A fibripositor deposits a collagen fiber into the extracellular space.

On page 553, Canty et al. show how cells use finger-like projections to lay down parallel arrays of collagen fibers that are many times the lengths of the cells.

Parallel arrays of long collagen fibers give tendons their resistance and tensile properties. Thousands of collagen molecules self-assemble into fibers, but the fibers need instruction from their environment for alignment—fibers in solution are randomly arranged. The work by Canty et al. shows how long, thin plasma–membrane extensions, called fibripositors, set up this parallel pattern in prenatal mouse tendons.

Microscopy studies showed that these fibripositors received mature, fully processed collagen fibers from Golgi-to-plasma membrane carriers (GPCs). The GPCs eventually fused with the tip of the fibripositor, which then retracted, leaving behind its fiber contents. More fibers were added to others at the base of the fibripositor, deep within the cell, near the trans-Golgi network. Collagen fiber assembly was seen two decades ago in microscopy studies, but at the time, the fibripositors appeared to be extracellular invaginations rather than an extension of the cell.

The GPCs and fibripositors both align with the long axis of the tendon, extending out into long extracellular spaces between adjacent cells. This implies that the layout of the cells in the tendon sets the parallel collagen pattern and explains why fibroblasts in culture are unable to produce parallel fibers. It is not yet clear how the cells attain the correct alignment, however.

The parallel pattern is set up before birth. After the mice are born, fibripositors are retracted, and collagen is secreted in a soluble form that is processed in the matrix and then added to the circumference of the fibers. In this way, the parallel arrangement is maintained without expending the energy required to make fibripositors. ▪