Small intestine microvilli

Vitamin B12 is special in as far as its absorption depends on the availability of several secretory proteins, the most important being the so-called intrinsic factor (IF). IF is produced by the parietal cells of the fundic mucosa in man and is secreted simultaneously with HC1. In the small intestine, vitamin B12 (extrinsic factor) binds to the alkali-stable gastric glycoprotein IF. The molecules form a complex that resists intestinal proteolysis. In the ileum, the IF-vitamin B 12-complex attaches to specific mucosal receptors of the microvilli as soon as the chymus reaches a neutral pH. Then either cobalamin alone or the complex as a whole enters the mucosal cell. 

To date, there is very little known about if and how phytochemicals modulate the metabolism of GIT tissues other than the liver. Of particular interest are the xenobiotic metabolizing enzymes of the GIT, which are involved with transformation of drugs and toxins. Whereas the metabolic activities of the resident microflora dominate in the large intestine, mucosal enzyme activities are more important in the small intestine where bacterial densities are lower and the villi and microvilli increase the area of exposure. 

The third mucosal layer is that lining the entire length of the small intestine and which represents a continuous sheet of epithelial cells. These epithelial cells (or enterocytes) are columnar in shape, and the luminal cell membrane, upon which the microvilli reside, is called the apical cell membrane. Opposite this membrane is the basal (or basolateral) plasma membrane, which is separated from the lamina propria by a basement membrane. A sketch of this cell is shown in Fig. 5. The primary function of the villi is absorption. 

The gut wall within the small intestine is particularly well adapted for its role as an absorptive surface. Absorption rate is proportional to the area of the surface that is available for absorption. Thus, the internal surface of the small intestine is folded towards the lumen of the gut. This folding increases the surface area of the gut by approximately 3-fold. In this area, the gut wall is covered with many fingerlike projections called villi, and these provide a further 10-fold increase in surface area. In addition, the gut wall epithelial cells are polarized such that on the luminal surface there are millions of microvilli providing a further 20-fold increase in surface area for absorption. In all, these surface area modifications provide an absorptive area which is some 600-fold higher than would be provided by a simple cylinder. Thus, the estimated surface area of the human gut is approximately 200 m2 [1],... 

Small-intestinal secretions. The glands of the small intestine (the Lieberkuhn and Brunner glands) secrete additional digestive enzymes into the bowel. Together with enzymes on the microvilli of the intestinal epithelium (peptidases, glycosidases, etc.), these enzymes ensure almost complete hydrolysis of the food components previously broken down by the endoenzymes. 

The inner surface of the small intestine is not smooth and flat but wrinkled into a large number of finger-like projections called villi, which project into the lumen. If we look at each villus under the microscope (Fig. 1) we find it, in turn, has small fingerlike processes projecting out into the lumen - the microvilli. The result of this is that the surface of the small intestine (which is only 300 cm in length - in the relaxed state after death it may measure 6-7 metres), is estimated to have an area of 250 m. It is obviously designed to absorb, particularly, nutrients and this is also where most of any drug taken by mouth is absorbed. 

Fig. 1. Longitudinal section of the intestine, showing the villi and microvilli which increase the surface area of the small intestine.

Figure 3.12 The mammalian gastrointestinal tract showing important features of the small intestine, the major site of absorption for orally administered compounds (A) liver (B) stomach (C) duodenum (D) ileum (E) colon (F) longitudinal section of the ileum showing folding, which increases surface area (G) detail of fold showing villi with circular and longitudinal muscles, (H) and (I) respectively, bounded by (J) the serosal membrane (K) detail of villi showing network of (L) epithelial cells, (M) capillaries, and (N) lacteals (O) detail of epithelial cells showing brush border or (P) microvilli. The folding, vascularization, and microvilli all facilitate absorption of substances from the lumen. Source From Ref. 1.

As mentioned above, the villi of the small intestine (Figure 1.2) house a dynamic, self-renewing population of the epithelial cells that includes absorptive cells (enterocytes), secretory cells, and endocrine cells. The thin lining (height 25 p,M height of the microvilli is 1.5 pM) of the columnar enterocytes is the only barrier between the intestinal lumen and the muscularis mucosa, which represents, in this context, the entire body interior. The entire epithelial lining of the intestine replaces itself every 3-5 d [128], It is the enterocyte and its neighboring cells where absorption processes occur and it will therefore be the focus of the mechanistic discussions below. 

The distal portion of the large intestine is the rectum. Rectal absorptive capacity is considerably less than that of the upper GI tract owing to a limited surface area, a result of the absence of microvilli. Also, the blood supply to colon and rectum is less than that to the small intestine. The rectal artery branching off the inferior mesenteric artery of the... 

The stomach has a surface epithelium composed of a single layer of columnar cells with few apical microvilli. The epithelial lining of the small intestine consists of a single layer of columnar cells with densely packed microvilli to promote absorption. The epithelium in the large intestine is similar to that in the small intestine except for the absence of villi in the large intestine. 

The intestine can compensate the 2.5 log units difference between it and the stomach by the increased surface area in the small intestines. The presence of microvilli (Figure 6.6b) in the intestine is an increase of 600-fold in surface area compared to a hollow tube of comparable length. Note that there is no absorption, except for water, in the large intestine. 

The large internal surface area of the small intestine is attributable to its length, folding, and the presence of villi and microvilli within its lumen. The villi contain capillaries and protrude into the lumen of the small intestine. There are approximately four to five million villi in the small intestine. Each villus has many microvilli as its outer surface (Figure 11.3). The microvilli represent the absorptive barrier of the small intestine. The stomach and large intestine do not contain villi and, therefore, have a small absorptive surface area compared with the small intestine. 

The lumen of the small intestine. The surface area of the small intestine is large due to its length, folding, millions of villi, and tens of millions of microvilli (shown in close-up view on right). 

The most important site of absorption of nutrients and drugs is the small intestine. Its extremely large surface area with many folds, villi, and microvilli, together... 

The gastrointestinal (GI) microflora plays an important role in the health status of people and animals. The GI tract represents a much larger contact area with the environment, compared to the 2 m2 skin surface of our body (van Dijk 1997). The mucosal surface of the small intestine is increased by forming folds, intestinal villi, and the formation of microvilli in the enterocyte resorptive luminal membrane. The resulting surface of GI system is calculated to be 150-200m2, therefore it provides enough space for the interactions related to digestion and for the adhesion to the mucosal wall. 

Absorption of compounds in the gastrointestinal tract occurs mainly in the small intestine with 3 different regions (duodenum, jejunum and ileum) and in the large intestine (colon). In the small intestine the uptake surface is increased by folds (3-times), villi (30-times) and microvilli (600-times). In the colon folds and villi are absent (Daugherty and Mrsny 1999). 

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