Collagen fibril bundles: a branching assembly unit in tendon morphogenesis.

The assembly, deposition and organization of collagen fibril bundles and their composite fibrils were studied during morphogenesis of the chick embryo tendon using electron microscopy, serial sections and computer-assisted three-dimensional reconstruction techniques. The 14-day chick embryo is a stage when tendon architecture is being established and rapid changes in the mechanical properties occur between days 14 and 17 of development. Tendon matrix structure develops from discrete subunits, bundles of collagen fibrils. The bundles branch; undergo a gradual rotation over several micrometers; are intimately associated with the cellular elements of the developing tendon; and form arborizing networks within and among fascicles. The organization of discrete fibril segments into bundles, during the establishment of tendon architecture and function, where the segmental fibrillar components could interact with the interfibrillar matrix as well as with adjacent fibrils would contribute to the stabilization of this structure. The observed gradual rotation of the bundles would serve to stabilize the immature bundle through the physical twining of the composite fibrils while the extensive branching of the bundles observed at 14-days of development and their intimate association with the cellular elements would provide a higher order of structure stabilization.