Villi absorptive cells

Villus absorptive cells in fetal small intestine of man Convoluted membrane - [1571... 

Figure 14 Ion transport pathways responsible for water flux across intestinal epithelia. Sodium absorption in villus tip cells (left) stimulates water absorption, while chloride channel exit in crypt cells (right) stimulates water secretion.

As the primary site of iron absorption, the duodenum shows a distinctive pattern of HFE expression HFE is highly expressed in crypt but not villus duodenal cells (Parkilla et al., 2001). As the absorption of ferrous iron from the diet is principally mediated by DMT-1 at the villus tip, a connection between abnormal HFE and overexpression of DMT-1 has been hypothesized. While the DMT-1 protein is expressed... 

Zinc homeostasis is largely regulated by its uptake and loss through the small intestine. Although a number of zinc transporters and binding proteins have been identified in villus epithehal cells, a full understanding of zinc absorption is not yet at hand. 

Figure 2 The structure of the absorptive cell of the intestinal lining reflects its specialized function in digestion. The absorptive cell is one of the two main cell types that constitute the epithelium of the villus. The other type is the goblet cell, which secretes mucus. Cells of both types are produced by cell division in the crypts and assume their mature structure during a 2-day migration to the villus tip. Exposed in transit to the action of digestive fluids, some of the membrane covering the absorptive cell may be sloughed off. The cell is abundantly supplied with mitochondria, which provide energy for metabolic...

The relative importance of each of these contributions to pool C is likely to be different in epithelial cells located at different points along the villus-crypt axis. The fact that cholesterol derived from synthesis and from the uptake of LDL is critically important for membrane formation and differentiation is suggested by the finding that 70-80% of total mucosal sterol synthetic activity and LDL transport activity are localized to the immature cells of the lower villus and crypt regions in both the proximal and distal intestine. In the mature absorptive cells of the upper villus in the jejunum, where most sterol absorption takes place, the rate of cholesterol synthesis appears to be suppressed. In the absence of fat absorption, cholesterol newly synthesized in these cells apparently is sloughed into the lumen and not reabsorbed. However, with active triglyceride absorption cholesterol synthesis in these cells is increased and a portion of this sterol appears in the intestinal lymph. Only under this condition does pool B apparently supply sterol for lipoprotein formation. 

Figure 8.2 Rat duodenal cells divide in the crypts of Lieberktihn and differentiate while migrating to the villus tips within approximately 48 h. The crypt cells take up iron from the blood, and are thereby able to sense the body s state of iron repletion. They migrate to the villus tips where this information determines their iron absorption capacity from the intestinal lumen. Adapted from Schumann et al., 1999, by permission of Blackwell Science.

More detailed knowledge of the mechanisms of iron absorption from the gastrointestinal tract of mammals has advanced by leaps and bounds in the last few years, with the cloning of key participants at both the apical and basolateral faces of the mucosal endothelial cells. The extraordinarily far-sighted contribution of William Crosby and Marcel Conrad, nearly 40 years ago (Conrad and Crosby, 1963 Crosby, 1963), reminded us of the key role of body iron status in determining the level at which iron absorption will be set within the cells of the crypts of Lieberkiihn when they differentiate and move up to the villus tips to play their key role in... 

Treatments which transiently open up intercellular gaps increase absorption significantly. In the villus, the loss of apical cells may cause large gaps at the tip,... 

The mucosal layer, comprising the lumenal surface of the small intestine, is responsible for the digestive and absorptive functions of the small intestine. The mucosal surface area is much larger than predicted for a simple cylinder. Circular folds account for this amplification. The mucosal surface area is extended further by fingerlike projections called villi and depressions called crypts. The villi are 0.5-1.0 mm in height. Each villus and crypt is lined by epithelial cells that are covered with many closely packed microvilli that project into the intestinal lumen. If the small intestine is viewed as a simple cylinder, its mucosal surface area would be on the order of half of a square meter. However, in reality, the mucosal surface area of the small intestine is approximately 250 square meters, comparable with size of a tennis court. 

Intestinal absorption of dietary iron. Ferrous iron is absorbed by the duodenal villus tip enterocytes mediated by divalent metal transporter-1 (DMTI). Iron transport mediated by DMTl of the apical surface and the basolateral tran.sporter at the basolateral surface are coupled to ferric reductase and ferroxidase that change the iron oxidation state, respectively. The degree of iron entry is determined by the level of DMTl and its level of expression is programmed in the crypt cells. The programming of the crypt cells is coupled to the body iron stores via transferrin-mediated and HFE protein-modulated iron transport. [Modified and reprinted with permission from B. R. Bacon, L. W. Powell,... 

Rotaviruses are double-stranded wheel-shaped RNA viruses. These strains cause diarrhea by infecting the enterocyts of the villi in the small intestine. Changes to the villi include shortening of villus height, crypt hyperplasia, and mononuclear cell infiltration of the lamina propria. Diarrhea results from decreased absorption across intestinal mucosal surface. ... 

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