Studies on the Fine Structure of Caeca in Domestic Geese

: The objective of this study was to investigate the villus distribution in the caeca of domestic geese based on the fine structure. The caeca of White Roman geese, 14-week old, were sampled and specimens were detected under photomicroscope and scanning electron microscope (SEM). The results indicated that the villi existed at the proximal caecum. The morphologies of these villi showed finger-like, peak-like or tongue-like shapes. The heights of the villi decreased far from the proximal caecum. No villi were found in the middle and distal caecum. It was obvious that the proximal segment was the main portion for absorbing food nutrients in the caeca. The caecal content particles were small and possessed a viscid character. The large particles filtered out at the proximal caecum just like a mesh. The surface of the middle caecum exhibited parallel ridges with no villi. There were band plicae circular shapes found in the middle caecum under scanning electron microsopy. (Asian-Aust. J. Anim. Sci. 2002. Vol 15, No. 7 : 1018-1021)


INTRODUCTION
Most birds possess caeca. However, in a few birds the caeca is small. Bee-eater birds have only a vestigial caeca with no function and caeca is absent in the swift (Mcllelland, 1989). Geese are herbivious and have a pair of welldeveloped caeca which were 54-60 cm in length (Chen et al., 1992). The caecum is the main digestive segment for degrading cellulose in the alimentary canal of geese (Clemens et al., 1975); Yang and Lin (1975) showed that cellulose digestibility decreased 21% in caecal ligated geese. It was suggested that the caeca of geese possessed the function for digesting cellulose. Caeca with a fine structure has been reported in some species of poultry, e.g. quail (Fenna and Boag, 1974), turkeys (Sudo and Duke, 1980) and chickens (Ferrer et al., 1991). However, the fine structure of caeca in geese has not been investigated. This study, therefore, was conducted to explore the villus distribution in the caeca using the photomicroscope and scanning electron microscope (SEM) in order to establish basal data on the fine structure of the caecum in geese.

Experimental materials
White Roman geese were fed a commercial diet. Water and feed were supplied ad libutum. At 14 wks of age, geese were slaughtered by neck vein bleeding. At necropsy, the caeca were immediately removed and divided into three parts, proximal, middle, and distal segments. The left caecum was taken for the histological detection and the right caecum for the SEM examination (figure 1).

Preparation of samples for SEM
Caecal samples were prepared for the SEM according to the method of Paulini et al. (1987) that was a modified method of Moore et al. (1988). All caecal samples were first rinsed with physiological salt water (0.9%), then fixed in 2.5% glutaric dialdehyde in phospate buffer (pH 7.3) for 2 h (Tsai and Wang, 1993). The specimens were postfixed in 1% Osmium tetroide 0.1 M phosphate buffer solution (pH 7.3) for 2 h. The samples were rinsed in the phosphate buffered solution 1 time and distilled water 3 times, for a duration of 10 min each. Thereafter, the samples were gradually dehydrated using increasing alcohol concentrations (30%, 50%, 70%, 80%, 90%, and 95%) for 15 min each and the absolute alcohol for the final 3 rinses. The samples were dried at the critical point dryer (CPD), mounted on aluminum stubs with double adhensive tape and coated with gold film about 20 nm thickness, and observed with SEM (Topcon 150S) at 15 kv.

Preparation of samples for histological detection
Caecal samples were placed into 10% buffered neutral formaldehyde solution pH 7.2 and shaken 24 h for fixation according to the method of Chiou et al. (1996). Then, all samples were dehydrated gradually by increasing concentrations of ethyl alcohol 60-100% for 1-1.5 h each. Those specimens were embedded in paraffin and then sliced into 6 µm strips. The slices were stained with hematoxylin and eosin, and mounted with mounting media. Thereafter, the specimens were placed into a photomicroscope (ZEISS AXIOSKOP) for histological observation. The histological detection results are showed in figure 3. In the proximal caecum, villi were found and had globet cells and crypts, and the muscle layer was thicker (figure 3-A, 3-B). No villi in the middle zone caecum was found and the muscle layer was thin with saw-shaped or parallel ridged tract lumen (figure 3-C, 3-D). The distal caecal muscle layer was thinner than that in the proximal caecum and its surface was saw-shaped and no parallel ridges or villi were found. (figure 3-E, 3-F).

