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Research Article

Domains of increased thickness in microvillar membranes of the small intestinal enterocyte

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Pages 170-177 | Received 20 Jan 2010, Accepted 22 Apr 2010, Published online: 14 Jun 2010
 

Abstract

The apical surface of the enterocyte is sculpted into a dense array of cylindrical microvillar protrusions by supporting actin filaments. Membrane microdomains (rafts) enriched in cholesterol and glycosphingolipids comprise roughly 50% of the microvillar membrane and play a vital role in orchestrating absorptive/digestive action of dietary nutrients at this important cellular interface. Increased membrane thickness is believed to be a morphological characteristic of rafts. Thus, we investigated whether the high contents of lipid rafts in the microvillar membrane is reflected in local variations in membrane thickness. We measured membrane thickness directly from electron micrographs of sections of fixed mucosal tissue. Indeed, mapping of the microvillar membrane revealed a biphasic distribution of membrane thickness. As a point of reference the thickness distribution of the basolateral membrane was clearly monophasic. The encountered domains of increased thickness (DITs) occupied 48% of the microvillar membrane and from the data we estimated the area of a single DIT to have a lower limit of 600 nm2. In other experiments we mapped the organization of biochemically defined lipid rafts by immunogold labeling of alkaline phosphatase, a well documented raft marker. Strikingly, the alkaline phosphatase localized to distinct regions of the membrane in a pattern similar to the observed distribution of DITs. Although we were unable to measure membrane thickness directly on the immunogold labeled specimens, and thereby establish an unequivocal connection between DITs and rafts, we conclude that the brush border membrane of the enterocyte contains microdomains distinguishable both by membrane morphology and protein composition.

Acknowledgements

We thank Lise-Lotte Niels-Christiansen for excellent technical assistance.

Declaration of interest: The work was supported by grants from the Novo-Nordic Foundation, the Beckett Foundation, the Augustinus Foundation, Brødrene Hartmanns Fond and Aase and Ejnar Danielsens Fond. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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