h. Large intestine 3

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1 (1) General features (a) Large intestine is last organ of digestive tract proper divided into 3 or 4 regions cecum appendix in humans colon rectum 1

2 b) No villi lumenal epithelium has microvilli This brush border is sometimes incomplete and difficult to detect. some absorption of nutrients, lower than small absorbs most of the remaining H 2 O h. Large intestine 2

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4 Large intestine has abundant intestinal glands simple tubular glands ; simple glandular epithelium ;mucus-producing cells like goblet cells but wider at base glands are densely-packed lamina propria between glands is composed of unusually cellular, dense C.T. 4

5 (d) Muscularis mucosae is thicker than in small intestine. (e) Lymphatic nodules are common in some areas usually form first in (the lamina propria) the mucosa and spread into the submucosa. 5

6 (f) Submucosa not significantly different from that of small intestine. (g) Muscularis externa ; incomplete outer, longitudinal layer inner, circular layer ; thick, subdivided into distinct masses. 6

7 (2) Special features (a) Cecum: first region of the large intestine 1. cecum is a side-pouch at the beginning of the large intestine cecum harbors commensal bacteria; help digestion of fibrous plant foods by producing cellulase an enzyme that almost no animals themselves can produce; Cellulose digestion in humans is insignificant 7

8 fibrous plant materials pass through the small intestine relatively undigested and lodge in the cecum the cellulose is broken down, exposing the cells to the other digestive enzymes produced by the microorganisms some of the digestion products are taken in by the microorganisms and some into the wall of the cecum Vita K; glucose animals with a higher proportion of fibrous plant materials in their diet tend to have a larger cecum. 8

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10 2. In humans most of the cecum is vestigial, vestigial portion is considered separate region of the large intestine, appendix. The remainder of the cecum in humans and the portion of the cecum in our slide of rhesus monkey cecum do not significantly differ from colon in structure. 10

11 (b) Appendix 1. The appendix is not found in most mammals found in humans a vestigial structure; lost much of the typical organization of tissues and layers characteristic of the large intestine primary function; immune defense evolutionarily become a part of the immunesystem. h. Large intestine 11

12 2. Mucosa of appendix a. Lumenal epithelium: simple columnar epithelium and scattered goblet cells; brush border absent. b. Intestinal glands: relatively sparse compared to other regions; cells with mucus vacuoles. c. Lamina propria: much more cellular than lamina propria of large or small intestine greater abundance of immunedefense cells d. Muscularis mucosae: unusually thin; sometimes discontinuous sometimes not close to bottoms of glands often not distinct except on high power. 12

13 3. Submucosa of appendix a. Mostly composed of lymphoid tissue (1) Dense lymphoid tissue. (2) lymphatic nodules are continuous with each other. b. More typical components of gut submucosa occupy only a minority of the layer's volume. c. The interface between submucosa and muscularis externa is not discreet. (1) bundles of smooth muscle fibers of the inner, circular layer of the muscularis externa are intermingled with dense connective tissue of the submucosa. h. Large intestine 13

14 4. Muscularis externa a. also, there may be some loss of the usual sharply defined layering pattern of the muscularis externa (inner, circular; outer, longitudinal). b. Smooth muscle tissue may be less compact than usual. c. At outer edge of muscularis externa there may be an intermingling of smooth muscle tissue with the dense connective tissue of the tunica adventitia. 14

15 5. Tunica adventitia a. no special features. 6. This loss of features typical of large intestine or of gut wall in general is due to the dominant immune-defense function. 15

16 (c) Colon 1. The colon is by far the longest region of the large intestine; much shorter than the small intestine ascending, transverse, and descending portions at the end of the descending portion is the junction with the last region of the large intestine, the rectum. h. Large intestine 16

17 2. The colon s main functions are: a. Further reabsorption of water from the lumen contents (1) Degree of further water reabsorption determines the consistency of the feces < normal diarrhea > normal constipation 17

18 (2) Several factors determine the colon's degree of water reabsorption speed with which the lumen contents are moved along the colon osmotic pressure of the solution in the lumen ability of the mucosa to absorb solutes of the solution in the lumen, level of infection of the lumenal epithelium water reabsorption can be severely impaired to the extent that severe dehydration occurs dehydration can be much more life-threatening than the infection itself. 18

19 b. Putrefaction of wastes Commensal bacteria present here also to break down some of the materials as yet not digested. c. Compaction of remaining undigested materials (feces)--in last portions of region. 3. Histological features of colon: none in particular; see description of general features of large intestine, 19

20 d) Rectum and recto-anal junction 1. The rectum is a short final region of the large intestine performs a bit more H 2 0 reabsorption and a bit more compaction of feces wall has receptors that when stretched (usually due to presence of fecal mass) sends impulse signals to the brain that result in the sensation of needing to defecate 20

21 2. In general the histological structure of the rectum is very similar to that of the colon mucosal submucosal folds are generally lacking, however. 3. Near the end of the rectum, where the rectum is continuous with the anal canal, the muscularis mucosae ends the inner, circular layer of the muscularis externa ends in a thickened mass; the internal anal sphincter. 4. Also in that vicinity the outer, longitudinal layer of the muscularis externa ends. h. Large intestine 21

22 5. Lumenal epithelium changes abruptly to the stratified squamous epithelium of the anal canal; intestinal glands end. 6. distant from the lumen and closer to the end of the anal canal a mass of skeletal muscle tissue external anal sphincter after infancy, is under voluntary control. 7. No distinct outer surface to the tunica adventitia. h. Large intestine 22

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