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Morphological bases for intestinal paracellular absorption in bats and rodents



Flying mammals present unique intestinal adaptations, such as lower intestinal surface area than nonflying mammals, and they compensate for this with higher paracellular absorption of glucose. There is no consensus about the mechanistic bases for this physiological phenomenon. The surface area of the small intestine is a key determinant of the absorptive capacity by both the transcellular and the paracellular pathways; thus, information about intestinal surface area and micro‐anatomical structure can help explain differences among species in absorptive capacity. In order to elucidate a possible mechanism for the high paracellular nutrient absorption in bats, we performed a comparative analysis of intestinal villi architecture and enterocyte size and number in microchiropterans and rodents. We collected data from intestines of six bat species and five rodent species using hematoxylin and eosin staining and histological measurements. For the analysis we added measurements from published studies employing similar methodology, making in total a comparison of nine species each of rodents and bats. Bats presented shorter intestines than rodents. After correction for body size differences, bats had ~41% less nominal surface area (NSA) than rodents. Villous enhancement of surface area (SEF) was ~64% greater in bats than in rodents, mainly because of longer villi and a greater density of villi in bat intestines. Both taxa exhibited similar enterocyte diameter. Bats exceeded rodents by ~103% in enterocyte density per cm2 NSA, but they do not significantly differ in total number of enterocytes per whole animal. In addition, there is a correlation between SEF and clearance per cm2 NSA of L‐arabinose, a nonactively transported paracellular probe. We infer that an increased enterocyte density per cm2 NSA corresponds to increased density of tight junctions per cm2 NSA, which provides a partial mechanistic explanation for understanding the high paracellular absorption observed in bats compared to nonflying mammals. Bats have longer and denser villi per unit intestinal nominal surface area (area of the bore tube) than rodents providing a larger total number of enterocytes which in turn produces more tight junctions. This differential intestinal morphology architecture gives the basis to explain an enhanced absorption of water‐soluble solutes through the paracellular route that compensates for shorter intestines and higher or similar energy requirements in bats than in non‐flying mammals.
Morphological bases for intestinal paracellular absorption
in bats and rodents
Antonio Brun
| Guido Fernández Marinone
| Edwin R. Price
| Lucas A. Nell
Beatriz M. V. Simões
| Alexandre Castellar
| Manuel Gontero-Fourcade
Ariovaldo P. Cruz-Neto
| William H. Karasov
| Enrique Caviedes-Vidal
Instituto Multidisciplinario de Investigaciones
Biológicas de San Luis, Consejo Nacional de
Investigaciones Científicas y Técnicas, San
Luis, Argentina
Departamento de Bioquímica y Ciencias
Biológicas, Facultad de Química, Bioquímica y
Farmacia. Universidad Nacional de San Luis,
San Luis, Argentina
Department of Forest and Wildlife Ecology,
University of WisconsinMadison, Madison,
Department of Biological Sciences, University
of North Texas, Denton, Texas
Department of Integrative Biology, University
of WisconsinMadison, Madison, Wisconsin
Departamento de Zoologia, Instituto de
Biociências, Universidade Estadual Paulista
Julio de Mesquita FilhoRio Claro, São Paulo,
William H. Karasov, Department of Forest and
Wildlife Ecology, University of Wisconsin-
Madison, 1630 Linden Drive, Madison, WI
Funding information
Argentina Consejo Nacional de Investigaciones
Científicas y Técnicas, Grant/Award Number:
PIP834; Argentina Universidad Nacional de
San Luis; Brazil CNPq, Grant/Award Number:
470128/2011-9; Fundaçao de Amparo a
Pesquisa do Estado de Sao Paulo, Grant/Award
Number: 2012/04610-5; U.S. National Science
Foundation, Grant/Award Number: IOS-
Flying mammals present unique intestinal adaptations, such as lower intestinal
surface area than nonflying mammals, and they compensate for this with higher
paracellular absorption of glucose. There is no consensus about the mechanistic
bases for this physiological phenomenon. The surface area of the small intestine is
a key determinant of the absorptive capacity by both the transcellular and the
paracellular pathways; thus, information about intestinal surface area and micro-
anatomical structure can help explain differences among species in absorptive
capacity. In order to elucidate a possible mechanism for the high paracellular
nutrient absorption in bats, we performed a comparative analysis of intestinal villi
architecture and enterocyte size and number in microchiropterans and rodents.
We collected data from intestines of six bat species and five rodent species using
hematoxylin and eosin staining and histological measurements. For the analysis
we added measurements from published studies employing similar methodology,
making in total a comparison of nine species each of rodents and bats. Bats pres-
ented shorter intestines than rodents. After correction for body size differences,
bats had ~41% less nominal surface area (NSA) than rodents. Villous enhancement
of surface area (SEF) was ~64% greater in bats than in rodents, mainly because of
longer villi and a greater density of villi in bat intestines. Both taxa exhibited simi-
lar enterocyte diameter. Bats exceeded rodents by ~103% in enterocyte density
per cm
NSA, but they do not significantly differ in total number of enterocytes
per whole animal. In addition, there is a correlation between SEF and clearance
per cm
NSA of L-arabinose, a nonactively transported paracellular probe. We
infer that an increased enterocyte density per cm
NSA corresponds to increased
density of tight junctions per cm
NSA, which provides a partial mechanistic expla-
nation for understanding the high paracellular absorption observed in bats com-
pared to nonflying mammals.
bats, enterocytes, nutrient absorption, rodents, small intestine surface area
Received: 3 May 2019 Revised: 19 June 2019 Accepted: 26 June 2019
DOI: 10.1002/jmor.21037
Journal of Morphology. 2019;111. © 2019 Wiley Periodicals, Inc. 1
In vertebrates, the small intestine is the major site of hydrolysis of
nutrient macromolecules and absorption of their breakdown products
via the transcellular and paracellular pathways (Karasov & Douglas,
2013). The surface area of the small intestine is a key determinant of
both of these processes. Villi, which are finger-like projections of the
intestinal wall inward into the lumen, increase intestinal surface area.
Earlier studies with a few bat species suggested that bats may have
greater villous surface area magnification than nonflying mammals
(Barry Jr., 1976; Makanya, Maina, Mayhew, Tsachanz, & Burri, 1997;
Mayhew, 1996). This paper considers villous features in a much larger
sample of mammals, and also within a structure/function context, in
order to better understand possible morphological and functional dif-
ferences between bats and other mammals.
Curiously, bats have small intestines that are significantly shorter
and have less overall nominal surface area (NSA; i.e., the area of the
smooth bore tube that ignores any effect of villi) compared with nonflying
mammals, but they have faster paracellular absorption of water-soluble
nutrients such as glucose per cm
NSA (Price, Brun, Caviedes-Vidal, &
Karasov, 2015). Unlike transcellular absorption, in which nutrients are
absorbed via membrane-bound nutrient transporters, paracellular absorp-
tion occurs by movement of nutrients through the tight junctions (TJs)
that link adjacent intestinal cells (enterocytes) (Pappenheimer & Reiss,
1987). Bat species have greater capacity for paracellular absorption than
do nonflying mammals (Price et al., 2015). It has been hypothesized that
this is a compensation for bats also having, compared with nonflyers,
smaller intestines that carry less digesta mass, which reduces the ener-
getic cost of flight and improves flight maneuverability (Caviedes-Vidal
et al., 2007). Even with smaller intestines, bats might achieve a greater
capacity for paracellular absorption by at least two mechanisms.
