Distribution of the elastic fiber and associated proteins in flexor tendon reflects function

The elastic fiber is known to be an important component of skin, lung, and vasculature. Much less is known about the distribution of elastin and elastic fiber-related proteins in connective tissues, yet genetic defects of elastic fiber constituents can lead to deficiencies in these tissues. For the first time, we determine the distribution of elastin, fibrillins 1 and 2, and microfibril-associated glycoproteins (MAGPs) 1 and 2 in the flexor digitorum profundus (FDP) tendon. Three functionally distinct regions of the FDP tendon, the fibrocartilagenous (FC) region, avascular/tensional (AV/T) region, and insertion region, were evaluated by immunohistochemical methods for these five proteins. Biochemical analysis of desmosine content, an elastin-specific cross-link, demonstrated the presence of elastin in each region, and this was verified histochemically. The fibrillins were found with elastin and also pericellularly with internal fibroblasts where elastin was not detected. Although there was overlapping distribution, fibrillin 2 was more prominent in the interior of the tendon while fibrillin 1 was prominent in outer cell layers that contained elastic fibers. Both MAGP-1 and -2 were found throughout the tendon, although the greatest abundance was near the tendon insertion to bone. Surprisingly, MAGP-1 demonstrated a filamentous appearance within the fibrocartilage that did not correspond to the fibrillin 1 or 2 or MAGP-2 staining pattern. Lastly, we have shown that a vincular membrane located along the dorsal surface of the tendon near the insertion has a very high elastin content and a unique interface with the tendon that consists of an elastic anchor within the tendon body.