Making tendons

Cells are fastidious about their internal conditions. But for scientists trying to decipher how collagen forms structures such as tendons and the cornea, the big question 30 years ago was how much control cells exert over their surroundings. In a tendon, for example, collagen molecules join end-to-end to yield fibrils, which line up alongside one another to create bundles. These amalgamations, in turn, cluster into fascicles. Most researchers thought that cells did little to aid the process beyond manufacturing collagen, according to Robert Trelstad (Robert Wood Johnson Medical School, Piscataway, NJ). The prevailing view, he says, “was that all the cell had to do was squirt this stuff [collagen molecules] into the intercellular space and—voila!—it would self-assemble.” Trelstad expressed his disagreement with that explanation in a ditty:

It took a decade to amass evidence that cells take a more active role. In the mid-1970s, his group's in vitro study revealed that collagen doesn't condense into fibrils in one fell swoop, as many researchers had argued. Instead, fibrils form in stages (Trelstad et al., 1976). Ten years later, he and colleague David Birk trained one of the new high-voltage electron microscopes on embryonic chick tendons. They identified three kinds of pockets in the membranes of collagen-producing fibroblasts (Birk and Trelstad, 1986). One was a deep, narrow crevice that held a single fibril, like a hair in a follicle. The second, wider groove cradled fibril bundles, while the third, even larger indentation held clusters of bundles.The observations suggested that the cell was orchestrating collagen condensation and fibril grouping by adjusting the contours of its membrane. Trelstad and Birk hypothesized that the vesicles that carry newly synthesized collagen stack up and then merge to form the deep crevices, where collagen molecules interlink into a fibril. The cell then manipulates the membrane that separates the pockets, allowing fibrils to merge into bundles and bundles to band together. When it comes to making a tendon, “cellular control of the early stages is essential,” Trelstad says. A recent study expanded on the findings, showing how the cell guides the fibrils into place using long protrusions termed fibropositors (Canty et al., 2004).