The barrier
function
Raj Murugesan
Technical Specialist
Raj Murugesan
Technical Specialist
Illu: anton_novik
Gut integrity —
the backbone
of performance
The
barrier
function
Livestock production today has to overcome major challenges, such as increased cost of feed
ingredients and disease outbreaks, in order to increase production efficiently to feed the
growing world population. In this context, gut health plays a critical role in influencing animal
productivity; in other words, it is considered the “backbone of performance”.
2
M
aintenance or enhancement of gut health
is far more complicated in commercially
raised food animals. Any factor that affects gut health will undoubtedly influence
the animal as a whole and consequently
change its nutrient uptake and requirements, resulting in altered performance.
The basis of gut health lies in its integrity, which is a highly complex process
encompassing the macro- and micro-structural integrity of the gut, the balance of
microflora, the status of gut-associated immune system and the energetic cost of metabolism. This article reviews the gut barrier
function and its effect on performance.
The gut harbours more than 650 different
species of bacteria, contains over 20 different hormones, digests and absorbs the
vast majority of nutrients, and accounts
for 20% of the body’s total energy expenditure, while being the largest immune
organ in the body.
Gut barrier function
The gastrointestinal tract is a vital
organ with conflicting functions, and
plays a major role in the digestion and
absorption of dietary nutrients. At the
same time, the gut constitutes the animal‘s most important barrier between
the internal and external environments.
The ability of epithelium to control the
uptake of molecules into the body is the
“gut barrier function”. The gut barrier
complex comprises the immunological,
physiological and physical barriers.
Gut epithelium exerts an important
immunological defence by the secretion
of mucus into lumen to bind, dilute and
wash away pathogens and noxious substances. The tight junctions, which interconnect the continuous epithelial cell
layer, restrict both transcellular and paracellular permeation of molecules, thus
constituting the principal component of
physiological barrier.
In addition, the epithelium itself,
consisting of the villi and crypts as well
as the epithelial and endothelial cell components, forms the physical barrier by
posing as a final blockade to the entry of
Th e b a r r i e r f u n c t i o n
external factors. With the coordination of
these individual barriers, the gut barrier
complex overcomes the challenges presented by stressors.
Stressors that could affect animal
health and production include pathogens, endotoxins, mycotoxins, as well as
processes that induce stress such as environment (hot or cold), feed withdrawal, feed change, vaccination, fasting and
transportation.
Immunological barrier
The epithelium covering mucus layer,
comprised of mucins, is the initial point
of contact between the host and gut microflora. Hence, mucins are the first line
of defence against pathogens, and they
communicate extensively with commensal and probiotic microbes. Mucins are
glycoproteins, which are categorised into
distinct families: gel-forming, soluble and
membrane-bound.
Binding to epithelial cells is the primary step for many enteric pathogens
to translocate into circulation as well as
to affect net fluid and electrolyte secretion. Therefore, the interruption of entero-pathogenic adherence could provide
therapeutic benefits to the host. By virtue of their negatively charged filamentous protruding structure, mucins act as
a selective barrier protecting the epithelial
cells.
Under normal circumstances, mucins
allow only minute quantities of intact
antigens to cross into the mucosa, where
they interact with the mucosal immune
system to down-regulate inflammation,
otherwise known as oral tolerance. However, mucin synthesis from goblet cells is
altered during stressful conditions to efficiently overcome the challenges.
Physiological barrier
Epithelial tight junctions are the primary component of gut physiological
barrier as they join epithelial and endothelial cells to each other and function as
a ‘‘fence’’. The barrier function of tight
junctions is the ability of epithelium and
endothelium-lined surfaces to differen-
3
Tight Junction
Villus
Epithelzelle
Microvilli
Transzellulärer
Weg
Figure 1. Transcellular and paracellular pathways in the intestinal
epithelium.
Stressors that could
affect animal health
and production include pathogens,
endotoxins, mycotoxins, as well as
processes that induce
stress such as environment (hot or cold),
feed withdrawal,
feed change,
vaccination, fasting
and transportation.
Figure 2. Villus and crypt in the intestinal tract.
Source: University of Waikato, 2011
tially restrict the passage of water, ions,
and larger solutes, based on size and
charge, through paracellular permeation.
The presence of or exposure to stressors compromises the integrity of the
tight junctions, leading to increased ion
conductance across the paracellular route
(Figure 1). This condition, commonly
known as “leaky gut”, essentially enables
pathogens, endotoxins and mycotoxins to
access the whole body including vital organs by reducing the integrity of the tight
junctions and increasing their translocation into circulation.
Physical barrier
The gastrointestinal epithelium undergoes morphologic and physiologic
changes immediately after birth/hatch,
increasing the surface area for digestion
and absorption which is essential to fully
express the animal’s genetic potential for
growth. The high plasticity of gut epithelium, such as changes in villus density,
villus height, crypt depth and rate of epithelial turnover, allows for the response
to challenges. Crypts are considered as
villus factories as they contain stem cells,
hence deeper crypts indicate faster tissue
turnover and higher nutrient demand for
new tissue.
However, increased tissue turnover
to permit the renewal of villi and crypts
is required in response to inflammation
from pathogens or their toxins (Figure 2).
Reduced villus height and increased crypt
depth (reduced ratio) lead to increased
endogenous secretion and reduced nutrient absorption, disease resistance and
performance.
Summary
There is a growing body of evidence
that stressors can have an impact on several components of the intestinal barrier
function and can adversely increase epithelial permeability. With the combination of the above-discussed barriers, the
gut protects itself as well as the animal
from various physical and physiological
stressors. Gut integrity is compromised
when the conditions are conducive for
increased presence of or exposure to
these stressors, which expose the animal
to a variety of challenges. These could be
either overt, clinical conditions or subtle,
sub-clinical conditions.
The sub-clinical challenges happen
on a day-to-day basis and do not exhibit
by any symptoms. Animals channel their
nutrients to overcome these challenges
through various means such as the activation of immune system, which otherwise
would have been used to increase growth
and productivity. This loss has been estimated to be 10-12% of the nutrients absorbed at any given time. Thus, it is imperative to understand that even a small
amount of stress could affect the animal’s
productivity, and maintaining an integral
and healthy gut is the primary step towards efficient production.
References are available upon request.
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ART_Nr08_Poultry_EN_0214_RMU
Parazellulärer
Weg
Crypt of
Lieberkühn
Muscularis
mucosa