Tendon cells in vivo form a three dimensional network of cell processes linked by gap junctions.

Tendons respond to mechanical load by modifying their extracellular matrix. The cells therefore sense mechanical load and coordinate an appropriate response to it. We show that tendon cells have the potential to communicate with one another via cell processes and gap junctions and thus could use direct cell/cell communication to detect and/or coordinate their load responses. Unfixed cryosections of adult rat digital flexor tendons were stained with the fluorescent membrane dye DiI to demonstrate cell shape. Similar sections were immunolabelled with monoclonal antibodies to rat connexin 32 or connexin 43 to demonstrate gap junctions and counterstained with propidium iodide to show nuclei, or the membrane stain DiOC7 to show cell membranes. Sections were examined with a laser scanning confocal microscope and 3-dimensional reconstructions were prepared from optical section series to demonstrate cell shape and the position of connexin immunolabel. Cells had a complex interconnected morphology with gap junctions at points of contact with other cells. Cell bodies contained the nucleus and extended broad flat lateral cell processes that enclosed collagen bundles and interacted with similar processes from adjacent cells. They also had long thin longitudinal processes interacting with the cell process network further along the tendon. Connexin 43 occurred where cell processes met and between cell bodies, whereas connexin 32 was only found between cell bodies. The results indicate the presence of a 3-dimensional communicating network of cell processes within tendons. The intimate relationship between cell processes and collagen fibril bundles suggests that the cell process network could be involved in load sensing and coordination of response to load. The presence of 2 different types of connexins suggests that there could be at least 2 distinct communicating networks

Tendons and ligaments - an overview

The structure, range of functions, blood supply, nerve supply, biochemical composition and development of tendons and ligaments are reviewed. The importance of their cells is often overlooked because of the obvious role of the extracellular matrix (ECM) in determining the physical properties of tendons and ligaments. However, it is emphasised that tendon and ligament cells have elaborate cell processes that form a three dimensional network extending throughout the extracellular matrix. The cells comrnunicate with each other via gap junctions that could form the basis of an important load sensing system allowing the tendon to modify its ECM. Tendons and ligaments have three specialised regions along their length - the myotendinous junction, the region where tendons change direction by wrapping around bony pulleys and the enthesis (bony insertion site). The myotendinous junction is a comrnon site of muscle strains and pulls, the wrap-around region is frequently fibrocartilaginous and a cornmon site for degenerative change, and the enthesis may be fibrous or fibrocartilaginous according to location, and is a common site for degenerative changes or 'enthesopathies'. Enthesis fibrocartilage is just one of a series of protective devices reducing wear and tear at insertion sites. Consideration is also given to the structure and function of tendon sheaths and to the dramatic effects of exercise and deprivation on tendons and ligaments - exercise strengthens, but even relatively short periods of immobilisation can dramatically weaken tendons and ligaments