Gut 1994; supplement 1: S1-S4 Si

Maintenance of normal intestinal mucosa:

function, structure, and adaptation Gut: first published as 10.1136/gut.35.1_Suppl.S1 on 1 January 1994. Downloaded from

J A Jankowski, R A Goodlad, N A Wright

Abstract Functions Diet is of fundamental importance in The two main functions of the gastrointestinal the healthy functioning of alimentary mucosa are concerned with digestion and epithelium, or during the disease pro- absorption of dietary nutrients and as a cess. Specifically, the incidence of 'non- defence against many noxious dietary sub- hereditary' gastrointestinal disease varies stances and bacteria. widely throughout the world, partly The digestive mechanisms of the gut reside because of different cultural and dietary at two levels. Luminal digestive enzymes habits. The aim ofthis review is to outline secreted by the pancreas reduce macro- the mechanisms that modulate normal molecular starch and protein to oligomeric mucosal development and growth in the forms (di and oligosaccharides, dipeptides small and large intestine. In addition a to hexapeptides, free amino acids, etc), brief mention will be made of morpho- which can undergo subsequent hydrolysis logical changes in certain pathological to monomers by a considerable number conditions. of hydrolases of the enterocyte brush (Gut 1994; supplement 1: S1-S4) border. In addition, Na+/K+ and ATPase pumps situated in the basolateral mem- brane of the enterocyte regulate salt and There is much evidence linking diet and main- water balance in the enterocyte and tenance of intestinal mucosal integrity. the overall process of salt and water absorp- Epidemiological data have, for example, tion. shown an association between Baker's yeast Mucosal defences consist of non-specific ingestion and incidence of Crohn's disease in barrier mechanisms and specific immuno- the small bowel and the inverse relation of logical responses. The first include production fibre consumption and cancer in the large of copious amounts of viscous mucin

bowel.I More direct evidence of cause and (3 1/day), there is also production of http://gut.bmj.com/ effect has been forthcoming from the study of an alkaline pH environment beneath the the cell biology and morphological character- mucin layer by active mechanisms, saliva, istics of normal and abnormal specimens in which acts as a buffer and lubricant, main- vivo.2 tenance of tight cell-cell junctions (im- Under normal circumstances new cells permeable to bacteria and large molecules), of the intestine spend the first days of their and rapid mucosal turnover quickens adaptive

lives in the cell division. Subsequently, cells responses and prevents bacterial trans- on September 26, 2021 by guest. Protected copyright. become mature and acquire various specialised location. functions associated with digestion and protec- Granulocytes, macrophages, and Paneth tion. Although, a certain proportion of cells cells act as intramucosal phagosomes, which will undergo programmed cell death (apopto- non-selectively, recycle medium sized intra- sis) or stochastic - that is, random - cell death cellular particles. as a result ofnecrosis or cell damage, most cells More specific immunological responses are lost by exfoliation into the lumen. The include the expression of IgA immuno- combination of these processes of maturation globulins on the apical luminal surfaces and and exfoliation result in the generation of an sensitised lymphocytes on Peyer's patches or organised morphological structure.2 The in T cell populations in intramucosal epi- Histopathology Unit, mucosa powers thelium associated tissue Imperial Cancer alimentary has considerable of (mucosa lymphoid Research Fund, Royal adaptation some of which are physiological that is 25% of mucosal cell mass). This last College of Surgeons and can give rise to specific morphological response can recognise very small epitopes J A Jankowski appearances. Some means of adaptation are (3 aminoacids). R A Goodlad inflammation (localisation of excess neutro- N A Wright phils and lymphocytes), restitution (repair of Gastrointestinal Unit, superficial breaches ofmucosal integrity by cell Structure: topographical organisation of University Departnent migration), hypertrophy (increase in cell intestinal epithelia - current theory of Medicine, Guy's number or size) and atrophy. (Table I) Hospital, London In abnormal states the mucosa must also be Recent theory suggests that the single most J A Jankowski able to repair mucosal integrity, which include important intestinal cells are the stem cells. Correspondence to: the reversible ulcer associated cell lineage, These may consist of a hierarchy of functional Dr J Jankowski, Imperial constitutive of cells, which have a decreasing capacity to Cancer Research Fund metaplasia (irreversible change Laboratories, Histopathology phenotype), dysplasia (abnormal differentiated produce a large family of descendants within Unit, Royal Postgraduate phenotype), and neoplasia (abnormal cell num- their own natural environment, throughout Medical School, London W12 OHS. ber and abnormal differentiated phenotype).3 the lifetime of the adult. S2 tJankowski, Goodlad, Wright

