| dc.description.abstract | The structural characteristics of the intestinal
tract of three species of bats namely the fruit bat
Epomophorus wahlbergi and the entomophagous bats
Miniopterus inflatus and Rhinolophus hildebrandti have
been examined macroscopically, with the light microscope
and with the scanning and transmission electron
microscopes. Stereological techniques and formulae have
been modified and utilized to estimate the surface
characteristics of the intestinal mucosa on two species of
bats. In all the bats examined, neither a caecum nor an
appendix was observed and the intestine was a narrow tube
of almost uniform diameter save for the rectum whose
diameter was greater than that of the rest of the
intestine. A colon was only observed in the fruit bat but
could only be distinguished from the rest of the intestine
from its characteristic mucosal surface after opening the
intestine. The villi in the cranial 20 % of the intestine
of the fruit bat branched and interconnected while those
occurring in the rest of the intestine were generally
finger-like discrete projections. In the insectivorous
bats, the villi were ridge-like and were transversely
oriented, the only deviation from this pattern being
observed in a small posterior part of the intestine of the
horseshoe bat. In the latter, the villi anastomosed
profusely forming shallow hexagonal compartments semblant
of the reticular cells of the ruminant stomach. In the two
insectivorous species of bats, a short segment of the
mucosa immediately posterior to the pylorus had numerous
hexagonal and cylindrical pits which were thought to be
either involved in enzyme secretion, absorption of
nutrients or both.
The ultrastructural picture of the intestinal mucosa
in the frugivorous bat showed remarkable cellular and
pericellular modifications that were absent in the
entomophagous bats. The enteric epithelium was made of
tall columnar cells between which were large intercellular
spaces. These columnar cells had large intracellular
vacuoles and sent long and tortuous cytoplasmic
projections (pseudopodia) into the intercellular spaces.
Adjacent cells were joined by means of desmosomes formed
between two apposing cytoplasmic processes. The role of
the cytoplasmic processes besides structural
reinforcement, was thought to be pinocytosis. In the
insectivorous bats, intercellular spaces were narrow and
epithelial cells were devoid of vacuoles and no
specializations were observed on the lateral membranes of
the absorptive cells. In both groups of bats, the
epithelial cells had numerous mitochondria distributed
over the apical and basal sides of the centrally located
nucleus.
Macroscopic morphometric comparisons showed that the
intestine of the frugivorous bat is longer than that of
the insectivorous bat but when intestinal length is
normalized with body mass, the insectivore had a longer
intestine. The microvillous dimensions (mean length,
diameter and surface) in the fruit-eating bat and
insectivorous bat showed no significant trends but such
trends were significant for segmental microvillous
numbers, microvillous amplification factors and segmental
surface areas with these values being highest in the
proximal intestinal segments and lowest in the posterior
segments of the intestine. In the fruit bat, the average
values for microvillous height, diameter and surface were
2.87 J..Lm0,.097 J..Lamnd 0.8739 J..Lmr2espectively with an
uncorrected total microvillous surface area of 2.50 X 1012
J..Lm2T.he mean microvillous height, diameter, and surface
area for the insectivorous bat were 1.09 J..Lm0,.088 J..Lamnd
0.3069 J..Lm2 respectively with an uncorrected absolute
intestinal surface area of 1.32 X 1011J..LmT2h.e microvillous
packing density and the absolute number of microvilli in
the fruit bat were 58 J..Lma-n2d 3.24 x 1012 respectively. In
the insectivorous bat the microvillous packing density was
88 J..Lmw-i2th an absolute number of 3.90 X 1011•
This study indicates that the chiropteran intestine
is structurally and hence functionally better adapted for
absorption than that of the land-based mammals for which
similar studies have been conducted. The qualitative and
quantitative results of this study indicate that the
frugivorous bat has a superior intestine than the
entomophagous bat. This may in part be explained in terms
of the differences in their energetic demands of flight
and the differences in the types of diet on which these
bats thrive. Although an attempt has been made to explain
the adaptive characteristics of the chiropteran intestine
and the observed species differences, conclusive
explanations of these peculiarities must not only await
broader morphometric studies on a wider range of species,
but also detailed observations of their ecological,
physiological and flight biomechanical characteristics. | en |
