A stereological comparison of villous and microvillous surfaces in small intestines of frugivorous and entomophagous bats: species, inter-individual and craniocaudal differences
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The extents of functional surfaces (villi, microvilli) have been estimated at different longitudinal sites, and in the entire small intestine, for three species of bats belonging to two feeding groups: insect- and fruit-eaters. In all species, surface areas and other structural quantities tended to be greatest at more cranial sites and to decline caudally. The entomophagous bat (Miniopterus inflatus) had a mean body mass (coefficient of variation) of 8.9 g (5 %) and a mean intestinal length of 20 cm (6 %). The surface area of the basic intestinal tube (primary mucosa) was 9.1cm2 (10%) but this was amplified to 48cm2 (13 %) by villi and to 0.13m2 (20 %) by microvilli. The total number of microvilli per intestine was 4´1011 (20 %). The average microvillus had a diameter of 89 nm (10 %), a length of 1.1 mm (22%) and a membrane surface area of 0.32 mm2 (31 %). In two species of fruit bats (Epomophorus wahlbergi and Lisonycteris angolensis), body masses were greater and intestines longer, the values being 76.0 g (18 %) and 76.9 g (4 %), and 73 cm (16 %) and 72 cm (7 %), respectively. Surface areas were also greater, amounting to 76cm2 (26 %) and 45cm2 (8 %) for the primary mucosa, 547cm2 (29 %) and 314cm2 (16 %) for villi and 2.7m2 (23 %) and 1.5m2 (18 %) for microvilli. An increase in the number of microvilli, 33´1011 (19 %) and 15´1011 (24 %) per intestine, contributed to the more extensive surface area but there were concomitant changes in the dimensions of microvilli. Mean diameters were 94 nm (8 %) and 111 nm (4 %), and mean lengths were 2.8 mm (12 %) and 2.9 mm (10 %), respectively. Thus, an increase in the surface area of the average microvillus to 0.83 mm2 (12 %) and 1.02 mm2 (11%) also contributed to the greater total surface area of microvilli. The lifestyle-related differences in total microvillous surface areas persisted when structural quantities were normalised for the differences in body masses. The values for total microvillous surface area were 148cm2 g-1 (20 %) in the entomophagous bat, 355cm2 g-1 (20 %) in E. wahlbergi and 192cm2 g-1 (17 %) in L. angolensis. This was true despite the fact that the insecteater possessed a greater length of intestine per unit of body mass: 22mmg-1 (8 %) versus 9–10mmg-1 (9–10%) for the fruit-eaters.