TABLE I Topographical organisation ofthe human gastrointestinal tract

Oesophageal epithelium Gastric mucosa Intestinal mucosa Colonic mucosa Epithelium Squamous Mucus secreting Mucus secreting Mucus secreting Proliferative com- Basal layer Foveola of gland Crypt base Crypt base Gut: first published as 10.1136/gut.35.1_Suppl.S1 on 1 January 1994. Downloaded from partment No of stem cells 25% of basal cells 10 cells in gland 4 cells in base 1 cell in base Differentiated Membrane lectins for Neutral mucin Brush border enzymes in Muc-1 mucin phenotypes protection villi Function Protection Acidification digestion, Digestion and absorption Water absorption, protection defence, fermentation

TABLE II Physiologically active growth regulatory specialised intestinal cells has yet to be fully agents elucidated and endocrine cells, which secrete Intrinsic factors many growth regulatory molecules and the gastrointestinal peptides goblet cells, which secrete novel peptides growth factors oncogens tumour suppressers lactated mitogens (including a large new family of molecules Luminal factors called trefoil peptides) are being intensively alimentary secretions, for example acid, bile enzymes, and exocrine hormones studied.6 diet (salts, sugars, and saturated fats) fibre, intestinal flora ingestion of carcinogens mesenchyme neurogenic influences LARGE INTESTINE Humoral factors systemic hormones, prostaglandins The base of the wide tubular crypts in the colonic mucosa may contain a single functional stem cell, with few potential stem cells1 as SMALL INTESTINE shown by radiobiological split dose experi- It has been estimated that there are about 4-40 ments.1 As in the small intestine crypt duplica- stem cells in the base of the crypt of tion may occur by bifurcations, from the apex Lieberkuhn in the adult (and possibly only 1 of the gland to the superficial compartments stem cell in early life) believed to occupy (crypt fission). Eccentric bifurcations (glan- positions 4-5 in the base of the intestinal dular buds) along the length of a gland may crypt.4 5 Furthermore, the population of stem occur also in normal and diseased mucosa. It is cells in a particular gland may be tightly con- possible that these represent aborted crypt trolled and if their number exceeds a 'pre- development resulting from the death of the determined quota for that epithelia' glands stem cell generating the bud. may undergo stochastic crypt fission (new gland formation). Radiobiological experiments on chimaeric animals suggest that high doses Growth regulation/adaptation of radiation result in clonogenic regeneration The molecules that act to maintain mucosal of crypts and in some cases adjacent crypts, architecture and function, originate from a http://gut.bmj.com/ from the basal surviving compartment. In variety of sites and hence have different routes addition after irradiation the surviving pro- of action. In this context molecules may be liferative cells phagocytose the adjacent dead subdivided according to their site of origin and cells. Cells containing 'phagosomes' (intra- transport; intrinsic to the mucosa (growth cellular organelles containing phagocytic factors), humoral supply (hormones), and vacuoles) are found within 36 hours of injury luminal origin (dietary nutrients and carcino- in all cell lineages, showing the common origin gens)7 8 (Table II). on September 26, 2021 by guest. Protected copyright. of all the intestinal cell types. Regulatory molecules intrinsic to the The daughter cells of the clonogenic stem mucosa include gastrointestinal peptides and cells, with the exception of the Paneth cells, gastrin. Of the many gastrointestinal peptides migrate up the crypt and laterally hence onto some have specific trophic actions on the the villus. intestinal mucosa. Gastrin stimulates gastric The crypt cell production rate is roughly con- acid secretion and possibly also intestinal stant throughout the length of the intestine growth because the DNA, RNA, and protein despite an increased crypt depth/villus height content of the epithelium increases. Gastrin ratio in proximal small intestine because more has been reported to increase proliferation crypts give rise to one villus in the proximal back to normal values in the 'atrophied' compared with the distal small intestine proximal colon and gastric body (but not (duodenal crypt villus ratio 7:1, ileal crypt antral) mucosa. Enteroglucagon may have villus ratio 4:1).2 4 similar actions in the small intestine although The differentiated small intestinal cells are the evidence is, in part, circumstantial.9 mainly confined to the top third of the super- Systemic hormones (for example, growth ficial compartment (villi). These cells secrete hormone and thyroxine) have been reported many brush border enzymes, which can be to increase small intestinal weight and may further enhanced by epidermal growth factor achieve this by modest mitogenic actions. in vivo. In addition transforming growth factor Prostaglandins are endogenous molecules a( expression is maximal in the superficial (long lived analogues have now been pharma- compartment. In this regard, reports suggest ceutically produced) and these molecules may that these growth factors act by way of be of benefit in the prophylaxis against non- their common receptor, epidermal growth steroidal anti-inflamatory drug induced intesti- factor receptor, either on the basolateral or nal ulcers. In this regard large differences in apical surfaces of intestinal cells. The role of proliferation are seen between human biopsy Maintenance ofnormal intestinal mucosa: function, structure, and adaptation S3

