Gap junctions of the medial collateral ligament: structure, distribution, associations and function.

Ligaments are composed of two major components: cells and extracellular matrix. The cells express gap junction proteins and are arranged into a series of rows that traverse the tissue, suggesting that all the cells of the tissue are functionally interconnected. The results of our study demonstrate that medial collateral ligament (MCL) cells do not have a uniform fusiform morphology or placement along a row of cells as previously suggested, but rather display a complex placement and form that weaves within the collagen matrix in a manner that is far more extensive and complex than previously appreciated. Within this morphological context, we find that MCL cells in vivo contain functional gap junctions (verified using fluorescence recovery after photobleaching) that are localized to sites of close cell-cell contact, and this pattern imparts or reflects a bipolarity inherent to each cell. When we studied ligament cells in conventional tissue culture we found that this bipolarity is lost, and the placement of gap junctions and their related proteins, as well as general cell morphology, is also altered. Finally, our study demonstrates, for the first time, that in addition to gap junctions, adherens junctions and desmosomes are also expressed by MCL cells both in vivo and in vitro and map to sites of cell-cell contact.

Communication between paired chondrocytes in the superficial zone of articular cartilage.

The regeneration and repair of cartilage damaged by injury or disease, a major goal of orthopaedic science, depends on understanding the structure and function of both the extracellular matrix and the chondrocytes. In this study, we explored the in situ organization and potential interactions between chondrocytes in the superficial zone of adult rabbit articular cartilage. Some chondrocytes in this zone were observed close together and appeared to be paired whereas others were solitary. The shared surfaces of a chondrocyte pair were separated by a narrow plate of extracellular matrix, into which extended small cytoplasmic projections from both cells. Furthermore, the spatial distribution of major cellular landmarks, such as the nucleus and centrosome as well as some intracellular proteins such as connexin-43, tended to be mirrored about this matrix plate. Fluorescence recovery after photobleaching revealed the fluorescent dye calcein-AM dye can pass between paired cells, and that the passage of this dye can be inhibited by the gap junction blocker octanol. These results illustrate that rapid cellular communication is possible between cells in the superficial layer of adult articular cartilage, which challenges the current thinking that these chondrocytes function in isolation.