One mechanism for increasing paracellular absorption per cm
small intestine is to have a higher number of TJs across which para-
cellular absorption occurs. This can be achieved by increasing villous
surface area with no change in the size of enterocytes, or by decreas-
ing enterocyte size with no change in villous surface area, both solu-
tions thereby yielding more cells and TJs per cm
NSA. Testing these
alternatives requires attention to intestinal villous morphology and
the size of villous cells, which has not been measured in most previous
studies. An alternative mechanism is that the TJs of bats are individu-
ally more permeable than those of other mammals, perhaps due to dif-
ferences in their protein makeup. This hypothesis is not directly
testable via histological measurements, but we will return to this pos-
sibility in our discussion.
Based on our initial studies (Caviedes-Vidal et al., 2008; Zhang
et al., 2015) we predicted that (#1) the surface enlargement factor
(SEF), which is the villous surface area relative to the NSA, would be
significantly higher in bats than rodents, effectively yielding similar or
greater total villous surface area in bats and thus compensating for
their lower NSA. We also predicted (#2) that there would be no signifi-
cant difference between bats and rodents in enterocyte size, effec-
tively yielding more TJs per cm
NSA in bats, and (#3) that across all
the species the rate of intestinal absorption per cm
NSA of para-
cellular probes would be positively correlated with SEF. Because our
prior studies on paracellular absorption in bats included species whose
primary dietary nutrients were carbohydrates (e.g., frugivores) or pro-
teins (e.g., insectivores) (Price et al., 2015) we sought for our broad
comparison of intestinal morphology a variety of rodent species with
different dietary habits (Table 1), as reliance on a few laboratory
model species would not suffice.
Using a phylogenetically-informed approach, we compared intestinal
morphology of 18 species of microchiropteran bats and rodents that
vary in size and diet, by producing new measurements on five bat spe-
cies and six rodent species and adding to existing published data
(Table 1). Species of rodents and bats, all relatively common, were
captured at night during the rainy season between October and
November 2012 and 2013 in the Mata do São José (a remnant intact
forest 15 km northeast of Rio Claro, São Paulo, Brazil). Rodents were
captured with Sherman and pitfall traps and bats were captured in
mist nets. Captured mammals were transported to the laboratory in
the Instituto de Biociências, Rio Claro, Universidade Estadual Paulista
and identified to genus and species based on morphological charac-
ters by coauthor A.P. Cruz-Neto (>20 years experience) and con-
firmed based on relevant local resources (Roberto dos Reis, Peracchi,
Batista, & Passos de Lima, 2017) and local authorities (A. Percequillo,
University São Paulo, SP, Brazil). All animal procedures adhered to
institutional animal use regulations and an approved animal use proto-
col (Universidade Estadual Paulista: protocol A1-2013).
On the morning following capture, animals were weighed and then
euthanized by overdose of CO
. Subsequently the gastrointestinal
tract was removed and the small intestine was dissected out and the
intestinal lumen perfused with ice cold 1% NaCl solution to remove
digesta. Intestines were then blotted dry, weighed, and measured for
length by holding one end of the intestine against a vertical ruler while
the other end was gently pulled until the intestine was taut. After
release, the length was measured and then the intestine was divided
in three equal portions, the proximal, middle, and distal regions. Each
whole portion except for 1 cm was carefully cut longitudinally with
small surgical blunt scissors, opened, cleaned with ice cold 1% NaCl
solution, and placed with the mucosa side up on a cold stainless steel
plate. Using a digital caliper, two measures of length and three of
width (circumference) were taken to estimate nominal surface area of
each region. The remaining segment from each region was immersed
in 10% formalin solution for 48 hr. Before embedding, tissue samples
were washed for 3 hr with distilled water every 0.5 hr, and
dehydrated through a graded series of ethanol solutions, and then
embedded in Paraplast Plus
. Cross sections were obtained (rotary
microtome) from each region, mounted on slides, stained with hema-
toxylin and eosin and covered with cover glasses. We orientated the
tissue sections as perpendicular as possible to the cutting angle to
ensure that most villi were cut in full longitudinal view on the sections.
TABLE 1 Gross morphometrics and diets of species that were used in this study. Data are presented as mean of the species ± SEM
Species Family (n) Body mass (g)
mass (g)
Nominal surface
area (cm
) Primary dietary nutrient References
Rodents Montane Akodont (Akodon
montensis; Thomas, 1913)
Cricetidae 4 32 ± 0.3 1.8 ± 0.1 44 ± 3.2 27.2 ± 1.3 Carbohydrate/protein This study
Pallid Delomys (Delomys
sublineatus; Thomas, 1903)
Cricetidae 3 72 ± 3.9 2.4 ± 0.3 36.5 ± 0.6 29 ± 1.9 Carbohydrate/protein This study
Russet Rice Rat (Euryoryzomys
russatus; Wagner, 1848)
Cricetidae 3 157.1 ± 7 5.1 ± 1.4 30.3 ± 2.9 39.7 ± 7.6 Protein This study
Black-footed Colilargo
(Olygoryzomys nigripes; Olfers,
Cricetidae 2 39 ± 22.9 1 ± 0.4 20.7 ± 5 12.9 ± 5.1 Carbohydrate This study
Paraguayan Rice Rat (Sooretamys
angouya; Fischer, 1814)
Cricetidae 2 103.2 ± 17.4 3.5 ± 0.1 47 ± 8.3 39.5 ± 7.1 Carbohydrate This study
Blackish Grass Mouse
(Thaptomys nigrita;
Lichtenstein, 1829)
Cricetidae 3 25.4 ± 1.1 1.6 ± 0.3 38.6 ± 4.1 19 ± 1.9 Carbohydrate/ protein This study
White-footed Mouse
(Peromyscus leucopus;
Rafinesque, 1818)
Cricetidae 3 25.7 ± 1 0.8 ± 0.1 23.6 ± 1.2 15.6 ± 1.8 Protein Price et al.,
Common Mouse (Mus musculus;
Linnaeus, 1758)
Muridae 6 37 ± 0.8 1.1 ± 0.1 47.5 ± 0.1 28.3 ± 5.1 Carbohydrate/ protein Zhang et al.,
Meadow Vole (Microtus
pennsylvanicus; Ord, 1815)
Cricetidae 5 43.5 ± 4.8 0.64 ± 0.1
24.6 ± 2.2 17.2 ± 1.7 Carbohydrate Price et al.,
Bats Black Mastiff Bat (Molossus
rufus; Geoffroy, 1805)
Molossidae 1 33.6 1.2 16.9 12.6 Protein This study
Little Brown Bat (Myotis
lucifugus;Le Conte, 1831)
Vespertilionidae 3 9.2 ± 1.9 0.44 ± 0.02 14.2 ± 1.5 6.8 ± 1.1 Protein Price et al.,
Pallas' Mastiff Bat (Molossus
molossus; Pallas, 1766)
Molossidae 4 13.5 ± 1.6 0.4 ± 0.1 10.2 ± 1.3 5.7 ± 0.6 Protein This study
Wagner's Mastiff Bat (Eumops
glaucinus; Wagner, 1843)
Molossidae 1 34.1 0.75 10.8 8.3 Protein This study
Common Vampire Bat
(Desmodus rotundus; Geoffroy,
Phyllostomidae 3 38.5 ± 3.4 0.8 ± 0.2 18.4 ± 0.6 10.4 ± 1.5 Protein This study
Seba's short-tailed Bat (Carollia
perspicillata; Linnaeus, 1758)
Phyllostomidae 4 17.9 ± 3.2 0.5 ± 0.1 8.5 ± 0.4 6.5 ± 0.4 Carbohydrate This study
Great Fruit-eating Bat (Artibeus
lituratus; Olfers, 1818)
Phyllostomidae 3 69.6 ± 5.7 2.5 ± 0.2 47.8 ± 2 28.2 ± 2.9 Carbohydrate Caviedes-Vidal
et al., 2008
Microphotographs were taken using an Olympus BX50 microscope
connected to a videocamera (HDCE-30C) and a PC-based image anal-
ysis system using Image J software ( RRID:
SCR_003070; [Schneider, Rasband, & Eliceiri, 2012]).
