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| A chimeric mouse with pups |
Identical twins arise when a zygote (a fertilized egg) begins
to divide. About five days after fertilization, it reaches the blastula stage and is implanted in the uterus,
but for unclear reasons it can be broken into two separate parts, which will
result in two independent embryos that may or not share the same placenta,
although they usually have a different amniotic sac. The two siblings who are
born share the same genetic endowment (the same DNA), except for possible
post-separation mutations.
In contrast, two non-identical twins arise
when two distinct eggs are fertilized, each by one spermatozoid, forming two
different blastulas, each of which is implanted in the uterus through a
placenta of its own. The two brothers will have different genetic endowments,
similar to those of two non-twin brothers, because they come from different
gametes.
But there is a third possibility: a chimera arises when two blastulas that would
normally give rise to two non-identical twins merge before being implanted in
the uterus and give rise to a single embryo and, consequently, to a single
individual possessing, in different cells, two different genetic endowments.
Thus, it may happen that a chimeric individual has (for example) the liver with
a genetic endowment and the kidneys with another. Typically, chimeras are
difficult to detect, unless (for example) just one of the blastulas would have given
rise to an albino, in which case the resulting chimeric individual may have
unequally pigmented skin. Even in this case, the cause could be different. It
could also happen (although it is very rare) that one of the two zygotes is
male (with X and Y chromosomes) and the other female (with XX chromosomes), in
which case part of the cells of the chimera would be male and another part
female.
A chimera can also be formed when a blastula
spontaneously absorbs a few cells detached from another living being, either
the mother, or one of the siblings sharing the maternal uterus. In this case we
speak of micro-chimeras, because the
number of cells with different genetic endowment is unbalanced, as one of the
two genetic endowments will be much more frequent than the other.
The frequency of chimeras depends on the
species. Thus, for example, in marmosets 95% of the individuals are chimeras,
although this number also includes micro-chimeras.
The Feb. 18th 2017 issue of Science News
contains a story titled Human-animal
chimeras created (this is the title in the paper magazine; in the web it is
different). What has been done? Incorporate human stem cells into embryos of
pigs and cattle, with the long-term intention of building living beings with a
human liver or heart, thus providing material for transplants. For the time
being, the results of the experiment have not been
very convincing: of 2075 pig embryos injected with human stem
cells, just 186 survived throughout the experiment, and of these, just 67 had
incorporated human cells. In addition, the chimeric embryos were
underdeveloped, which seems to indicate that the injection of human cells had
interfered negatively in their development.
But these are temporary technical limitations,
which will surely be solved in the future. The real issue is that such
experiments pose important ethical problems. For
example, if the human stem cells are not well controlled, we could end up
creating a pig with a human brain, or a sow with human ovaries, that if
fertilized with human semen, would give birth to a baby of our species. Are we
willing to let this research lead to such results?
In some countries, research with chimeras made
up of human cells mixed with animals is prohibited. In the United States, using
federal funds is forbidden, but private or state funding is permitted.
Faced with this situation, Françoise Baylis, an
expert on bioethics at Dalhousie University in Halifax, Canada, believes that
research on human-animal chimeras will eventually be banned because of the faulty assumption that human life is more valuable than that
of non-human beings.is And he adds this:
The hope that one can
‘forever’ avoid the tough ethical questions by simply ensuring that the
nonhuman animals are not ‘substantively humanized’ is flawed (short-sighted),”
In other words, this bioethicist regrets that
these investigations may be impeded, because human life is not, according to
him, more valuable than (for example) that of a cockroach. And yet I don’t
think that any biologist will deny that the human brain is the most complex
object in the universe (as far as we know, leaving aside the possible existence
of extraterrestrial life). Baylis therefore merely repeats without argument the
supreme dogma of materialistic biology:
All species of living
beings are equivalent, none is superior to others.
This dogma is clearly anti-scientific, because
it relies exclusively on ideology and refuses to take into account the abundant
scientific evidence that man is a species different and superior to all others,
evidence that I summarized in the most read article in my blog: Is man
just an animal?
Manuel Alfonseca
Thanks for this informative post. I had seen some stories about human-animal chimeras, but was not aware that chimera occur naturally in mammalian development.
ReplyDeleteIt is strange how the anti human ideology has crept into science -- the false assumption is that there are no substantial differences between things.