Absorptive surface of the small intestine of three species of passerine birds with different dietary regimen: A comparative stereological study
Abstract
Morphology of the absorptive surface reflects the diet. Comparative stereology was done on the absorptive surface of the small intestine of similar sized, nectarivorous, omnivorous and granivorous species of passerine birds using light and electron microscopy. The aim was to determine differences that can be attributed to their disparate diets. The dimension of absorptive cells, the microvillous length, diameter and packing density were estimated. The absorptive cells are the same in the three species. The remarkable difference was the height and packing density of the microvilli. The significantly, shortest, microvilli were in the granivorous species which measured 1.11 ± 0.58 µm, intermediate in the omnivorous species, measuring 4.30 ± 0.23 µm and the nectarivorous species had significantly, tallest microvilli with a mean length of 8.025 ± 1.66 µm. The packing density was significantly lower (≤ 52.0 ± 4 per µm2 ) in the granivore, intermediate in the omnivore (95.0 ± 8 per µm 2 ) and significantly larger (≤ 109.0 ± per µm2 ) in the nectarivore. The absorptive surface in the small intestine of the nectarivore is greatly enhanced by the tall and densely packed microvilli and this type of absorptive surface is suitable for maximal absorption of liquid diet. Thus this species is more adapted to its liquid diet. The granivore has very short microvilli and a longest intestine allowing for thorough digestion and absorption and this suggests that the granivores are more adapted to their granivorous diet. The omnivore combines the qualities of the other two species.
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Introduction
Stereology, a quantitative anatomical method was developed to correlate structure and function of various organs1 by extrapolating three dimensional structural quantities from simple counts made on two dimensional slice images. The images may take various forms: physical, optical sections; magnetic resonance imaging (MRI) and computer tomographic scans (CT scans). Mayhew2 wrote that the primary mucosal surface is anisotropic i.e. it is essentially cylindrical and has a preferred longitudinal orientation however the overall villus surface is isotropic (shows no preferred direction or orientation in space). The morphology of the alimentary tracts of mammals reflects their diets. 3 Avian gastrointestinal anatomy may vary due to differences in the diet between species. Some birds have a varied diet while others, such as honeyeaters and granivores, have restricted diets. Nectarivores (Brown honeyeaters, Hummingbirds, Sunbirds) feed exclusively on one diet, nectar4,5 granivores (Finches) feed mainly on grass seeds.
Conclusion
It can be concluded from this study that histologically, the cells are the same in all three species. The appearance of the absorptive chief cells is consistent with that of the mammalian and avian absorptive chief cells. The small intestine is longest in the granivore while the other two species have similar length. The villous surface is largest in the granivore than that of the nectarivore. The omnivorous species has the largest amplification factor. The microvilli are taller and more densely packed in the omnivore than those of the granivore while they are shorter when compared to that of the nectarivore. The absorptive surface area in the nectarivore is attributable to the very tall and densely packed microvilli and is more adapted to the liquid diet.