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 University of Illinois researchers say the domestic pig is ideal
for these studies because their brain size, rate of development,
and digestive system are excellent analogues for human newborns.
They know a lot about pig brains, having built the first – and
recently, the second – complete, MRI-based atlases of the organ.
They’ve used the first to study the effects of numerous
nutritional interventions in pigs. But some critics say they
can’t be sure those outcomes reflect reality. After all, these
research subjects are raised in carefully controlled
environments, not mingling with siblings and mom in
farm-standard farrowing crates.
So just how much does the pig’s early rearing environment affect
brain development?
“We've been countering a lot of criticism about whether the
development of the pig brain is the same in our laboratory as it
is on any pig farm. And the answer is yes. We now have
indisputable evidence to say that the brains of pigs raised in
an artificial environment grow and develop in the same way
structurally as those of pigs raised by their mother,” says Ryan
Dilger, associate professor in the Department of Animal Sciences
and senior author on a new study in Frontiers in Neuroscience.
In the study, the research team brought 2-day-old piglets to
their facility, known as the Piglet Nutrition and Cognition
Laboratory, which is outfitted with large individual enclosures
that let pigs see, smell, and hear others in adjacent pens.
Dilger says the high ambient temperature and ability for pigs to
socialize is important.
“In our nutritional studies, we want to keep them separated to
avoid cross-contamination of the bacteria found in their colon,
which is collectively known as the microbiota. But the pigs
still get to express social behaviors by seeing, hearing, and
smelling each other,” he says. “And in many of our studies, we
let the pigs out of their enclosures to socialize each day, so
they get to have a piggy party each afternoon.”
Another subset of piglets stayed with their littermates and
mothers in farrowing crates on a research farm on the U of I
campus. At 4 weeks of age, when pigs have developed enough that
they no longer need to solely drink milk, the artificially
reared pigs moved back to the farm and were group-housed with
their sow-reared counterparts. So, in the end, the pigs only
lived in different environments for the first four weeks of life
and from that point forward, all pigs were treated the same.
All pigs were anesthetized and scanned in a state-of-the-art
magnetic resonance imaging (MRI) machine at 1, 2, 3, 4, 8, 12,
18, and 24 weeks of age. The researchers assessed brain macro-
and microstructure of the artificially reared and sow-reared
pigs using the new pig brain atlases for young and adolescent
pigs.
Not only did the pigs eat and grow at the same
rates in the two rearing environments, their brain development
was equivalent overall as well. The researchers found no
differences in absolute volumes of the whole brain, gray matter,
white matter, cerebrospinal fluid growth, or microstructural
changes (neuronal connections between brain regions) over time
in the two groups.
Joanne Fil, doctoral student in the Neuroscience Program at
Illinois and first author of the study, says, “We looked at a
surrogate measure of myelin, myelin water fraction, which
reflects the fat and protein surrounding neurons and helps them
communicate more effectively. Humans and pigs develop a lot of
myelin significantly after birth, so if we see more myelination,
then we assume the brain is maturing at a different rate.
Longitudinally, there were no differences between the myelin
water fraction in the two groups.
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“We did see slight differences in the rate of
development, with artificially reared pigs having a slightly higher
rate of myelin development than the sow-reared pigs, but in the end,
pigs raised in either environment reached the same place when it
came to brain growth.”
The researchers also compared the pigs’ memory in behavioral tests.
The pigs were presented with two stationary toys to play with – one
they had previously been able to investigate and one they had never
seen before. If they spent more time checking out the new toy, that
was evidence they remembered the older one and therefore had encoded
a memory.
At a couple of early time points, 4 and 8 weeks, pigs in the
sow-reared group had slightly greater object recognition. The
authors suggest that might have been due to greater peer interaction
in the sow-reared environments. However, the differences were slight
and temporary.
“Behavior is always more variable, more subjective. That’s why we
like the objective and structural measures of the brain we can
evaluate by MRI,” Dilger says. “And what we found is that when both
sets of pigs are healthy and we've met their nutritional
requirements, the rearing environment didn’t appear to influence
brain development. Their brains effectively grew the same.”
In addition to supporting the continued use of laboratory rearing
environments in pig neuroscience research, the study provides new
data on pig brain development over time.
For half a dozen years, the first pig brain atlas served as the
definitive reference for researchers around the world. But that
atlas was based on 4-week-old pigs and couldn’t be easily
extrapolated to older animals. Not only do the new atlases include
one for 12-week-old pigs, the rearing environment study provides
absolute volume measures for pigs at many more time points, up to
adulthood.
“This provides a lot of foundational data where somebody could come
back and ask how many cubic millimeters is the pig brain at a
particular age. We can model that based on what we have here,” Fil
says. “Our objective was comparing the two rearing environments, but
also providing what is expected for a certain age of pig. That helps
because we can start to develop nutritional interventions for
specific ages and understand what parts of brain development can be
influenced by nutrition.”
Dilger adds, “There's a lot of power in the pig as a model for
biomedical research, and we’re showing that by bringing together
engineering and agriculture, which are central to our mission as a
land-grant institution. We use lots of mice in research on our
campus, but we're also very good at working with pigs. We’re mixing
the biomedical world with the agricultural world, to ultimately
benefit both pig and human nutrition.
The article, “Influence of rearing environment on longitudinal brain
development, object recognition memory, and exploratory behaviors in
the domestic pig (Sus scrofa),” is published in Frontiers in
Neuroscience [DOI: 10.3389/fnins.2021.649536]. Authors include
Joanne Fil, Sangyun Joung, Courtney Hayes, and Ryan Dilger. Funding
was provided by Nestlé.
The Department of Animal Sciences is in the College of Agricultural,
Consumer and Environmental Sciences at the University of Illinois.
[Sources: Ryan Dilger, Joanne Fil
News writer: Lauren Quinn] |