Nominal surface area (NSA) of each intestinal region was calcu-
lated from the product of the circumference of the serosal surface
times the length of the intestinal region, and these regional values
were summed to calculate values for whole intestine. From each
region, we measured villus height (V
; from base to tip), villus width
), and the width of the crypts (C
). We took ~10 such measure-
ments per region, resulting in ~30 measurements per individual. We
measured only those villi that were cut in their midline, from tip to
base, as verified by observations of similarly sized and shaped
enterocytes. Enterocyte diameter (E
) was calculated as the inverse of
the number of enterocyte nuclei per unit length counted in sections
of villi where only enterocytes were present. Three sections from dif-
ferent villi per intestinal region were measured at 400×magnification
using ImageJ software (Schneider et al., 2012).
Three of the measurements were used to estimate the mucosal to
serosal surface area enlargement factor (SEF) using the following
equation according to Kisielinski, Willis, Prescher, Klosterhalfen, and
Schumpelick (2002):
Villous surface area (VSA) for a region was thus the product of the
respective regional measures of NSA and SEF, and these regional
values were summed to calculate values for whole intestine. Addi-
tional morphometrics were calculated using these measurements, as
described in Table 2. To avoid inflation of degrees of freedom by
repeated measurements within individuals, means were calculated for
individual animals and those means were used to calculate species
We made comparisons of our histological data with previously
reported functional measurements on the absorption of paracellular
probes. These probes, usually L-arabinose (M
150) but occasionally L-
rhamnose (M
164), are carbohydrates that are similar in size to glu-
cose but have no affinity for membrane transporters. Therefore, the
absorption of these probes provides an estimate of the paracellular
portion of glucose absorption. Whole-animal absorption of these pro-
bes (sometimes called fractional absorption) was measured using
standard pharmacokinetic techniques, in which animals were orally
dosed with the probes. Tissue-level absorption of these probes was
also measured in situ using surgical preparations on anesthetized ani-
mals, in which a buffer was circulated through an isolated section of
the small intestine. This experimental setup has the benefit of provid-
ing data on absorption of the paracellular probes normalized to cm
nominal intestinal surface area while controlling for any interspecies dif-
ferences in transit time. These data are presented here as clearance,
which is simply absorption rate divided by the concentration of the
probe in the buffer (clearance is linearly related to absorption).
TABLE 1 (Continued)
Species Family (n) Body mass (g)
mass (g)
Nominal surface
area (cm
) Primary dietary nutrient References
Brazilian Free-tailed Bat
(Tadarida brasiliensis; Geoffroy,
Molossidae 8 14 ± 0.6 0.4 ± 0.1 16.7 ± 0.1 8 ± 0.5 Protein Zhang et al.,
Big Brown Bat (Eptesicus fuscus;
Palisot de Beauvois, 1796)
Vespertilionidae 3 15.9 ± 3.9 0.73 ± 0.04 12.4 ± 0.6 7.48 ± 2 Protein Price et al.,
Length listed in this table is the length of the small intestine of rodents but of the whole intestine of bats, which lack a large intestine or is short and difficult to distinguish from the small intestine
macroscopically (e.g., E. fuscus).
This value from (Derting & Noakes, 1995).
Comparing our morphological measurements to these previously
reported data allowed us to relate structure to function.
2.1 |Statistical analyses
Phylogenetically informed methods were used for nearly all analyses, and
all analyses were carried out in R version 3.5.3 (R_Development_Core-
Team, 2019). We extracted the phylogeny for all species (Figure S1) from
the TimeTree database at (Hedges, Dudley, & Kumar, 2006).
All P-values were two-tailed and calculated using the equal-tail bootstrap
P-value (Hedges et al., 2006; MacKinnon, 2009) based on parametric
bootstrap replicates (2,000) of models (Ives, Midford, & Garland Jr., 2007;
MacKinnon, 2009). We used the same parametric bootstrapping to calcu-
late 95% confidence intervals for figures. Code and raw data can be
found at
Phylogenetic linear regressions were performed using the phy-
lolmpackage (Tung Ho & Ané, 2014). We used Pagel's lambda (Pagel,
1999) as the model for the regression error terms. We regressed the
following measurements on clade (a dummy variable where 1 indicates
the species is a bat): intestinal length, nominal surface area (NSA), villous
surface area (VSA), total number of enterocytes, and fractional absorp-
tion. We regressed the following measurements on clade, with separate
regressions for each intestinal segment: intestinal diameter, villus height,
villus width, crypt width, surface enlargement factor (SEF), enterocyte
diameter, and enterocyte density (enterocytes per cm
NSA). Lastly, we
regressed SEF on diet, where each diet type (herbivorous, omnivorous,
carnivorous) was a dummy variable with a value of 1 indicating a species
belonged to that group; P-values are presented only for omnivorous and
carnivorous groups and indicate whether they differ from herbivores.
We included log
(body mass) as a covariate in models where it was
significant (p< .05). We normalized fractional absorption (f)bytotal
intestinal surface (NSA ×SEF)(i.e.,f/((NSA ×SEF)). All dependent vari-
ables other than crypt width, enterocyte diameter, and villus width were
-transformed before taking species means; this was to reduce skew
in residuals and to improve linearity with independent variables.
In testing prediction #3, we calculated the Pearson correlation
coefficient between intestinal clearance and SEF. We used correlation
instead of regression because both variables were subject to error. To
account for phylogenetic signal, we used the function cor_phylo in the
R package phyr(Zheng et al., 2009) that can conduct parametric
bootstrapping of model fits. We included measurement error for both
parameters as their standard error within each species; for species
with only one individual, we used the average standard error among
species with >1 individual.
Bats had significantly shorter intestines than rodents (p= .003;
Figure 1a) and lower NSA (p< .001; Figure 1b). Bat intestinal diame-
ter, after controlling for body mass, was not significantly narrower
than that of rodents in any of the three segments assessed, nor were
differences detected along the intestine (proximal p= .527, middle
p= .959, distal p= .912; Figure 1c).
Bats and rodents differed in their villous morphology (Figure 2).
Bats exhibited significantly longer villi than rodents in middle and dis-
tal segments (proximal p= .177, middle p= .001, distal p< .001;
Figure 2a). In rodents, the villus height declined more quickly along
the length of the intestine than in bats (Figure 2a). Villus widths in
bats were significantly narrower than in rodents in proximal and mid-
dle segments (proximal p= .012, middle p= .004, distal p= .145;
Figure 2b). Similarly to villus heights, villus widths in rodents declined
along the longitudinal axis of the intestine much more noticeably than
in bats (Figure 2b). The width of crypts, which are located between
villi and contain immature nonabsorptive cells, did not change along
the segments of the intestine of rodents, and only the proximal region
was slightly narrower than the rest of the regions for bats (Figure 2c).
The comparison between clades shows that rodents have significantly
wider crypts than bats in all segments except the proximal region,
where the model had difficulty converging (see Uncertainty in regres-
sion of proximal crypt width on cladein the Data S1; proximal
p= .089, middle p= .014, distal p< .001; Figure 2c). This effectively
spaces out the villi more and reduces villous density in the rodents.