specimens taken from the edge of an ulcer and TABLE III Growth factor and trefoil peptide expression from an unaffected region ofthe mucosa. Non- in the alimentary tract steriodal anti-inflammatory drugs seem to Peptides

inhibit regeneration by abolishing this pro- Gut: first published as 10.1136/gut.35.1_Suppl.S1 on 1 January 1994. Downloaded from liferative difference. Prostaglandins may Epithelial mucosa EGF TGFa pS2 hSP protect the mucosa from this inhibitory effect Oesophagus normal oesophagus when given prophylactically. 1012 Available Barrett's metaplasia + ++/+++ ++ -/+ data suggest, however, that prostaglandins act adenocarcinomas + + ++ ++ ++ Stomach by complex mechanisms and may entail a normal mucosa -/+ ++ + + favourable modulation of cell proliferation, metaplasia + ++/+++ ++ ++ adenocarcinomas + + +++ ++/+++ ±++ membrane permeability, and cell migration.'3 Small intestine Proliferation effects induced by prosta- normal ++ - + UACL +++ +++ +++ +++ glandins are also seen in the small bowel, but Colon are far less noticeable in the colon. normal + + -/+ -/+ hyperplastic polyps +/+ + _/+ +++ _ Alimentary secretions such as gastric acid and adenocarcinomas + _/+ + + bile acids produce dose related increased mito- genic responses in the oesophagus, stomach, Peptide expression: -absent, + expressed weakly, + + expressed moderately, + + + expressed maximally. and small intestine and carcinogenic responses EGF=epidermal growth factor, TGFao=transforming growth in the colon. factorot, pS2=oestrogen inducible breast cancer associated peptide, hSP=human spasmolytic polypeptide, UACL=ulcer Growth factors may produce more complex associated cell lineage. patterns of epithelial adaptation compared with other growth regulatory molecules. In this context their actions seem to be tissue, dose, small bowel by certain dietary regimens. In and disease specific because they can modulate the large bowel many oncogens and onco- proliferation, differentiation, and expression of suppressors have been reported, c-ras, OCC, other novel peptide molecules.2 6 and p53 have been most implicated in the serial Transforming growth factor ot peptide and progression of dysplasia-neoplastic sequence.'4 its mRNA are expressed throughout the length Dietary components (amines, saturated fats, of the intestinal mucosa on the tips of the villi. and ingested carcinogens) have been reported to Epidermal growth factor on the other hand is stimulate growth independent of their normally present only in small amounts on the nutritional value. In this context sodium superficial surface of colonic epithelium. The chloride and glucose (salts and sugars) usually physiological significance of these growth produce proliferative responses in line with factors in humans is not yet known, in part, nutritional demand, however, other salts such because the precise localisation of their as magnesium chloride may produce colonic receptor in the crypts and on the superficial hypertrophy. Dimethylamine, amino-methyl- enterocyte cell membranes has still to be eluci- phenylimidazopyridine (phIP), and saturated dated (apical or basolateral).6 It may also be of fats exert proliferative responses and are also