levels of success, the following are also problems
associated with cloning:
–
–
–
–
–
Tumours
Genetic defects
Overgrowth syndrome
Premature aging (genetic age)
Massive quantities of human eggs would be required to
clone humans
– Potential insertion of genes that cause problems
– Reduction in adaptability due to decreased genetic
variation
Attitudes to cloning
Public attitudes to animal cloning are mixed.
Some people support the concept because they believe that
it will benefit people by providing a source of tissues for
transplantation or other products.
Other people oppose the concept for various reasons, such
as their belief that cloning is interfering with nature.
When people are questioned about the cloning of human
beings, there is a very high level of opposition to it.
Some governments, including Australia, have banned
experiments directed to producing human clones and
leaders of some religious groups have opposed human
cloning.
The Prohibition of Human Cloning Act 2002, passed by the
Australian Parliament in December 2002, bans human
cloning. This Act took effect on 16 January 2003.
Uses for cloning
Replacing organs or other tissues
Infertility
Replacing a lost child
Creating donor people
Gene therapy
Saving endangered species
Reversing the aging process
Ethical questions associated with cloning
Is cloning ethical for humans?
If you had a clone, would it be your child or your delayed
twin?
What about all the duds?
What about creating clones for organs?
Who should decide who is cloned?
Who should have access to cloning technology – only
people with good genes?
Who should pay for cloning?
Should we be allowed to use this technology to design our
children?
Who does this genetic information belong to?
Should people with genetic diseases be cured? Should they
have children?
Cloning Plants
Cloning of plants can occur both naturally and artificially.
Natural cloning occurs through cuttings, runners and
suckers.
Artificial cloning of plants involves the culturing of a piece
of adult plant. As this piece grows, it can be further
subdivided so that a large number of genetically identical
plants can be produced from the original piece.
If large numbers of plants are produced through natural or
artificial cloning, the members of the resulting population
are genetically identical.
As a result, these populations have very limited genetic
variation compared with a population that has been
produced by sexual reproduction.
Bananas are a natural example of plants reproducing by
cloning. Having a 3N genome, bananas are unable to
successfully undergo meiosis.
Transferring genes between species
Under normal conditions, genes of one species can be transferred
to only another member of that species, for example, from
parents to offspring.
Transfer of genes between different species normally does not
occur.
The restrictions that normally prevent gene transfer between
different species are known as the ‘species barrier’.
These restrictions include the inability of different species to mate
and the inability of gametes from one species to fertilise those of
another species.
Genetic engineering technology, however, has made this species
barrier irrelevant.
Genetic engineering technology allows the genetic material to be
manipulated and enables genes to be transferred between any
two species.
Examples of these gene transfers include:
– the transfer of a human gene into bacteria
– the transfer of a human gene into cows
– the transfer of a bacterial gene into cotton plants and the transfer of a
jellyfish gene into mice.
Any organisms that possess a ‘foreign’ gene or segment of
‘foreign’ DNA in their genome as a result of human
experimentation are termed transgenic organisms (TGOs).
Transferring genes between species
The introduction and incorporation of external
DNA into a cell can result in permanent genetic
changes.
– If the cells concerned are prokaryotic cells, such as
bacterial cells, they are said to be transformed.
– If the cells are eukaryotic cells, when external DNA is
added to the cells they are said to be transfected.
Various techniques exist for transferring genes
into a host cell. These include:
– micro-injection of the DNA of a gene into a cell, such as
an egg cell or a somatic cell
– transfer using a virus, either a retrovirus or an
adenovirus, to carry the gene
– use of an electric pulse (electroporation)
– use of ballistics (the ‘gene gun’)
These various techniques are ‘hit-and-miss’.
Cloned transgenic animals
In the past, when scientists wished to create a genetically modifi
ed transgenic mammal, the only method available was microinjection of the DNA of the gene concerned into newly fertilised
eggs. The eggs were then implanted into females and the
embryos were allowed to develop to term. It was only after the
baby mammals were born that they were tested to see if the gene
had been taken up.
Success rates using this method were not high, perhaps just one
in 100.
The successful application of artificial cloning to transgenic
animals in 1997 has overcome this problem.
Soon after development begins, the cells of a transgenic embryo
can be artificially separated so that the single cells then develop
into a number of identical organisms.
Human genes and genes of other species have been engineered
into mammalian cells, such as hamster cells and mice cells.
Human genes have also been engineered into mammalian clones,
such as cattle, sheep and goats.
These events are not possible under the normal evolutionary
processes.
Cloned transgenic animals
George and Charlie were the
first transgenic cows to be
artificially cloned. They were
derived from cattle body cells
that had been genetically
altered to incorporate the
human gene for a particular
blood protein known as serum
albumin.
