Chapter 1: Colon
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Colon
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Late Effects of Radiation on Normal Tissues: Nonstochastic Effects
Edward L. Alpen, in Radiation Biophysics (Second Edition), 1998
Small and Large Intestine
The small and large intestine are anatomically similar except for the extensive proliferation of the microvilli of the small intestine. The large intestine is without villi and, except for the goblet cells, which are the mucus secreting cells of the large intestine, it is a smooth surface. There is an extensive submucosal connective tissue and smooth muscle development in both small and large intestine that is richly supplied with vascular elements.
The early effects of irradiation of the small intestine have been examined in great detail in Chapter 10. These acute effects will not be reexamined here, except to report that, as we might expect, the large intestine is not involved in the acute changes induced by irradiation because of the lack of the rapidly turning over crypt cells found in the small intestine.
Vascular and connective tissue changes are predominant in the late responses of all portions of the intestine. The earliest microscopically visible precursor of the late effects in the intestine is the alteration of endothelial cells in small vessels, accompanied by extensive thrombosis. All these changes in the microvasculature are precursors to the familiar late syndrome outlined for other parts of the gastrointestinal tract. These changes are fibrotic thickening of the submucosal tissues, vascular insufficiency, and generalized fibroatrophy. Again, stenosis or complete intestinal blockage can occur (Geraci et al., 1977).
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Adaptations of Gut Structure to Function in Herbivores*
Peter Langer, Robert L. Snipes, in Physiological Aspects of Digestion and Metabolism in Ruminants, 1991
b. The Cecum
This diverticulum of the large intestine is differentiated close to the aperture of the ileum into the large intestine (ostium ileocaecale). This opening is often characterized by folds and even a spiral muscular architecture, i.e., a sphincter, that prevents reflux of digesta from the cecum into the ileum. It is probable that this closing mechanism is enhanced by compressible venous cushions in the tela submucosa of the ileocecal junction (Ferraz de Carvalho et al. 1972). In some species, an open connection exists between cecum and colon as, e.g., in man. In other species there is a narrow aperture between these two parts of the large intestine. In the horse, an ostium caecocolicum (NAV 1973) is found (Fig. 4), and it has also been demonstrated in the guinea pig (Snipes 1982a).

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Figure 4. Colon and cecum of the horse. Permanent strictures: 1 through 3.
The cecum can be free of teniae, haustra, and semilunar folds (especially in the zoophagous mammals), but these structures may also often be present. In many rodents, e.g., the Muridae, the colon is free of teniae, but they are present in the cecum of these animals (Behmann 1973). Highly complex and differentiated structures can be found in the rodent family Spalacidae (mole-rats) (Snipes, in press) as well as in the cecum of lagomorphs, such as in the genus Ochotona (pika) (Yamasaki and Komatsu 1971) or in the rabbit cecum (Snipes 1978). The latter has also been investigated with respect to functional aspects (Fioramonti and Ruckebusch 1974).
A narrow part of the cecum (caecum angustius, Kostanecki 1926) represents a lymphatic organ in Primates and Lagomorpha (Kostanecki 1926; Hill and Rewell 1948; Arvy 1972; Scott 1980). According to Arvy (1972), the so-called appendix vermiformis is of minimal functional importance, but in the Lagomorpha, it is considered a functionally important lymphatic organ. In the latter mammalian order, a detailed light- and electron-microscopical account of the lymphatic material of the rabbit appendix vermiformis has been presented by Snipes (1978).
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Carcinoma of the ovary
Martin C. Powell MD, MRCOG, FRCS(Ed), ... E. Malcolm Symonds MD, FRCOG, in Magnetic Resonance Imaging in Obstetrics and Gynaecology, 1994
Primary and recurrent carcinoma of the rectum and colon
A primary lesion of the large bowel may present as a pelvic mass, and therefore may be mistaken for an ovarian lesion. MRI can help to distinguish between the two different cancers by the pattern of signal intensities as well as the tumour morphology and site. In Figure 8.6 is an example of a tumour involving the sigmoid colon; the tumour has partially obstructed the bowel, with a dilated segment of bowel proximal to the tumour. The tumour is well-demonstrated on a T1/T2 sequence and its morphological appearance is unlike that observed with ovarian carcinomas.

