Complexus adhaerentes, a new group of desmoplakin-containing junctions in endothelial cells: The syndesmos connecting retothelial cells of lymph nodes

We describe, in bovine and human tissues, a new kind of adhering junctions ('complexus adhaerens') which occur in certain vascular endothelia and, by morphological and compositional criteria, are neither desmosomes nor adhering junctions of the zonula (fascia) adhaerens category. A particularly abundant and complex subtype among the 'complexus adhaerens' junctions is characteristic of the endothelium-related, stellate cells that form the three-dimensional filter meshwork of lymph node sinus. These 'retothelial' cells are multiply interconnected by junctions which are highly polymorphic in shape and size, often show interruptions and branches and can be traced for several micrometers. Their dense cytoplasmic plaque contains plakoglobin and desmoplakin but is practically negative for vinculin and α-actinin. In contrast to true desmosomes, including those of the dendritic reticulum cells of lymph node follicles, the retothelial junctions are negative for any of the known desmosomal cadherins (desmogleins and desmocollins) and also for E- and M-cadherin. These cells, however, are rich in cadherin-5 which colocalizes with desmoplakin and plakoglobin in numerous, though not all sites, indicating that here cadherin-5 may contribute to the formation of a desmoplakin plaque. The complex arrangement of the retothelial form of complexus adhaerens, for which the term 'syndesmos' is proposed, has been reconstructed from electron micrographs by a computer-aided design. We also report that desmoplakin-positive complexus adhaerens junctions constitutively occur in various lymphatic endothelia of many other tissues, most prominently in submucosal lymph vessels, and discuss the possible function of this kind of junctions as a means to fortify cell connections between the tenuous endothelial cells and, in the lymph node sinus, to support their filtering functions.