Clint, Arnold and Danny were
the first goats to be artificially
cloned from adult goat cells.
They were also transgenic as
their cells contained the spider
gene for silk production.
Mira, Mira and Mira were three
genetically identical female
goats produced by artificial
cloning of an adult goat cell that
incorporated a human gene that
controls production of a protein
that prevents blood clotting. The
milk produced by these goats
contains this human protein.
Difference between transgenic organisms
and genetically modified organisms
The term genetically modified organism (GMO) refers to any
organism whose genetic makeup has been artificially changed.
So, all transgenic organisms are GMOs but not all GMOs are
transgenic organisms (TGOs).
GMOs include organisms whose genotypes have been modified but
the modification does not involve insertion of gene(s) from a
different species.
Such modifications can include the switching off (or silencing) of a
gene that is normally active in an organism.
For example, in 2004, an American biotechnology company
started taking orders for genetically modified cats to go on sale in
2007.
– The gene that will be ‘silenced’ is one that is normally active in cells of
a cat’s skin and its salivary glands and it controls production of a
protein, known as cat allergen, that is shed by a cat.
– Cats engineered to have this gene ‘silenced’ will not produce this
protein and, as a result, people who usually suffer from cat allergies
will not show allergic symptoms in the presence of these genetically
modified cats.
Gene therapy
Another human intervention that has the potential to
change natural evolutionary processes is gene therapy.
Gene therapy is a process by which a faulty allele in an
organism is replaced by the normally functioning allele of
the gene concerned.
It is a technique that aims to treat inherited disorders by
directly targeting the genotype.
This is in contrast to conventional treatments for inherited
disorders that act at the level of the phenotype by
ameliorating the symptoms of the disorder.
Gene therapy provides the prospect for treating inherited
disorders for which no treatment presently exists.
Gene therapy
Technical difficulties must be solved:
– How can a gene be targeted to cells of the affected tissue?
– How can a gene be targeted to a position where it does not
interfere with the function of another essential gene?
In addition, ethical issues must be resolved:
– Should gene therapy be restricted to somatic tissues only, so that the
introduced gene is not transmitted to the next generation?
At present, gene therapy affecting germline cells is banned.
Before gene therapy is permitted, there must be an assessment of
the safety of the patient and the general public and the expected
benefit to the patient is compared with the likely risk.
At present, gene therapy aims to add copies of the normal allele
of a gene into the cells of a target tissue, switching them on to
produce the functional protein that is missing in a person with a
particular disorder, such as a clotting factor in persons with
haemophilia, or a tumour suppression agent in persons suffering
from certain cancers.
Stem Cells
Stem cells are undifferentiated or precursor cells that have the ability to
differentiate into many different and specialised cell types, such as nerve
cells, blood cells, bone cells, heart cells, skin cells and so on.
The first human stem cells were identified in the 1960s and these cells
were in the bone marrow One type of stem cell in the bone marrow can
differentiate into red blood cells, white blood cells and platelets.
Since then, stem cells have been found in other human tissues, such as
fat tissue, in skin and in the circulating bloodstream, but in very low
numbers.
In 1998, scientists discovered how to isolate stem cells from embryonic
tissue.
Stem cells also exist in other mammalian species and have been widely
studied in mice.
Because stem cells have the potential to differentiate into specialised cells
of various kinds, their potential use to replace faulty or dead cells is great
and much research is presently occurring.
Stem cells are described as totipotent, pluripotent or multipotent in
terms of their power or potency to produce various cell types.
Stem Cells
Stem Cells
In September 2005,
scientists at the University
of California reported that
following the injection of
human stem cells from
nerve tissue into the spinal
cords of paralysed mice, the
test group of mice displayed
better mobility than the
non-injected controls after
just nine days and after four
months, the test group of
mice could walk.
The stem cells migrated up
the spinal cord and
developed into different
kinds of cells including
those cells that form
insulating layers of myelin
around nerve cells.
Injured spinal cord of mouse following injection
of human stem cells. These stem cells
developed into myelin-producing cells that form
a wrapping (green) around nerve cells (red) (see
the areas marked by arrowheads).
Other nerve cells remained without a myelin
wrapping (see the areas indicated with arrows).
Types of Stem Cells
Stem cells can also be grouped as follows:
embryonic stem cells that
– can be obtained from the inner cell mass of an early embryo
known as a blastocyst.
– A single cell is isolated from the inner cell mass of a blastocyst
and is grown in culture, dividing by mitosis to produce a
culture of stem cells.
