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1 Scool of Cell and Molecular Biosciences, University of Newcastle upon Tyne, Medical School, Framlington Place, Newcastle upon Tyne, United Kingdom
* To whom correspondence should be addressed. E-mail: n.l.simmons{at}ncl.ac.uk.
The functional organisation of the bovine rumen epithelium has been examined by electron and light microscopy combined with immunocytochemistry in order to define a transport model for this epithelium. Expression of connexin-43, an integral component of gap-junctions, the tight-junction molecules claudin-1 and ZO-1 and the catalytic
-subunit of the Na+-K+/ATPase were demonstrated by SDS/PAGE and Western blotting.
From the lumen surface four cell layers can be distinguished, namely the stratum corneum, the stratum granulosum, the stratum spinosum and the stratum basale. Both claudin-1 and ZO-1 immunostaining showed plasma membrane staining, which was present at the stratum granulosum with decreasing intensity through the stratum spinosum to the stratum basale. The stratum corneum was negative for claudin-1 immunostaining. Transmission electron microscopy confirmed that occluding tight junctions were present at the stratum granulosum.
Plasma-membrane connexin-43 immunostaining was most intense at the stratum granulosum and decreased in intensity through stratum spinosum and stratum basale.
There was intense immunostaining of the stratum basale for the Na+-K+/ATPase, with weak staining of the stratum spinosum. Both the stratum granulosum and the stratum corneum were essentially negative. Stratum basale cells also displayed a high mitochondrial density relative to more apical cell layers.
We conclude that epithelial barrier function may be attributed to the stratum granulosum, whilst cell-cell gap junctions allow diffusion to interconnect the barrier cell layer with the stratum basale where Na+-K+/ATPase is concentrated.
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