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Fig. 8.6. Coronal sagittal view. Primary carcinoma of the sigmoid colon. (a) (TR 1000 ms TE 40 ms); (b) (TR 1500 ms TE 120 ms); (c) (TR 1500 ms TI 100 ms).
In addition, large bowel tumours appear to have a different pattern of signal intensities. The signal from the tumour decreases relative to the surrounding tissues as the T2 weighting lengthens, contrasting the pattern with primary ovarian cancer. Recurrence of large bowel tumours involving the ovary can be interpreted by MRI to be a primary ovarian cancer. In Figure 8.7, a large-mass lesion is visible arising from the pelvis, possessing an intermediate signal on a T2-weighted image. The T1-weighted images, taken in the coronal plane of view, demonstrated the tumour to have both cystic and solid elements, compatible with the appearance of an ovarian primary.

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Fig. 8.7. Sagittal coronal view. Colonic metastasis in the ovary. (a) T1,/T2 (TR 1000 ms TE 40 ms); (b) T1, (TR 2000 ms TI 600 ms). (Cystic areas within tumour become more apparent on T1-weighted sequence.)
MRI is unable to distinguish a bowel recurrence in the ovary from a primary tumour. This is a problem shared with the histopathologist, as the histological appearances may also be very similar (Langley, 1973). This is particularly true with mucinous and endometrioid ovarian tumours, and cancers of the colon metastatic to the ovary.
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Comparative Aspects of Digestion in Nonruminant Herbivores
H. Hörnicke, in Invited Lectures, 1982
Microbes
The fermentation regions of the large intestine contain microbial populations, sometimes in concentrations comparable with those in the ruminant forestomachs. Some species also harbour protozoa. Their function and necessity is unknown. We are also ignorant about the reasons why in some species (e.g. guinea pig and horse) protozoa are regularly present in caecal contents but in others (e.g. rabbit) they are absent.
Caecum and colon act as continuous flow fermenters. When the flow is too low then microbial growth ceases because nutrients are lacking and metabolites accumulate. When the flow is large in relation to microbial propagation the microbes are flushed out and disappear from the fermentation chamber. By means of the separation mechanism in the proximal colon previously mentioned, some animals like rabbit and lemming can maintain a high passage rate without losing their microbes. They are largely returned by the colicocaecal reflux of small particles (Björnhag, 1981).
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THE DIGESTIVE SYSTEM (II)
S. BRADBURY M.A., D.Phil., in Hewer's Textbook of Histology for Medical Students (Ninth Edition), 1973
Vermiform Appendix (Fig. 20.26)
The appendix is a diverticulum of the large intestine and its walls have the same general structure except that the longitudinal muscle coat is evenly distributed round the circumference. It is characterized by a great increase in the lymphoid tissue, the nodules occupying a large part of both mucous and submucous coats: the muscularis mucosae is rather deficient. The numerous eosinophil cells often present in both mucous and submucous connective tissue may indicate some pathological change. The glands are much less closely packed than in the large intestine: they are most numerous in early life and tend to disappear in old age. The lumen of the appendix itself is often obliterated in later life.

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FIG. 20.26. A transverse section through the vermiform appendix. Note the presence of large prominent nodules of lymphoid tissue in the submucosal region.
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Surgical Pathology of Endocrine Tumours of the Alimentary Tract
Paul D. Lewis, in Surgical Endocrinology, 1993
Large bowel
Well-differentiated carcinoids of the large bowel occur principally in the rectum as polypoid masses. Carcinoids in more proximal colon tend to be large and invasive, penetrating the bowel wall and involving regional lymph nodes. Multiplicity is uncommon with rectal carcinoids. As with classic carcinoids, a characteristic yellow colour is seen in gross specimens after formalin fixation; however, reactivity for silver reagents is often negative. Staining for non-specific neuroendocrine markers is positive, and hormones demonstrable by immunocytochemistry include somatostatin, glucagon, substance P, YY peptide, gastrin, calcitonin and PP. Positive immunostaining for prostatic acid phosphatase, a potential source of diagnostic confusion, has also been described [26]. Rectal carcinoids do not cause the carcinoid syndrome.
Ganglioneuromas may occur in the rectum, whilst diffuse ganglioneuromatosis [27] may develop in association with von Recklinghausen's disease or multiple endocrine neoplasia type 2B. It may also occur on its own. In this condition there is an overgrowth of the neural plexus of the small bowel, the proliferation of nerve elements thickening the bowel and disorganizing the mucosa. This condition may affect small bowel as well as colon and rectum.
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Enteric Neuroimmune Interactions
Jackie D. Wood, in Immunophysiology of the Gut, 1993
C Vascular Component
The submucosal vasculature of both small and large intestine represents a third effector system (Fig. 1) that is controlled and regulated by the minibrain in the intestine. Activation of submucosal neurons dilates submucosal blood vessels in both small intestine and colon (Nield et al., 1990; Vanner and Surprenant, 1991). The available evidence suggests that the enteric neurotransmitters are vasoactive intestinal peptide, substance P, and acetylcholine, each of which can evoke vasodilation. In all likelihood, the coordinated secretory and motor behavior of the intestine involves a component of vasodilation. This would be expected to support the secretory component of the response program.
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Blood: catabolism of haemoglobin
Eric D. Wills, in Biochemical Basis of Medicine, 1985
27.5 Excretion of bile pigments and bacterial metabolism
During their passage through the small and the large intestine, the bile pigments are attacked by bacteria that can reduce or oxidize the carbon atoms of the molecule. The result of the series of complex metabolic changes is that the bilirubin molecule excreted is substantially changed from that passed into the intestine. The main excretory product is stercobilin, but doubtless complex mixtures of this and several other pigments occur in the faeces (Fig. 27.7).