http://gut.bmj.com/ some significance that epidermal growth factor associated with the induction of premalignant is also a lactated mitogen, which can modulate lesions in distal colonic sites.'5 16 normal alimentary development in the breast The production of short chain fatty acids by fed infant.'4 the colonic luminal microflora may have the Recently the discovery of novel peptides most potent mitogenic effects. These include with a clover leaf structure (trefoil peptides) acetate, butyrate, and propionate of which have been reported to have growth regulatory butyrate is preferentially metabolised as a sub- on September 26, 2021 by guest. Protected copyright. properties. They have some structural homol- strate ofoxidative metabolism by colonocytes.17 ogy with the epidermal growth factor/trans- In addition other trophic factors are increased forming growth factor a, insulin like growth in response to fibre ingestion such as mucosal factor family, suggesting, in part, a common expression of epidermal growth factor. 18 growth regulatory function. Two peptides, in Furthermore, the epithelial tissue beneath particular, human spasmolytic polypeptide the mucosa, the mesenchyme, not only pro- (hSP) and an oestrogen inducible breast vides structural support and nutrition but is cancer associated peptide (pS2) have been also reported to be important in the normal identified in specific morphological compart- development of intestinal mucosa, in part, by ments in the alimentary tract. Expression of regulating the ratio of mucosal proliferation hSP alone is seen in Brunner's glands in the and differentiation. Finally, neurogenic duodenum but has mitogenic actions on colo- influences are also important in the normal rectal cells. pS2 is not commonly identified in development of intestinal mucosa as denerva- the normal intestinal mucosa, although some tion of mucosa may lead to mucosal atrophy. positive cells are seen in areas with strong expression for transforming growth factor at. Furthermore pS2 mRNA and protein expres- Disease adaptation sion is upregulated in regions of the alimentary In general terms abnormal conditions of the mucosa receiving mucosal damage (Table III). small intestine result in failure of digestive Oncogenes and tumour suppressors have also processes (for example, lipid malabsorption gained much attention recently because they caused by Crohn's disease, salt and water are integral to both normal cell division and imbalances may stem from acute changes in also neoplastic potential. colonic mucosal permeability caused by Furthermore a recent report has suggested xenobiotics such as cholera toxin). Abnormal that oncogene expression may be induced in the conditions of the colon (colonic adenomas), S4 J7ankowski, Goodlad, Wright

however, may be the result of failure of cancer 1 Willett W. The search for the causes of breast and colon cancer. Nature (Lond) 1989; 338: 389-93. surveillance and defence. In both these cases 2 Wright NA, Alison MR. The biology of epithelial cell increased mucosal proliferation may be populations. Vol 2. Oxford: Clarendon Press, 1987. associated with changed expression of growth 3 Jankowski J, Wright NA. Epithelial stem cells in gastro-