Villous dimensions and crypt width, which is inversely related to vil-
lous density, combine to determine the SEF (Equation (1)). Bats
exhibited a significantly larger SEF for all segments and this difference
became greater from the proximal to the distal intestinal segments
(proximal p< .001, middle p< .001, distal p< .001; Figure 3). SEF signif-
icantly varied with a species' body mass only in the proximal segment
(proximal p= .022, middle p= .322, distal p= .329; Figure 3), and did
not vary with a species' primary dietary nutrient (omnivore p= .122,
TABLE 2 Calculation of morphological features in each intestinal region
Eq. Feature Equation References
2 Number of villi, N
NSA/(basal area of a villous-crypt
unit) = NSA/(π×Vw+Cw
Kisielinski et al. (2002)
3 Surface area of single villus, A
Kisielinski et al. (2002)
4 Number of enterocytes per villus, N
Surface area of single villus/area of single
enterocyte = A
5 Total number of enterocytes N
6 Density of enterocytes per cm
Abbreviation: NSA, nominal surface area.
carnivore p=.505).WithgreaterSEF in bats one might expect them to
have more structural components and thus a heavier intestine. Among
the species in Table 1 intestine mass correlated positively with NSA
=46.0,p< .001) and was greater in bats than rodents, though not
significantly (F
=1.9,p= .19 by analysis of covariance [ANCOVA]
on log
transformed values). The greater SEF in bats compensated for
their significantly lower NSA (Figure 1(b)), effectively yielding VSA over
the entire small intestine that did not differ significantly between
rodents and bats (p= .922; Figure 4).
Enterocyte diameter did not differ along the length of the small
intestine in either clade (Figure 5a) and did not differ significantly
Intestinal length (cm)
10 40 160
Body mass (g)
NSA (cm2)
Proximal Middle Distal
10 40 160 10 40 160 10 40 160
Body mass (
Intestinal diameter (cm)
FIGURE 1 Intestinal gross morphometrics. (a) Length of small
intestine in rodents and whole intestine for bats expressed in
centimeters (cm). (b) Intestinal nominal surface area (NSA) in the
small intestine of rodents and whole intestine for bats of each
species expressed in squared centimeters (cm
). (c) Mean diameter
of bat and rodent species for each intestinal segment in
centimeters (cm). In all figures, variableswereregressedonbody
mass, points are species means, lines are phylogenetic-regression
confidence intervals. All axes are on the log scale
Proximal Middle Distal
10 40 160 10 40 160 10 40 160
Body mass (g)
Villus height (mm)
Villus width (mm)
Crypt width (mm)
Rodent Bat
FIGURE 2 Measurements of intestinal (a) villus height, (b) villus
width, and (c) crypt width (in mm). Points are species means, lines are
phylogenetic-regression fits for rodent and bat clades, and gray
envelopes are 95% confidence intervals. For (a), the y-axis is on the
scale, while the x-axis is on the log
scale for all figures. Facet
labels indicate the intestinal segment. Variables were regressed on
body mass for facets with solid lines
between rodents and bats except in the middle segment where it was
slightly smaller in bats (proximal p= .314, middle p= .047, distal
p= .632; Figure 5a). Based on this result, along with the significantly
greater SEF in bats, the enterocyte number per cm
of NSA (Equation (6))
was significantly greater in bats than in rodents (proximal p= .004, mid-
dle p< .001, distal p< .001; Figure 5b). Considering that the rodents had
greater NSA than the bats (Figure 1b), the total number of enterocytes,
and hence TJs, for the entire small intestine did not differ significantly
between bats and rodents (p= .318; Figure 6).
Clearance was positively correlated with SEF, although this was
only marginally significant (Pearson's r= 0.581, p= .103; Figure 7(a)).
This model was influenced very strongly by a single species (Tadarida
brasiliensis; Figure S3), and when it was re-fit without T. brasiliensis,
the correlation was much higher (r= .923, p= .004). Given such a
strong influence by a single point, we conclude that there is probably
a significant correlation between clearance and SEF. Whole-animal
fractional absorption (f) of paracellular probes, normalized to total vil-
lous surface area (VSA), was greater in bats (p< .001; Figure 7b). The
reason these ratios have negative slopes when regressed vs. body
mass, with bats higher than rodents, is that the numerator, f, is body
mass independent but higher in bats (Table S8) whereas the denomi-
nator, VSA, increases with body mass and does not differ between
bats and rodents (as shown in Figure 4). Though the comparison of
paracellular absorption in Figure 7 used the probe molecule L-arabi-
nose, results would be similar for some other probe molecules. For
example, fof another nutrient-sized probe molecule, L-rhamnose, was
also significantly higher in bat species than rodent species (respec-
tively, 0.74 ± 0.08, n= 3 bat species versus 0.18 ± 0.03, n= 3 rodent
species; p= .005, t-test on arcsin(square root (f)); Table S8).
All phylolmregression fits that included clade as a covariate
showed very weak phylogenetic signal, while the only regression
that did not include clade (SEF on diet) showed a moderate signal
(Tables S1S4). The phylogenetic signal for the correlation between
SEF and clearance was high, particularly for SEF,althoughtherewas
a high degree of uncertainty in the signal for both parameters
(Table S6).
Flight is a very energetically demanding mode of locomotion, contrib-
uting to high daily energy demands, but its structural prescription for
low weight also may shape an aspect of fliers' digestive apparatus in a
way that runs counter to that system's role in providing fuel to meet
high energy demands. For example bats (and birds), compared to ter-
restrial mammals, present small intestines with smaller NSA, and also
present shorter retention times of digesta (Price et al., 2015). These
features reduce the average load of digesta carried and improve flight
performance but should also jeopardize the digestion and absorption
of nutrients. Despite this, bats do not appear to suffer reduced nutri-
ent assimilation, and achieve similar or even greater digestive effi-
ciency compared to non-flying mammals, all while meeting equal or
higher daily energy demands (Price et al., 2015). The current study
was designed to understand the mechanistic basis of this feat.
Our findings suggest that bats can entirely compensate for 41.2%
lower NSA, compared with rodents in our sample, through differences
in features of villi. Our comparisons of cross-sections of villi of 18 spe-
cies indicated that bats, compared with rodents, have significantly
taller villi (Figure 2a) that are thinner (Figure 2b) and are spaced more
closely together because the crypts between them are narrower
(Figure 2c). We think there was little opportunity for differences in
food absorption to cause differential dilation of villous structures or
alterations in enterocyte sizes (Pappenheimer, 2001) because we pre-
pared tissues beginning the morning following nighttime capture and
animals were not fed. According to the model that we used to
Proximal Middle Distal
10 40 160 10 40 160 10 40 160
Body mass (
Surface enlargement factor (SEF)
Rodent Bat
FIGURE 3 Surface enlargement factor (SEF) across the intestinal
segments. Points are species means, lines are phylogenetic-regression
fits for rodent and bat clades, gray envelopes are 95% confidence
intervals, and facet labels indicate the intestinal segment. Variables
were regressed on body mass for facets with solid lines. Both axes are
on the log
10 40 160
mass (
Villous surface area (cm2)
FIGURE 4 Total villous surface area of the intestine regressed on
body mass. Points are species means, lines are phylogenetic-
regression fits for rodent and bat clades, and gray envelopes are 95%
confidence intervals. Both axes are on the log
estimate how villus features influence surface area (Equation (1)), the
SEF of bats was 63.9% greater than in rodents. SEF greater by >59%
in bats compared to nonflying mammals was reported earlier by (Barry
Jr., 1976) and by Mayhew and coworkers (Makanya et al., 1997; May-
hew, 1996; Mayhew & Middleton, 1985), relying on much smaller
numbers of species and using stereological methodologies different
from ours but uniform within their respective research groups. We
found that the total intestinal villous surface area did not differ sig-
nificantly between bats and rodents (Figure 4), consistent with our
first prediction that via differences in villous features, bats compen-
sated for lower NSA than rodents. Intestine mass as a function of
NSA was not significantly higher in bats than rodents, perhaps
differences in the ratio of intestinal mucosa mass to serosa mass.
