LANDIS, W.J.*; SILVER, F.H.: Structure and Function of Normally Mineralizing Avian Tendons

Leg tendons of some avians normally calcify. The gastrocnemius tendon of the domestic turkey, Meleagris gallopavo, is an example. Its structure and biomechanical properties have been examined to model the adaptive nature of tendon to external forces, including the means of mineral deposition and the functional role of mineralization in this tissue. Structurally, the distal gastrocnemius bifurcates into two smaller proximal segments that mineralize progressively. Mineral deposition occurs near the bifurcation and proceeds in a distal-to-proximal direction along the segments toward lateral and medial hip muscle insertions. Extracellular vesicles and type I collagen fibrils mediate tendon mineral formation. Biomechanical analyses indicate lower ultimate tensile stresses, tensile strength, and moduli for the distal gastrocnemius compared to the flat, mineralized proximal segments. Tendon mineralization appears stress-induced, a result of high tensile forces placed by the muscles on the proximal gastrocnemius. Such forces are thought to be transduced to cells through membrane α and β integrin subunits. Flexible sites on collagen containing charged GER and GER-like regions are suggested to bind to integrins α1β1 and α2β1. Thus, mechanical forces may be transduced to tenocytes through collagen-integrin complex formation. Mechanotransduction may lead to unfolding of flexible sites on collagen, permitting electrostatic calcium and phosphorus binding and subsequent mineral nucleation and growth in the tendon. Functionally, mineralization increases stored elastic energy by preventing collagen flexible regions from stretching. Mineralization thereby limits tendon deformation, provides greater load-bearing and preserves muscle energy during animal locomotion.