Comparative morphological and stereological studies on the gastrointestinal tract of three species of Australian passerines

The intestine was lined by enterocytes with microvilli which were longest in the nectarivore, shortest in the granivore and intermediate in the omnivore. The gastrointestinal anatomy was analysed quantitatively using stereology. Volumes of the stomach and the proportions of the various layers of the stomach, discernible at light microscopic level, were similar in the three groups. However, the omnivorous silvereyes were found to have a significantly larger volume density of the muscular layer of the proventriculus than the other species. The granivorous zebra finches had a significantly larger surface area of the primary mucosal tube of the proventriculus than the other species. The granivorous zebra finches had a significantly larger volume of the muscular layer of the ventriculus as well as a significantly thicker gastric cuticle. Intestinal volumes and all other variables were similar in the nectarivore and the omnivore. Most variables were significantly larger in the granivorous zebra finch than in the other species with the exception of the villous amplification factor in the duodenum. This was significantly larger in the omnivore than the other two species. The larger values in the granivorous zebra finch was due to the significantly longer segments of the intestinal tract than in the other species. Ultrastructurally, the nectarivorous brown honeyeaters had significantly taller microvilli than the other two species and a significantly larger packing density than the granivorous zebra finches. There was no statistical difference in the overall surface area due to microvilli in all segments of the intestine in the three different dietary groups. The villi and microvilli jointly enlarge the basic mucosal tube more than 75-fold in the nectarivores, 83-fold in the granivores and 106-fold in the omnivores. The avian gastrointestinal tract was characterised using lectin cytochemistry Concanavalia agglutinin (Con A) and Arachis hypogea agglutinin. Peanut agglutinin (PNA). The apical cell boundaries of surface epithelial cells of both the proventriculus and ventriculus, the duct and glandular neck cells of the proventriculus, the intestinal brush borders, the surface epithelial cells of the sheep abomasum and the chief cells of the sheep abomasum were stained by Con A and PNA. The intracytoplasmic granules of the duct and glandular neck cells of the proventriculus, those of the chief or principal cells of the ventriculus, those of the goblet cells of the ileum, colon and cloaca and those of the chief and surface epithelial cells of the sheep abomasum were stained by PNA. Gastro-intestinal endocrine cells and the parietal cells of both the avian gut and the sheep abomasum were not stained by either lectins. The cytochemical results indicated that those cells stained by the two lectins have both a-D-mannose or a-D-glucose and P-galactose- (l,3)-N-acetylgalactosamine residues in their membranes. Those stained by either of the two lectins had either of the two sugars as components of their cell membrane and their granules. Goblet cells were heterologous. The duct and glandular neck cells of the proventriculus, the chief cells of the ventriculus, the goblet cells of the ileum, colon, cloaca, and the chief and surface epithelial cells of the sheep abomasum had similar intracytoplasmic granules. The results indicate that the nectarivores and the granivores are adapted to their various diets. The adaptive structures of the nectarivores to their liquid diet are the smaller proventriculi, with more branching and anastomosing, spiral, mucosal folds. These provide a large surface area for the accommodation of its more liquid diet. The small, sac-like, less muscular ventriculus lined by a jelly-like gastric cuticle also reflects that the liquid diet does not need any trituration before absorption. The shorter intestines, with the tallest and most densely packed microvilli provided a maximal absorptive surface for rapid absorption of sugars. The adaptive structures of the granivores to their solid granivorous diet were the thin and longer proventriculi, with reduced and scattered mucosal folds, which only acidify the digesta. The much heavier, highly muscular ventriculus, lined by a tough, abrasive resistant, gastric cuticle reflects the degree of trituration needed by the granivorous diet. The much longer intestines with shorter, wider and less densely packed microvilli, enhance digestion and absorption of nutrients provided by the solid granivorous diet. The adaptive features of the omnivores that allow them to accommodate any diet are the intermediate proventriculi which are not as long as those of the granivores but are longer than those of the nectarivores. The spiral mucosal folds are reduced in size and scattered in the proventriculi. The ventriculi of this species are moderately muscled, with a semi solid gastric cuticle. The intestines of this species are shorter but have longer and more densely packed microvilli than those of the granivores. These accord with the silvereye’s omnivorous diet. Thus the height and packing density of the microvilli seen in the intestine of the birds examined reflects, and is adapted to, their diet. The granivore and the nectarivore are specialist of their different diet. If the environment or the habitat of the three species of birds is altered, the omnivorous species may adapt successfully to the altered environment and the available diet. However, the other two species may not adapt successfully