Birds also do not differ from non-flying mammals in small intestine
mass and we argued elsewhere that the reduced intestinal size
(i.e., NSA) of flying vertebrates does notdirectlyreducebodymass,
but rather decreases the amount of digesta carried and the reten-
tion time of digesta (Price et al., 2015).
Although the intestinal surface has multiple functions, the functional
significance of the greater SEF in bats is probably larger for paracellular
absorption of water-soluble nutrients such as sugars and amino acids
than for membrane-based enzymatic hydrolysis of substrates, transmem-
brane mediated absorption of nutrients or transmembrane passive
absorption of lipids. The latter three processes benefit from further sur-
face area elaboration afforded by microvilli on the surface of enterocytes.
But paracellular absorption occurs between intestinal cells and is not
influenced by differences in surface area elaboration by microvilli. One
mechanism for increasing paracellular absorption per cm
small intestine
is an increase in number of TJs across which paracellular absorption
occurs. Our second prediction was that there would be no significant dif-
ference between bats and rodents in enterocyte size, effectively yielding
greater enterocyte density and more TJs per cm
NSA in bats due to
their greater SEF. As predicted, there was no significant difference in
enterocyte size between the taxa, except for a marginal difference in the
middle segment (Figure 5a). Total number of cells per cm
of NSA was
102.8% greater in bats than rodents (Figure 5b). We think that our esti-
mates for enterocyte diameter and density for bats and rodents are rea-
sonable, considering comparisons with a few published values. For
example, our estimates of enterocyte diameter ranged 4.79.6 μm
(Figure 5a), similar to reported values for rat (4.2; Mayhew, 1996), mice
(6.8 [Abbas, Hayes, Wilson, & Carr, 1989]), and chickens (9.5 [Karcher &
Applegate, 2008]). Our estimates for enterocyte density of rodents
ranged 11.354.7 million/cm
NSA, bracketing a published estimate for
laboratory rats (39.7 million/cm
[Zoubi, Mayhew, & Sparrow, 1995]).
Because higher cell density should correspond to a higher number
of TJs across which paracellular absorption occurs, our third
Proximal Middle Distal
10 40 160 10 40 160 10 40 160
Body mass (
Enterocyte diameter (µm)
Enterocyte density (cm 2)
Rodent Bat
FIGURE 5 Diameter and density of enterocytes along the small
intestine for rodents and whole intestine for bats. (a) Average
enterocyte diameter in micrometers (μm). (b) Mean densities of
enterocytes per square centimeter of nominal surface area (NSA)
along the intestine (in millions of enterocytes per cm
). Points are
species means, lines are phylogenetic-regression fits for rodent and
bat clades, and gray envelopes are 95% confidence intervals. In (b),
the y-axis is on the log
scale, and the x-axis is on the log
scale for
both figures. No variables were regressed on body mass
10 40 160
Body mass (
Total enterocytes
FIGURE 6 Total number of enterocytes (in billions of
enterocytes) in the small intestine for rodents and bats, regressed on
body mass. Points are species means, lines are phylogenetic-
regression fits for rodent and bat clades, and gray envelopes are 95%
confidence intervals. Both axes are on the log
prediction was that the clearance of paracellular probes would be pos-
itively correlated with SEF. Accordingly, we combined data from this
study with clearance measurements we previously reported for some
of the bat and rodent species. Consistent with our prediction, clear-
ance appears positively correlated with SEF (Figure 7a). Thus, it seems
plausible that a functional significance of enhanced SEF is greater
paracellular absorption of water-soluble nutrients such as glucose and
amino acids. Bats have higher capacity for paracellular absorption
(Price et al., 2015), and a greater density of TJs afforded by greater
SEF could be a mechanistic explanation. It might be useful in the
future to test this further by focusing on measures of TJ proteins
(e.g., claudins, cadherins, tricellulin, ZO occludins; [Gunzel & Yu,
2013]), or their mRNA, as an index to TJ density cm
Paracellular absorption was higher in bats than in rodents for both
the probe molecules L-arabinose (Figure 7) and L-rhamnose (Table S8).
Researchers hypothesize that paracellular absorption might occur via
both a higher capacity porepathway and a very low capacity leak
pathway (Van Itallie et al., 2008; Zihni, Mills, Matter, & Balda, 2016),
and paracellular absorption generally declines with increasing molecu-
lar size due to size discrimination of the pathway, as predicted by pore
theory (Chediack, Caviedes-Vidal, Fasulo, Yamin, & Karasov, 2003).
Hence, as expected, fractional absorption (f) of the smaller-sized L-
arabinose (molecular radius ~3.1 Å) was greater than the larger-sized
L-rhamnose (~3.7 Å). In both taxa fdeclines due to limiting pore radius,
which may be ~4 Å [Van Itallie et al., 2008; Zihni et al., 2016]. The
lower capacity leakpathway is thought to accommodate much larger
molecules (up to 60 Å; [Van Itallie et al., 2008; Zihni et al., 2016]), but
such large probe molecules have never been tested in bats, so we
know little about the leak pathwayin bats in relation to their
greater SEF.
An alternative, but not mutually exclusive, mechanistic explanation
for higher paracellular absorption is that there are differences in the
proteins that makeup the TJs and influence the permeability of each
TJ (Price et al., 2014, 2015). We suspect that there are differences in
TJ permeability between bats and rodents, because the ratio of
whole-animal fractional absorption to whole intestine VSA (our proxy
of number of tight junctions) in bats still exceeds that of rodents
(Figure 7b). Moreover, the transit time is likely quite rapid in bats
(i.e., food spends less time in the gut of bats compared to rodents, and
therefore has less time for absorption) (Buchler, 1975; Klite, 1965;
Price et al., 2015), which also suggests that the TJs of bats must be
more permeable than those of rodents.
Overall, our comparative analysis of bats and rodents identified
significant differences between the taxa in villous features that lead
to higher SEF in the bats, and we have suggested a functional conse-
quence of this differencehigher paracellular absorption. It is impor-
tant to note that our model to estimate how villus features influence
surface area (Equation (1)) assumes that villi are radially symmetrical.
It may overestimate SEF if villus width and crypt width differ in the
longitudinal direction along the intestine, and future studies can
improve on our approach if they consider the specific three-
dimensional shape of villi and model them as closely as possible. Our
approach also simplistically assumed that all mucosal intestinal cells
are columnar enterocytes, whereas in fact intestinal mucosa includes
other cell types (e.g., goblet, entero-endocrine, and Paneth cells;
[Cheng & Leblond, 1974] whose TJ's have different permeability fea-
tures (Pearce et al., 2018). But, the vast majority of intestinal cells are
enterocytes (e.g., >90% in laboratory rats; (Cheng & Leblond, 1974)),
which we assume is also the case in bats, and so we expect that our
conclusions based on the relative differences using our simplifying
assumptions should hold up. TJ permeability also can differ between
bi- and tri-cellular junctions (Krug, 2017), but we are not aware of any
10 40 160
Surface enlagement factor (SEF)
mass (
L-arabinose clearance (µl min 1cm 2)Absorption 103/ intest. area (cm 2)
Clade: Rodent
Bat Diet: Carb
FIGURE 7 (a) Intestinal clearance of the paracellular probe
L-arabinose versus SEF in nine species of bats and rodents for which
both measures were available. The unfilled triangle indicates the
influential bat species for the correlation. Both axes are on the log
scale. SEF data are listed in this paper, and clearance data and sources
are in Table S8. (b) Whole-animal fractional absorption of paracellular
probes divided by total intestinal surface area (NSA ×SEF) regressed
on body mass. Lines are phylogenetic-regression fits for rodent and
bat clades, and gray envelopes are 95% confidence intervals. Both
axes are on the log
scale. Morphological data are listed in this paper,
and those for fractional absorption and their sources are in Table S8.
