Functional Organisation of the Bovine Rumen epithelium

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 α-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.