Method development and characterization of chick embryo tendon mechanical properties

Tendons are involved in multiple disorders and injuries, ranging from birth deformities to tendinopathies to acute ruptures. The ability to characterize embryonic tendon mechanical properties will enable elucidation of mechanisms responsible for functional tendon formation. In turn, an understanding of tendon development could inform approaches for adult and embryonic tendon tissue engineering and regenerative medicine. The chick embryo is a scientifically relevant model that we have been using to study Achilles (calcaneal) tendon development. Chick embryo calcaneal tendons are challenging to mechanically test due to small size and delicate nature, and difficulty distinguishing embryonic tendons from muscle and fibrocartilage using the naked eye. Here, we developed and implemented a "marking protocol" to identify and isolate calcaneal tendons at different stages of chick embryonic development. Mechanical testing of tendons isolated using the marking protocol revealed trends in mechanical property development that were not observed with tendons isolated by naked eye (eyeballing). Marked tendons exhibited non-linear increases in tensile modulus and ultimate tensile strength, whereas eyeballed tendons exhibited linear increases in the same properties, reflecting a need for the marking protocol. Furthermore, the tensile mechanical properties characterized for marked tendons are consistent with previously reported trends in cell length-scale mechanical properties measured using atomic force microscopy. This report establishes new methodology to enable tensile testing of chick embryo tendons and provides new information about embryonic tendon mechanical property development.