For both figures, points represent species means; point shape
indicates diet (Carb= herbivorous or omnivorous,
Protein= carnivorous), and color indicates clade
information on these in rodents versus bats and for now we assume
no major differences due to that.
We see in our data and in the clearance data (Figure 7a) a possible
effect of diet on SEF and arabinose clearance that can only be
explored with studies on more species. Within each clade, animals
that digest mainly protein seem to have lower values of SEF and arabi-
nose clearance. This may make functional sense, because the amino
acid products of protein digestion tend to have smaller molecular
masses and radii than the monosaccharide products of carbohydrate
digestion. Studies in bats and rodents have shown that smaller para-
cellular probes (such as creatinine, which models the paracellular
absorption of amino acids) are absorbed to a much greater extent than
larger, glucose-sized paracellular probes (Dominguez & Pomerene,
1945; Lundholm & Svedmyr, 1963; Pappenheimer, 1990; Price et al.,
2014; Price et al., 2016; Turner, Cohen, Mrsny, & Madara, 2000). It is
plausible that there has been less natural selection for high SEF and
paracellular permeability in carnivores and insectivores compared to
herbivores and omnivores. We caution, however, that we do not yet
understand the relative contributions of individual amino acids versus
the larger di- and tri-peptides to protein absorption (Adibi, 2003; Price
et al., 2015).
Funded by United States National Science Foundation IOS-1025886
to WHK and EC-V, Brazilian CNPq Processo 470128/2011-9 and
Fundaçao de Amparo a Pesquisa do Estado de Sao Paulo 2012/
04610-5 to APC-N, Universidad Nacional de San Luis 2-0814 and
Consejo Nacional de Investigaciones Científicas y Técnicas PIP834 to
EC-V, and Department of Forest and Wildlife Ecology, UW-Madison.
Antonio Brun
William H. Karasov
Enrique Caviedes-Vidal
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Additional supporting information may be found online in the
Supporting Information section at the end of this article.
How to cite this article: Brun A, Fernández Marinone G,
Price ER, et al. Morphological bases for intestinal paracellular
absorption in bats and rodents. Journal of Morphology. 2019;
... The villi enlarge surface area, and the surface enlargement factor (SEF) for villi is greater in bats than in nonflying mammals (Fig. 1B) because bats have taller villi that are thinner and are spaced more closely together (crypts between them are narrower) (Brun et al., 2019b). Birds appear also to exceed mammals in villous SEF (Fig. 1B). ...
... The body size range considered here is up to ~10 kg, considering the size range of most flying birds and bats. NSA is about 35-40% lower in the flyers (Brun et al., 2019b;Lavin et al., 2008). Regression plot adapted from (Price et al., 2015). ...
... Stereological methods are used to determine how, and to what extent, villi increase surface area, through the calculation of villous surface enlargement factor (SEF), defined as villous SA/NSA. For illustration purposes the outline of villi relative to that of intestine circumference is shown, but SEF more typically factors in measures of villi heights, widths, and density (or indices thereof) (Brun et al., 2019b). Phylogenetically informed comparative analyses found that SEF was independent of body mass in mammals and birds and higher in flyers than nonflying mammals (Brun et al., 2019b;Lavin et al., 2008) (higher by 59% and 44% for bats and birds, respectively, in data shown here). ...
The small intestine of mammals and birds exhibits fascinating variation across taxa, body size, and life history features such as locomotion and diet. In the intestine's brush border membrane (BBM), hydrolases are more abundant than transporters in both mammals and birds, but there are differences among the groups in abundance of certain hydrolases and possibly in transporters. For example, mammals express two α-glucosidases, sucrase-isomaltase (SI) and maltase glucoamylase (MGAM), whereas songbirds we studied have only SI, and the chicken expresses SI plus another α-glucosidase that functions similarly to MGAM but is not a true ortholog. For intestinal absorption of sugars and amino acids, small fliers rely on a paracellular pathway to a greater extent than do nonflying mammals, which rely more on transporters. Possibly having evolved in fliers as compensation for lower intestinal nominal surface area (NSA), the fliers' reliance on paracellular absorption is supported by their greater villous surface enlargement that leads to more (per cm² NSA) tight junctions and greater clearance of passively absorbed compounds. To match digestive capacity to nutrient load, a positive relationship is often observed between dietary intake of macronutrients and intestinal activity of the enzymes and transporters of their respective constituents. In enterocytes, rapid, fine-tuned adjustment to high dietary carbohydrate and protein involves rapid, specific correlated increase in activity and abundance of hydrolases and transporters in the BBM and increases in their mRNA.
... It has been speculated that bats use the paracellular pathway for nutrient absorption as a compensatory mechanism for the lower intestinal nominal surface area when compared to non-flying mammals (Caviedes-Vidal et al., 2008). This passive mechanism occurs through the tight junctions between adjacent enterocytes, and is advantageous because it is a faster mechanism, that does not require energy expenditure to absorb amino acids and monosaccharides (Brun et al., 2019). In addition, even having shorter intestines, the absorptive intestinal area is increased by higher villi and a larger area of microvilli than in rats, for example (Makanya et al., 1997). ...
... The intestinal length was within the range previously described for other bat species, ranging from 10.2 cm in the insectivorous bat Molossus molossus to 47.8 cm in A. planirostris (Brun et al., 2019). The intestinal coefficient was obtained to correlate the investment in intestines proportionally to the body mass of each species. ...
Full-text available
In view of the great diversity of dietary habits among bats, and the need for morphological adaptations in their digestive system, this study characterized and compared the morphology of the small (SI) and large intestines (LI) of the bats Artibeus planirostris (Phyllostomidae: Stenodermatinae) and Diphylla ecaudata (Phyllostomidae: Desmodontinae), and interpret their morphologies in the context of dietary differences. We hypothesized that diet could influence the morphological characteristics in these two species by means of a more complex intestinal morphology in A. planirostris than in D. ecaudata. The intestines were histologically processed, stained, and analyzed. Despite body mass differences, total intestinal length were almost two folds higher in A. planirostris than in D. ecaudata, and the intestinal coefficient, which quantifies the investment in intestines in relation to the body mass, wasn’t statistically different. Macroscopically, no distinction was observed between the SI and LI, and few differences allowed to distinguish histologically the regions of the SI in both species. We found a typical organization of Peyer’s patches only in D. ecaudata. Enteroendocrine cells were more frequently observed at caudal parts of the small intestine in both species. In A. planirostris, the SI diameter, the thickness of its wall, and the thickness and percentage occupied by the mucosal layer, as well as the wall thickness of its LI, were greater than those in D. ecaudata. Circular folds were observed only in the LI of A. planirostris. We speculate that these differences represent strategies to increase the contact surface available to optimize nutrient uptake in A. planirostris; while the hematophagous feeding habit of D. ecaudata, predominantly proteinaceous and semifluid, has a gut wall characterized with less morphological complexity and specializations; which confirms our hypothesis. Based on the similarities and differences founded, it can be concluded for both species that the reduction in the anatomical structures, associated with the microscopic findings, allows to optimize the absorption of nutrients and water in these bats when compared to other mammals; a similar pattern to that found in other chiropterans.
... These adaptations generally reduce immune reactivity to pathogens, particularly viral pathogens, but they may also alter host filtering of microorganisms more generally. Additionally, weakened tight junctions between enterocytes increase paracellular nutrient absorption to support the energetics of flight but also allow more microbial transfer across the gut epithelium [162][163][164][165] . To avoid constant immune activation as a result of these exposures, bats have increased immune tolerance and resilience to microbial infections [166][167][168] . ...