DISCUSSION
The goose proximal caecum in this study showed that the villi were like that of chicken (Ferrer et al., 1991). These findings were different from Chiou et al. (1996) and Yu et al. (1998) who did not find villi in the caeca of geese with SEM. The different results may be due to sampling from a different position. This report took the sample from proximal middle and distal caecum in this study. However, Yu et al. (1998) took the samples from one position that might be middle or distal caecum. This study indicated that the height of the villi decreased as position moved far from the proximal caecum. No villi were found in middle and distal caecum. These results concur with the findings in turkey and in Japanese quail reported by Sudo and Duke (1980) and Fenna and Boag (1974), who indicated that turkeys possessed well developed villi in the proximal caeca and the Japanes quail showed elongated villi filling the lumen in the proximal caeca. Hill (1971a,b) reported that the proximal part of the caeca was the main digestive portion for absorbing nutrients in domestic fowl. Sudo and Duke (1980) also pointed out that the mucosa of the proximal caecum in turkeys possessed well developed villi and this caecum portion would appear to have a greater potential for nutrient absorption than the distal segment. The present study also suggests a possibility that the proximal caecum might be absorptive function in geese.
The caecal contents in these geese had a smooth, viscid character which is in agreement with the findings of Fenna and Boag (1974)   birds were smooth and viscid because proximal caecum was just like a mesh that filtered out the large particles. Bjornhang and Seperber (1977) also found dry matter particle sizes larger than 0.2 mm were 3% in caeca, 38-45% in the small intestine, and 50% in the colon in turkey. In this study, the observed smooth character of caecal contents was associated with, or perhaps responsible for, the reduction in the particle size in the caecal contents. Globet cells produced mucus (Young and Heath, 2000) and were affected by medicine, e.g. Cyclosporine that increased the mucin volume (Phillips et al., 2000). The caecal villi have globet cells that may produce mucus, there by affecting the viscid content character in the caeca.
The histological specimens from the proximal caecum of geese exhibited the same character as that discussed in Fenna and Boag (1974) who stated that the Japanese quail caecum possessed villi and the middle zone of the caecal structure showing parallel ridges. Comparing histological specimen photographs (figure 3-C) with the findings in SEM (figure 2-E), the parallel ridges are plicae circulares that end at the distal caecum. These parallel ridges increase the caecal wall surface area (Fenna and Boag, 1974;Ferrer et al., 1991). The histological specimens from the middle geese caecum showed parallel ridges like the Japanese quail (Ferna and Boag, 1974), but no villi in this segment were found in SEM (Yu et al., 1998). Long band plicae circulares drew digesta into the caecum and delayed the digesta rate (Stevens and Hume, 1995). Plicae circulares may have a blocking function when the caecal contents are emptied. The proximal end was narrow. The caecal contents of geese emptied once every 4 h (Yang and Lin, 1975), with not all contents emptied once a day, so plicae circulares were correlated to content digestion. Whether this phenomenon relates to nutrient absorption or another physiological function is unknown. This study suggest that further research is required to resolve this problem.
Histological specimens from the caecum showed tract lumen in the middle and distal caecum that had a saw-like shape. SEM examination showed a wire net structure.

ACKNOWLEGEMENTS
This study was funded by National Science Council NSC 86-2313-B005-023 in Taiwan. We give our thanks and appreciation to instructor Cheng-Chung Lin, and Ching-Lin Shyu, Miss Shu-Hui Chang, and Mr. Ching-Chang Cheng of the Department of Veterinary Medicine for their help with sample preparation for histological detection and photo microscopic examination.