DescriptionThe mechanical properties of the spinal cord dictate its response to traumatic loading conditions, and also provide important cues to cellular constituents that regulate behavior such as growth and differentiation. After initial connections are established, the structure and composition of the human spinal cord continues to significantly change during development, both pre-natally and during the early years of life. As such, the mechanical properties of the spinal cord are likely to also change, which would potentially alter both the physical tolerance of the spinal cord to injury as well as the regulatory mechanostructural cues that encourage or inhibit neural differentiation and axon growth.
Previous studies have quantified the properties of fully developed adult spinal cords from the rat, cat and human. This study quantifies the mechanical properties of the chick embryo spinal cord during a period of rapid growth and development which partially parallels the development of the post-natal human infant.
Quasistatic uniaxial tensile testing to failure was performed on chick embryo spinal cords at 0.001s-1. Samples were tested at embryological days (E) 14, 15, 16 and 18. Spinal cords demonstrated non-linear stress-strain behavior that was modeled with a 1-term Ogden hyperelastic strain energy density function. Stiffness and ultimate tensile stress (UTS) were observed with increasing development.
Stress-relaxation viscoelastic testing was also performed on spinal cords from the same development days at a loading rate consistent with those experienced during trauma (ramp to 7.5% stretch at ~19.5s-1 and hold for 10 seconds). All spinal cords demonstrated significant relaxation, and the behavior was modeled with a linear series of 4 exponential decay time constants. Statistical analysis indicated that the viscoelastic properties did not change between the days tested. Regardless of the maximum stress reached from the ramp phase, all cords tested relaxed ~72.5% with ~68% of this relaxation occurring within the first 30ms. The changes in the stiffness and UTS in the developing chick embryo spinal cord suggest similar changes in the developing human spinal cord, which points to the need for age-specific injury tolerance criteria.