ANATOMICAL AND HISTOCHEMICAL STUDIES OF THE DIGESTIVE SYSTEM OF THE AFRICAN GIANT RAT

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Department of Medicine

ABSTRACT
Forty African giant rats (AGRs), (Cricetomys gambianus) were used for morphometric, morphologic, histologic and histochemical studies. The various segments of the gastrointestinal tract (GIT) were weighted, measured and photographs taken. Transverse sections of different segments of the GIT were stained with Haematoxylin and Eosin for normal histological studies. For histochemical studies, transverse sections were stained with Alcian Blue, Periodic Acid Schiff and Alcian Blue-Periodic Acid Schiff to determine their nature of secretions. The large intestine was found to have the highest mean weight (19.98 ± 0.39 g) followed by the stomach (19.47 ± 0.26 g) and then the small intestine (17.19 ± 0.30 g), respectively. Of all the digestive glands, the liver was found to have the highest weight (21.29 ± 0.43 g). The small intestine was observed to be the longest (109.17 ± 28.68 cm) followed by the large intestine (75.57 ± 1.78 cm).Out of all the intestinal segments (small and large), the jejunum was observed to be the longest (72.26 ± 2.39 cm) followed by the colon (57.30 ± 1.58 cm).Grossly, the AGR had two upper and two lower incisor teeth and three upper and three lower molar teeth. The parotid and mandibular salivary glands were found to be non-lobulated and lobulated, respectively. The esophagus was found to lie dorsal to the trachea throughout its length. The stomach was made up of glandular and non-glandular parts with a constriction separating the two parts. Esophageal epithelium was non-keratinized and devoid of lamina muscularis and glands. The stomach was observed to be divided into two parts (non-glandular and glandular) with an isthmus separating them. The non-glandular part had papillae which were covered with stratified squamous epithelium and resembled the rumen of ruminants. The isthmus was made up of keratinized stratified squamous epithelium with no glands. The glandular stomach had mucous secreting epithelium with gastric pits and glands at the mucosa. The small intestine was made up of the 8 duodenum, jejunum and ileum. The duodenum had villi that were covered by simple columnar epithelial cells. The duodenal (Brunner’s) glands were seen on the upper part of the submucosa. The jejunum had intestinal glands (crypts of Lieberkuhn), while the ileum had villi, aggregate of lymphatic nodules (Payer’s patches) and intestinal glands (crypts of Lieberkuhn).The large intestine was observed to be made up of the caecum, colon and rectum. They all had similar basic structures and none had villi. The mucosal surfaces were smooth with goblet cells. Histochemical studies of the GIT showed a positive response to AB, PAS, and AB-PAS with the exception of the esophagus that was negative to all the reactions. The jejunum and ileum were AB positive and PAS negative. The parotid salivary glands consisted entirely of serous acini as they were PAS positive while the mandibular salivary glands consisted of both serous and mucous acini as they were AB and PAS positive. The gall bladder had the shape of a balloon and was found on the visceral surface of the liver. The liver was observed to be divided into four lobes. The gross and histological features of the stomach of the AGR which resembled the abomasum and rumen of ruminants may be responsible for its feeding habit (omnivorous).The thickening of the tunica muscularis of the colon and rectum has been correlated with the temporary storage and expulsion of fecal materials. Although the esophagus had no mucous secreting glands, the mucous produced by the salivary glands and the enzymes of surface origin may protect the mucosal surface and also provides digestive functions for the esophagus. The production of acidic and neutral mucins in the stomach and duodenum is to maintain the p.H of the chyme. The presence of large number of mucous secreting cells provides a mucous layer around the fecal pellets facilitating its release and protecting the epithelium.