The anucleate prismoid fiber cells of the eye lens are densely packed to form a tissue in which the plasma membranes and their associated cytoplasmic coat form a single giant cell-cell adhesive complex, the cortex adhaerens. Using biochemical and immunoprecipitation methods in various species (cow, pig, rat), in combination with immunolocalization microscopy, we have identified two different major kinds of cortical complex. In one, the transmembrane glycoproteins N-cadherin and cadherin-11 [which also occur in heterotypic (`mixed') complexes] are associated with α- and β-catenin, plakoglobin (proportions variable among species), p120ctn and vinculin. The other complex contains ezrin, periplakin, periaxin and desmoyokin (and so is called the EPPD complex), usually together with moesin, spectrin(s) and plectin. In sections through lens fiber tissue, the short sides of the lens fiber hexagons appear to be enriched in the cadherin-based complexes, whereas the EPPD complexes also occur on the long sides. Moreover, high resolution double-label fluorescence microscopy has revealed, on the short sides, a finer, almost regular mosaicism of blocks comprising the cadherin-based, catenin-containing complexes, alternating with patches formed by the EPPD complexes. The latter, a new type of junctional plaque ensemble of proteins hitherto known only from certain other cell types, must be added to the list of major lens cortex proteins. We here discuss its possible functional importance for the maintenance of lens structure and functions, notably clear and sharp vision.

The long-term fate of stem cells depends on their interaction with the niche. Mesenchymal stem cells (MSC) from human bone marrow have been demonstrated to differentiate into various tissues, such as bone, cartilage, muscle and fat. As the interaction with the microenvironment plays a major role in differentiation, we have characterized the cell-cell contact among MSC. We have demonstrated the occurrence of a novel kind of adhering junction, consisting of slender, villiform-to-vermiform cell projections (processus adhaerentes). In this study, we have systematically analyzed the molecular composition of these junctions. A panel of antibodies specific for various components of tight junctions, gap junctions, adherens junctions and desmosomes was used. The expression of these antigens was verified by light and electron microscopy and by biochemical analysis, including immunoprecipitation and RT-PCR. MSC from two different sources were analyzed: MSC obtained from bone marrow aspirates from healthy voluntary donors and additionally MSC originating from umbilical cord blood donated for scientific research. We have also shown the presence of vimentin-positive retothelial cells (which are probably the source of MSC) in situ in fresh bone marrow samples. We demonstrate that MSC were interconnected by occasional gap junctions and frequent adhering junctions. Additionally, we found a unique molecular composition of these adhering junctions, as they comprise the transmembrane glycoproteins cadherin 11, N-cadherin and syndecan-1, together with the cytoplasmic plaque proteins α- and β-catenin and p120ctn. Constitutive complexes of these molecules have been identified. Our data indicate that MSC communicate with each other through junctions and junctional complexes. We hypothesize that MSC can embark on alternative differentiation pathways with specific junctional and cytoskeletal patterns. Characterization of and understanding the role of such intercellular contacts and their correlation to specific differentiation programs are being conducted.