Developing general principles of host–microorganism interactions necessitates a robust understanding of the eco-evolutionary processes that structure microbiota. Phylosymbiosis, or patterns of microbiome composition that can be predicted by host phylogeny, is a unique framework for interrogating these processes. Identifying the contexts in which phylosymbiosis does and does not occur facilitates an evaluation of the relative importance of different ecological processes in shaping the microbial community. In this Review, we summarize the prevalence of phylosymbiosis across the animal kingdom on the basis of the current literature and explore the microbial community assembly processes and related host traits that contribute to phylosymbiosis. We find that phylosymbiosis is less prevalent in taxonomically richer microbiomes and hypothesize that this pattern is a result of increased stochasticity in the assembly of complex microbial communities. We also note that despite hosting rich microbiomes, mammals commonly exhibit phylosymbiosis. We hypothesize that this pattern is a result of a unique combination of mammalian traits, including viviparous birth, lactation and the co-evolution of haemochorial placentas and the eutherian immune system, which compound to ensure deterministic microbial community assembly. Examining both the individual and the combined importance of these traits in driving phylosymbiosis provides a new framework for research in this area moving forward.
... But, how could the smaller bats have compensated for high TT in the intestine to acquire the energy content of food? In food processing, digestion is the initial step to decomposing complex molecules into simpler ones that can be later absorbed by the intestine (Price et al. 2014;Brun et al. 2019). This is achieved by hydrolytic enzymes that are bonded to the apical section of the enterocytes (Zhang et al. 2015;Price et al. 2015). ...
Full-text available
Bats are unique among mammals in that they have evolved the capacity to fly. This has generated strong selective pressure on the morphology and function of their digestive system. Given that in bats intestinal length and nominal surface-area are proportional to body mass, this trait importantly relates to explaining some of their digestive characteristics. We described the relationship between digestive traits and body mass of four species of bats of the family Vespertilionidae living in a montane ecosystem in central Mexico. We calculated food transit time, apparent dry matter digestibility, and defecation rate in feeding trials under captive conditions. We also: 1) built a model of the relationship between digestive traits and body mass to determine if this association was consistent within the members of the family Vespertilionidae, and 2) mapped these traits along the phylogeny to explore how digestive characteristics may have evolved. In our feeding trials, body mass was positively related to transit time and negatively related to apparent dry matter digestibility. The model predicted accurately the transit time in bats with body mass < 20 g. The phylogenetic approach suggested that over the evolutionary history of the family, transit time decreased as digestibility increased. Because of the results obtained here, it is likely that for most bats of the family Vespertilionidae, adaptations in digestive traits to process food have followed evolutionary changes in their body mass. We discuss these findings in a physiological and ecological context.
... Villus length and crypt width determine the intestinal absorptive surface and nutrient absorption. A surface area of the small intestine is a key determinant of the absorptive capacity [61]. Moreover, in the assessment of the intestinal response to various diets, the determination of the villus to crypt ratio is also effective [62]. ...
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The aim of the experiment was to assess the effect of dietary alpha-ketoglutarate (AKG) supplementation on performance, serum hormonal indices, duodenum and jejunum histomorphometry, meat quality characteristics, bone quality traits and cartilage degradation in laying hens with a mature skeletal system. Forty-eight 30 week-old Bovans Brown laying hens were randomly assigned to a control group or the group fed the basal diet plus 1.0% AKG. The experimental trial lasted 30 weeks. The supplementation of AKG increases blood serum content of leptin, ghrelin, bone alkaline phosphatate and receptor activator of nuclear factor kappa-Β ligand, while osteoprotegerin and osteocalcin decrease. While dietary AKG was given to laying hens negatively influenced villus length, crypt depth, villus/crypt ratio and absorptive surface area in duodenum and jejunum, these changes have no effect on feed intake, weight gain, nor laying performance. In breast muscles, no significant changes in skeletal muscle fatty acid composition were observed, however, a higher shear force and decreased cholesterol content following AKG supplementation were noted, showing the improvement of muscle quality. While dietary AKG supplementation did not affect the general geometric and mechanical properties of the tibia, it increased collagen synthesis and enhanced immature collagen content. In medullary bone, an increase of bone volume fraction, trabecular thickness, fractal dimension and decrease of trabecular space were observed in AKG supplemented group. The trabeculae in bone metaphysis were also significantly thicker after AKG supplementation. AKG promoted fibrillogenesis in articular cartilage, as indicated by increased cartilage oligomeric matrix protein immunoexpression. By improving the structure and maintaining the proper bone turnover rate of highly reactive and metabolically active medullar and trabecular bones AKG showed its anti-osteoporotic action in laying hens.
... The absorption of small nutrient molecules, such as simple sugars, free amino acids, and peptides, occurs directly through the paracellular and transcellular pathways 57,58 . The first is important in bats 56,59 . Food rich in small nutrient molecules should result in a fast and high assimilation, even with short intestines, as we observed. ...
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Frugivorous bats often possess short intestines, and digest rapidly. These characters are thought to be weight-saving adaptations for flight. The hypothesis that they limit digestive efficiency was tested by assaying glucose and protein in fecal samples of a free-ranging bat, and in fruit of its main food plant. To assure the correct calculation of digestive efficiencies, seeds were used as a mass marker for nutrients in fruit and feces. Glucose represents 32.86%, and protein 0.65%, of the nutrient content of fruit. Digestive efficiencies for these nutrients respectively are 92.46% and 84.44%, clearly negating the hypothesis for glucose. Few studies have quantified protein in fruit. Instead, “crude protein”, a dietary parameter solely based on nitrogen determinations, is used as a surrogate of protein content. This study shows that, for fruit consumed by bats, crude protein estimates typically are much greater than true protein values, implying that a large fraction of the crude protein reported in previous studies consists of free amino acids. The rapid digestion of frugivores has the potential to limit protein digestion, thus it may require free amino acids for efficient assimilation of nitrogen; therefore, the crude protein approach is inadequate for the fruit that they consume because it does not differentiate free amino acids from protein. Adding simple sugars and free amino acids, instead of protein, to fruit reduce metabolic costs for plants. Direct assimilation of these small nutrient molecules increases digestive and foraging efficiencies. Both factors contribute to the persistence of the mutualism between plants and frugivores, with community-wide repercussions.
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Background: Mammalian small intestinal tight junctions (TJ) link epithelial cells to one another and function as a permselective barrier, strictly modulating the passage of ions and macromolecules through the pore and leak pathways, respectively, thereby preventing the absorption of harmful compounds and microbes while allowing regulated transport of nutrients and electrolytes. Small intestinal epithelial permeability is ascribed primarily to the properties of TJs between adjoining enterocytes (ENTs), because there is almost no information on TJ composition and the paracellular permeability of nonenterocyte cell types that constitute a small but significant fraction of the intestinal epithelia. Results: Here we directed murine intestinal crypts to form specialized organoids highly enriched in intestinal stem cells (ISCs), absorptive ENTs, secretory goblet cells, or Paneth cells. The morphological and morphometric characteristics of these cells in organoids were similar to those in vivo. The expression of certain TJ proteins varied with cell type: occludin and tricellulin levels were high in both ISCs and Paneth cells, while claudin-1, -2, and -7 expression was greatest in Paneth cells, ISCs, and ENTs, respectively. In contrast, the distribution of claudin-15, zonula occludens 1 (ZO-1), and E-cadherin was relatively homogeneous. E-cadherin and claudin-7 marked mainly the basolateral membrane, while claudin-2, ZO-1, and occludin resided in the apical membrane. Remarkably, organoids enriched in ENTs or goblet cells were over threefold more permeable to 4 and 10 kDa dextran compared to those containing stem and Paneth cells. The TJ-regulator larazotide prevented the approximately tenfold increases in dextran flux induced by the TJ-disrupter AT1002 into organoids of different cell types, indicating that this ZO toxin nonselectively increases permeability. Forced dedifferentiation of mature ENTs results in the reacquisition of ISC-like characteristics in TJ composition and dextran permeability, suggesting that the post-differentiation properties of TJs are not hardwired. Conclusions: Differentiation of adult intestinal stem cells into mature secretory and absorptive cell types causes marked, but potentially reversible, changes in TJ composition, resulting in enhanced macromolecular permeability of the TJ leak pathway between ENTs and between goblet cells. This work advances our understanding of how cell differentiation affects the paracellular pathway of epithelia.
