The axonal coupling to the glia matrix was hypothesized to contribute to the transition from non-affine (independent) to affine (interdependent) behavior of axonal kinematics. The effect of spinal cord growth on axonal kinematic behavior was investigated with a chick embryo spinal cord model. Chick spinal cords at different development stage (E12, E14, E16, and E18) were stretched to different levels (0, 5, 10, 15, and 20%). The tortuosity distribution of axons at each developmental stage and each stretch level was characterized. Axonal deformation showed increasing coupled behavior with development and growth. The experimental results did not follow ideal affine nor non-affine behavior. A ‘switching’ model was then employed and the values of parameters of the ‘switching’ model were determined by minimizing the difference between experimental results and predicted results. The ‘switching’ model predicted the experimental results more accurately. This percentage of axons that exhibit purely non-affine behavior decreased with development, indicating more non-affine manner at early developmental stages. Thus axons exhibit increasing affine deformation as developing and growth progress in chick embryos.

We identified the role of axonal coupling to glia on axon kinematics by disrupting the myelination of axons. This was done by introducing GalC antibody or ethidium bromide (EB). Pure rabbit IgG and saline were used as a control respectively. Following each injection, spinal cords were incubated until E18 and two different stretch levels were applied (5, or 15%). Following EB and GalC injections, spinal cords showed predominant demyelination. Glial cells, including astrocytes and oligodendrocytes were disrupted following EB injection, but not GalC injection. Saline or pure rabbit IgG did not cause any change to the glia and myelination of axons. The transition from affine to non-affine behavior was detected from myelinated spinal cord compared to demyelianted spinal cord. The shift was very modest in spinal cord following GalC injection, though significant in spinal cord following EB injection. The results demonstrate that the role glial is important. We finally characterized the material properties of myelinated and demyelinated spinal cords. Higher ultimate stress and greater shear modulus were observed for myelinated spinal cords compared to demyelinated spinal cords. Greater strain at ultimate stress was also observed for spinal cords following GalC injection compared to EB injection. The results indicated that spinal cords were stronger when myelinated vs. demyelinated, as well as with astrocytes vs. without astrocytes. Alteration in spinal cord compositions affected the mechanical properties of the tissue, and might affect the strain transfer from tissue to microscopic cells as well.