Tension is required for fibripositor formation.

Embryonic tendon cells (ETCs) have actin-rich fibripositors that accompany parallel bundles of collagen fibrils in the extracellular matrix. To study fibripositor function, we have developed a three-dimensional cell culture system that promotes and maintains fibripositors. We show that ETCs cultured in fixed-length fibrin gels replace the fibrin during ~6 days in culture with parallel bundles of narrow-diameter collagen fibrils that are uniaxially aligned with fibripositors, thereby generating a tendon-like construct.

Tendon development requires regulation of cell condensation and cell shape via cadherin-11-mediated cell-cell junctions.

The ability of tendon to transmit forces from muscle to bone is directly attributable to an extracellular matrix (ECM) containing parallel bundles of collagen fibrils. Although the biosynthesis of collagen is well characterized, how cells deposit the fibrils in regular parallel arrays is not understood. Here we show that cells in the tendon mesenchyme are nearly cylindrical and are aligned side by side and end to end along the proximal-distal axis of the limb.