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Epithelia and endothelia separate different tissue compartments and protect multicellular organisms from the outside world. This requires the formation of tight junctions, selective gates that control paracellular diffusion of ions and solutes. Tight junctions also form the border between the apical and basolateral plasma-membrane domains and are linked to the machinery that controls apicobasal polarization. Additionally, signalling networks that guide diverse cell behaviours and functions are connected to tight junctions, transmitting information to and from the cytoskeleton, nucleus and different cell adhesion complexes. Recent advances have broadened our understanding of the molecular architecture and cellular functions of tight junctions.
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Flying vertebrates (birds and bats) are under selective pressure to reduce the size of the gut and the mass of the digesta it carries. Compared with similar-sized nonflying mammals, birds and bats have smaller intestines and shorter retention times. We review evidence that birds and bats have lower spare digestive capacity and partially compensate for smaller intestines with increased paracellular nutrient absorption. ©2015 Int. Union Physiol. Sci./Am. Physiol. Soc.
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Flying vertebrates have been hypothesized to rely heavily on paracellular absorption of nutrients to compensate for having smaller intestines than non-flyers. We tested this hypothesis in an insectivorous bat (Myotis lucifugus) and two insect-eating rodents (Onychomys leucogaster and Peromyscus leucopus). In intact animals, the fractional absorption of orally dosed L-arabinose (Mr 150) was 82% in M. lucifugus, which was more than twice that of the rodents. Absorption of creatinine (Mr 113) was greater than 50% for all species and did not differ between M. lucifugus and the rodents. We also conducted intestinal luminal perfusions on anesthetized animals. Absorption of L-arabinose per nominal surface area in M. lucifugus was nearly double that of the rodents, while absorption of creatinine was not different among species. Using an everted sleeve preparation, we demonstrated that high concentrations of L-arabinose and creatinine did not inhibit their own uptake, validating their use as passive, paracellular probes. Histological measurements indicated that M. lucifugus has more cells, and presumably more tight junctions, per nominal surface area than P. leucopus. This seems unlikely to explain entirely the higher absorption of L-arabinose in M. lucifugus during perfusions, because L-arabinose absorption normalized to the number of enterocytes was still double that of P. leucopus. As an alternative, we investigated tight junction gene expression. M. lucifugus had higher expression of claudin-1 and -15, and lower expression of claudin-2 relative to P. leucopus. Expression of claudin-7 and occludin did not differ among species. Taken together, our results support the hypothesis that bats have evolved higher paracellular nutrient absorption than non-flying animals, and that this phenomenon might be driven by both histological characteristics and differences in tight-junction gene expression.
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We developed a linear-time algorithm applicable to a large class of trait evolution models, for efficient likelihood calculations and parameter inference on very large trees. Our algorithm solves the traditional computational burden associated with two key terms, namely the determinant of the phylogenetic covariance matrix V and quadratic products involving the inverse of V. Applications include Gaussian models such as Brownian motion (BM) derived models like Pagel's lambda, kappa, delta and the early-burst model; Ornstein-Uhlenbeck models to account for natural selection with possibly varying selection parameters along the tree; as well as non-Gaussian models such as phylogenetic logistic regression, phylogenetic Poisson regression and phylogenetic generalized linear mixed models. Outside of phylogenetic regression, our algorithm also applies to phylogenetic principal component analysis, phylogenetic discriminant analysis or phylogenetic prediction. The computational gain opens up new avenues for complex models or extensive resampling procedures on very large trees. We identify the class of models that our algorithm can handle as all models whose covariance matrix has a 3-point structure. We further show that this structure uniquely identifies a rooted tree whose branch lengths parametrize the trait covariance matrix, which acts as a similarity matrix. The new algorithm is implemented in the R package phylolm, including functions for phylogenetic linear regression and phylogenetic logistic regression.
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Changes in gut capacity may be important for a species adapting to increased energy requirements or decreased food quality in a seasonal environment. We conducted a comparative study of seasonal changes in gut capacity in two rodent species with diets of different types. Although the lengths and masses of gut organs differed between species within a season, the species did not differ in the types of gut changes that occurred from summer to winter. All organs except the colon had significantly heavier wet and dry masses in winter than in summer. No significant differences in organ lengths, volumes, or surface areas occurred with season. Increased mass of the small intestine was due to large increases in the mass of the mucosa and smaller increases in the mass of the serosa. In winter, Microtus pennsylvanicus had significantly lower body mass than in summer. Peromyscus leucopus had no change in body mass in winter but may have used torpor as an energy-conservation mechanism. Energy-conservation adaptations in each species may have minimized the need for large changes in the gut organs.
The tricellular tight junction (tTJ) is a potential weak point of the paracellular barrier. For solving the proportional contribution of the tTJ, ion conductances and macromolecule permeabilities were analyzed in cell lines of different leakiness. MDCK II, Caco-2, and HT-29/B6 cells were subjected to two-path impedance spectroscopy and morphological analyses in order to calculate the contribution of the tTJ to paracellular and total ion conductivity. The contribution to macromolecule permeability was evaluated by tricellulin overexpression or knockdown. Tricellulin-dependent macromolecule passage was comparably regulated in leaky and tight epithelia, but relative and absolute ion permeabilities of the tTJs were different. Assuming a minimal (50 pS) and maximal (146 pS) conductivity per single tTJ, the possible range of contribution of the tTJ to paracellular ion conductance amounted to only 0.3-1.1% in the leaky cell line MDCK II, but 3-25% in the moderately tight cell line Caco-2, and not less than 29% in the tight cell line HT-29/B6. In these cells, this resulted in a contribution to total epithelial conductance of 9-32%. In conclusion, in leaky epithelia the bicellular TJ accounts for nearly the entire paracellular ion conductance, whereas in tight epithelia the low bicellular TJ conductance has large impact on the tTJ.
Studies on birds have led to the hypothesis that increased intestinal absorption between enterocytes (paracellular) evolved as a compensation for smaller intestinal size in fliers, which was perhaps selected to minimize the mass of digesta carried. This hypothesis predicts that bats will also exhibit relatively reduced intestinal size and high paracellular absorption, compared with nonflying mammals. Published studies on three bat species indicate relatively high paracellular absorption. One mechanism for increasing paracellular absorption per cm2 small intestine (SI) is increased number of tight junctions (TJs) across which paracellular absorption occurs. To our knowledge, we provide the first comparative analysis of enterocyte size and number in flying and nonflying mammals. Intestines of insectivorous bats Tadarida brasiliensis were compared with Mus musculus using hematoxylin and eosin staining method. Bats had shorter and narrower SIs than mice, and after correction for body size difference by normalizing to mass3/4, the bats had 40% less nominal surface area than the mouse, as predicted. Villous enhancement of surface area was 90% greater in the bat than in the mouse, mainly because of longer villi and a greater density of villi in bat intestines. Bat and mouse were similar in enterocyte diameter. Bats exceeded mice by 54.4% in villous area per cm length SI and by 95% in number of enterocytes per cm2 of the nominal surface area of the SI. Therefore, an increased density of TJs per cm2 SI may be a mechanistic explanation that helps to understand the high paracellular absorption observed in bats compared to nonflying mammals. J. Morphol., 2014. © 2014 Wiley Periodicals, Inc.