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Fig. 27.7. Bacterial degradation of the bile pigments in the intestine.
Me, methyl; Et, ethyl; Pr, propionate; V, vinyl.
A proportion of the pigments is reabsorbed after bacterial degradation from the large intestine and excreted as urobilin or urobilinogen into the urine, and it is this which gives urine its characteristic colour.
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THE COURSE OF PROTEIN DIGESTION AND AMINO ACID ABSORPTION IN PIGS
S. Buraczewski, in Advances in Animal and Comparative Physiology, 1981
5 Large intestine
Although amino acids may be absorbed from the large intestine of new born pigs /Rerat, 1978/, the absorption which occurs in older animals appear to be small. In pigs of about 70 kg liveweight with isolated caecal pouches absorption of amino acids was observed after introduction of various solutions of amino acids /Olszewski and Buraczewski, 1978/. Intensive absorption of serine was observed at different concentrations of hydrolysed protein and also to some extent of threonine, lysine, tyrosine, arginine, histidine and aspartic acid. There was little or no absorption of other amino acids.
Measurements of the digestibility of amino acids in the ileum were summarized by Just et al. /1980/. He concluded that the dietary concentrations of crude fibre and crude protein had a significant effect on the apparent ileal digestibility of crude protein and that the digestibility differed considerably among the amino acids.
Part of the crude protein leaving the small intestine is further digested and absorbed in the large intestine. Therefore, the digestibility of crude protein at the end of gastrointestinal tract is usually higher than at the ileum. Just et al. /1980/ found that on average 8% of crude protein and amino acids in the diet, disappeared in the hind gut. Zebrowska /1978/ noted that 5 to 25% of ingested amino acids disappears in the hind gut, depending on the diet.
The amino acids which enter the hind gut may be transformed or deaminated thus making them unavailable to the animal /Zebrowska, 1978; Buraczewski, 1980/.
In experiments in which pigs receiving a protein-free diet were infused with casein or casein hydrolysate into the terminal ileum or caecum /Zebrowska, 1973b, Zebrowska, 1978/ it was evident that although proteins or amino acids were digested in the hind gut, most of the absorbed nitrogen was excreted in the urine. The evidence suggests that the activity of the microflora leads to deamination, and the eventual production of urea is excreted in the urine, or that amino acids are used for synthesis of microbial protein which may be voided in the feces /Mason, 1979/. This bacterial synthesis is demonstrated in some cases by a larger excretion of methionine in the feces than the amount entering the large intestine /Wünsche et al., 1979/.
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Arthropod and Helminth Parasites*
Gary L. Hofing, Alan L. Kraus, in The Biology of the Laboratory Rabbit (Second Edition), 1994
c LIFE CYCLE
Adult T. affinis are found in the cecum and large intestine. Ova are shed in feces, hatch, and develop to the infective larval stage while outside the host. Infection is through ingestion of infective larvae. The prepatent period is 10–11 days (Dixon, 1965b). Adults of both T. retortaeformis and T. ransomi are found in the small intestine. Life cycles are probably similar to that of T. affinis.
Adult G. strigosum are found in the stomach and produce eggs which pass with feces. Hatching occurs in 32–34 hr. Larvae develop to the infective third stage in 4–6 days. Infection is by ingestion. According to Flynn (1973), patency is reached in 12 days; however, both Cabaret (1981) and Nickel and Haupt (1986) report a prepatent period of at least 5 weeks. Adults live approximately 6 months (Cushnie, 1954).
Adult L. noviberiae and Nematodirus spp. are found in the small intestine. The life cycle is probably direct but details are unknown.
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