Surface enlargement in the large intestine of the koala (Phascolarctos cinereus): morphometric parameters
Snipes, RL, Snipes, H & Carrick, FN 1993, Australian Journal of Zoology, vol. 41, pp. 393-397.
Multiple parameters of the morphology of the koala’s large intestine were measured. A computer-aided planimeter was used to find the basal surface area of the caecum and colon, and the microscopically visible folds/plicae were measured to determine the surface enlargement of the mucosa. The basal surface areas were 890.6 cm2 for the caecum (32.0% of the entire intestine), 1434.8 cm2 for the colon (51.5%) and the entire intestine was 2785.0 cm2. An average of 8 and 12-14 longitudinal folds were found striping the caecum and proximal colon, respectively, which increased the surface area significantly. The surface areas for the caecum, colon and entire intestine, respectively, when including the extra surface from the folds were 3659.8 cm2 (45.9%), 3854.0 cm2 (48.3%) and 7973.4 cm2. Respective total surface areas were determined by multiplying the basal surface area by the surface enlargement factor. These were 10,979.5 cm2 (44.9%), 9808.5 cm2 (40.1%) and 24,464.7 cm2 for the caecum, colon and entire intestine, respectively.
The caecum was observed to be folded in on itself, with one end behind the spleen and attached by its own section of the mesentery. The origin of the section of the mesentery was at the dorsal wall retroperitoneal to the stomach, with other duplicatures to the small intestine and colon. The caecal artery was observed to approach the mid-portion of the caecum, then dividing into three equal parts, one running towards the ileocaecal junction, the second onto the wall of the mid-caecum and the last towards the blind-end of the caecum.
Compared to other species of the same size, the size of the koala’s caecum is very large, supporting the hypothesis that koalas have the most highly developed caecum of studied mammals. Interestingly, the calculated surface enlargement factor of 8 (based on limited koala data and median values from eutherian mammals) is the same as for humans in other studies. The ratio of the measured surface area of large intestine to small intestine in koalas is 5, which is exceptionally high and indicates the extreme importance of the hindgut in the koala’s nutritional strategy. This is three times the ratio of the rabbit, the highest ratio previously calculated. Volume ratios indicate fermentative function, with koalas having a ratio of 130, compared to rabbits at 71.2 in other studies. The ratio of measured surface area to volume gives the surface area per unit volume. The koala’s caecum has a ratio of 22.5, compared to the rabbit’s 2.2, voles at 15.5 and dwarf hamsters at 9.8. The previous studies had suggested a high ratio indicated a metabolically active species, which is not the case for the koala.
The study represents the first time empirical values for size of the large intestine were collected for the koala. An understanding of the anatomy of the koala intestine improves our knowledge of its metabolism and diet.
Summarised by Laura Wait
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