intestinal morphogenesis, adaption and carcinogenesis. Gut: first published as 10.1136/gut.35.1_Suppl.S1 on 1 January 1994. Downloaded from factors. Increased expression of transforming Semin Cell Biol 1992; 3: 445-56. 4 Potten CS, Loeffler M. Stem cells: attributes, cycles, spirals, growth factor a is associated with more severe pitfalls and uncertainties. Lessons for and from the crypt. mucosal inflammation in Crohn's disease Development 1990; 110: 1001-20. 5 Loeffler M, Stein R, Wichman HE, Potten CS, Kaur P, especially in the ulcer associated cell lineage, Chwalinski S. Intestinal cell proliferation I. A compre- while increasing dysplastic change in tubulo- hensive model of steady-state proliferation in the crypt. associated with decreased Cell Tissue Kinet 1986; 19: 627-45. villous polyps is 6 Jankowski J, Wright NA. Gene expression in the alimentary transforming growth factor a expression. tract; modulation of epithelial proliferation, differentia- are due, tion and peptide transcription by growth regulatory pep- These apparently paradoxical results tides. Current Medical Literature-Gastroenterology 1992; 11: in part, to the dual actions of growth factors, 78-85. which can induce differentiation or stimulate 7 Goodlad RA, Wright NA. Changes in intestinal cell prolif- eration, absorptive capacity and structure in young, adult proliferation, or both according to dose, tissue and old rats. JAnat 1990; 173: 109-18. sensitivity, and presence of disease. 8 Goodlad RA, Wilson TG, Lenton W, Wright NA, Gregory H, McCullagh KG. Intravenous but not intragastric uro- In conclusion, the alimentary tract has gastrone-EGF is trophic to the intestinal epithelium of several functions in addition to digestion of parentarally fed rats. Gut 1987; 28: 573-82. 9 Goodlad RA, Ratcliffe B, Fordham JP, Ghatei MA, Domin food and preventing the transfer of many J, Bloom SR, et al. Plasma enteroglucagon, gastrin and molecular or biological agents that can, and peptide YY unconventional and germ-free rats refed with a fibre-free of fibre-supplemented diet. Q J Exp Physiol do, prove harmful to host. 1989; 74: 437-42. The gastrointestinal mucosa exhibits 10 Goodlad RA, Lee CY, Levin S, Wright NA. Effects of the prostaglandin analogue misoprostol on cell proliferation regional differences in morphology from proxi- in the canine small intestine. Exp Physiol 1991; 76: mal small intestine to distal colon (proximal - 561-6. as cells migrate 11 Levi S, Goodlad RA, Lee CY, Stamp G, Walport MJ, distal gradient). In addition Wright NA, et al. Inhibitory effect on non-steriodal anti- luminally they mature and express proteins inflammatory drugs on mucosal cell proliferation associated with gastric ulcer healing. Lancet 1990; 336: primarily associated directly or indirectly with 840-3. specific absorptive functions (vertical epithelial 12 Levi S, Goodlad RA, Stamp G, Lee CY, Walport MJ, gradient). Wright NA, et al. Effects of non-steriodal anti-imflamma- tory drugs and misoprostal on gastric and duodenal The regulation of this mucosal structure epithelial proliferation in patients with arthritis. Gastro- requires a complex cascade of 'growth regula- enterology 1992; 102: 1605-11. 13 Dembinski A, Konturek SJ. Effects of E, F, and I series tory' molecules, which exert local and distant prostaglandins and analogues on growth of gastro- effects by autocrine, endocrine, paracine, intestinal mucosa and pancreas. Am Jf Physiol 1985; 248: 170-5. exocrine routes. 14 Fearon ER, Vogelstein B. Agenetic model for colorectal There is some evidence that this heirachy of tumorigenesis. Cell 1990; 61: 759-67. 15 Scheppach W, Sommer H, Kirchner T. Effect of butyrate control may experience adaptive changes enemas on the colonic mucosa in distal ulcerative colitis. during aging in humans resulting in increased Gastroenterology 1992; 103: 51-6.

16 Chinery R, Goodlad RA, Wright NA. Soy polysaccharide in http://gut.bmj.com/ mucosal proliferation (chronological gradient). an enteral diet: effects on rat intestinal cell proliferation, Furthermore, growth control mechanisms may morphology and metabolic function. Clin Nutr 1992; 11: also become mutated or induced as a result of 277-83. 17 Goodlad RA, Ratcliffe BR, Fordham JP, Wright NA. Does environmental influences. Such aberrations dietary fibre stimulate intestinal epithelial cell prolifera- increase the expression of positive regulatory tion in germ-free rats? Gut 1989; 30: 820-5. 18 Goodlad RA, Raja KB, Peters TK, Wright NA. The effects molecules and decrease expression of growth of urogastrone-EGF on small intestinal brush border inhibitory molecules (pathological gradients). enzymes and mitotic activity. Gut 1991; 32: 410-1. on September 26, 2021 by guest. Protected copyright.