– Embryonic stem cells are pluripotent; this means that they can
give rise to many different cells types found in a mammal,
such as blood cells, skin cells and liver cells.
adult stem cells
– (more accurately called somatic stem cells) that can be
obtained from various sources such as bone marrow, skin and
umbilical cord blood
– Somatic stem cells are multipotent; this means that they can
give rise to certain cell types such as various kinds of blood
cells or various kinds of skin cells. Cord blood, for example,
contains mainly bloodcell- producing stem cell
Therapeutic cloning for stem cell therapy
Depending on its purpose, cloning can be separated into
reproductive cloning and therapeutic cloning.
– Reproductive cloning - purpose of the cloning is to produce a
new organism.
– Therapeutic cloning – purpose of the cloning is to produce
stem cells for use in treatment.
Therapeutic cloning involves the creation, through the nuclear
transfer technique, of an embryo for the purpose of obtaining
stem cells from that embryo.
These stem cells are intended for use in treating a patient who
has a spinal cord injury or brain injury or has suffered a stroke or
has a degenerative disease.
Therapeutic cloning for stem cell therapy
The cell that provides the nucleus in therapeutic cloning is a
healthy cell from the patient who is to receive treatment.
As a result, the embryo that is created is a genetic match to the
patient and these cells will not cause an immune response.
Therapeutic cloning raises possibilities for new treatments for
diseases.
Application of this technique would mean that normal evolutionary
pressures of natural selection will no longer act on people with
particular disorders.
However, therapeutic cloning also raises major ethical issues.
Ethical issues
The use of early embryos as a source of stem cells raises many ethical
issues since establishing an embryonic stem cell line destroys an embryo.
In December 2002, the Research Involving Human Embryos Act 2002 was
passed in the Australian Parliament.
This Act established a framework that regulated the use of ‘excess’
embryos. An ‘excess’ embryo is one that:
– was originally created by artificial reproductive technology for use in IVF
procedures, and
– has been identified in writing by all ‘responsible people’ as being in excess to the
needs of the couple for whom the embryo was first created.
Provisions of the Research Involving Human Embryos Act 2002 include the
following:
– only persons holding a special licence may carry out research on embryos
– where that research may damage or destroy the embryo, only excess embryos
created before 5 April 2002 may be used
– embryos cannot be created solely for research purposes.
The provisions of the Research Involving Human Embryos Act 2002 are
monitored by the National Health and Medical Research Council (NHMRC)
Licensing Committee.
Under the provisions of this Act, therapeutic cloning is not permitted in
Australia.
Genetic screening
Genetic screening is a procedure in which a DNA sample is
analysed to detect the presence of one or more alleles
associated with an inherited disorder.
Genetic screening may be carried out as follows:
– adult screening, to identify carriers of an inherited disease
where a couple wish to determine if one or both of them can
transmit an inherited disease to their children
– embryo biopsy or pre-implantation genetic screening, in which
a single cell is removed from an embryo conceived by IVF to
determine that the embryo will not later be affected by certain
inherited diseases
– pre-natal screening, to identify the genetic status of a fetus
where a specific inherited disorder is suspected to be present,
using chorionic villus sampling or amniocentesis
– predictive screening, to identify persons at risk of developing a
late onset disease, such as Huntington disease
Technology in human reproduction
The development of technology has given humans the ability to
manipulate reproduction:
These advances fall into two categories:
– Those that prevent human conception
vasectomy (vas deferens cut and sealed)
tubal ligation (fallopian tubes cut and sealed)
contraceptive pill
intra-uterine device (IUD)
condom with spermicide
diaphragm with spermicide
condom alone (rubber sheath over penis)
diaphragm alone (cap over cervix)
spermicides alone
– Those that assist human conception
Donor insemination
IVF — in-vitro fertilisation
Surrogacy
Gamete intrafallopian transfer
Intracytoplasmic sperm injection
Each of these technologies has varying degrees of effectiveness.
Issues associated with assisted
reproduction
There are social, moral and legal considerations associated
with assisted reproduction as these techniques are dealing
with the creation of human life.
Should we interfere with nature?
When does life begin?
What should happen to frozen embryos on the death or divorce of
the parents?
Recent legal issues related to reproductive technologies include
questions such as:
– Should a woman be allowed to use the frozen sperm of her dead
husband?
– Should sperm be collected from a dead person because a family
makes the request?
The law needs to change as the technology changes and debate
continues on many of the issues that arise.
Points to consider
Humans change their environment more
than any other species, and knowingly or
unknowingly affect, at times, the course of
evolution.
Biological, cultural and technological
evolution are today interrelated through
modern practices in agriculture and
medicine.
In modern medicine, genetic screening,
gene therapy and cloning can be viewed
as intervening in human evolution, with
raises ethical issues.