Mechanical, compositional, and structural properties of the post-natal mouse Achilles tendon

Abstract

During post-natal development, tendons undergo a well orchestrated process whereby extensive structural and compositional changes occur in synchrony to produce a normal tissue. Conversely, during the repair response to injury, structural and compositional changes occur, but in this case, a mechanically inferior tendon is produced. As a result, the process of development has been postulated as a potential paradigm through which improved adult tissue healing may occur. In this study we measured the mechanical, compositional, and structural properties in the post-natal mouse Achilles tendon at 4, 7, 10, 14, 21, and 28 days old. Throughout post-natal development, the mechanical properties, collagen content, fibril diameter mean, and fibril diameter standard deviation increased. Biglycan expression decreased and decorin expression and fiber organization were unchanged. This study provides a new mouse model that can be used to quantitatively examine mechanical development, as well as compositional and structural changes and biological mechanisms, during post-natal tendon development. This model is advantageous due to the large number of genetically modified mice and commercially available assays that are not available in other animal models. A mouse model therefore allows future mechanistic studies to build on this work.

FIGURE 1

A custom built grip holder was used to ensure that the Achilles tendon was slack between the grips during manipulation which prevented unwanted damage prior to testing.

FIGURE 2

Body weight increased with increasing age in post-natal mice. Each data point represents an individual mouse. All ages were significantly different from each other as demonstrated through a 1-way ANOVA with Tukey’s post hoc (results not shown on graph).

FIGURE 3

The cross-sectional area of the Achilles tendon increased throughout development. Vertical bars represent mean of the parameter and the error bars represent standard deviation. p < 0.05.

FIGURE 4

Representative load vs. displacement curves of the ramp to failure tensile test of mouse Achilles tendon. Curves demonstrate general trends seen during post-natal development.

FIGURE 5

Biglycan expression was initially high in 4 days old Achilles tendon but then rapidly decreased while decorin expression remained constant throughout development. Vertical bars represent mean of the parameter and the error bars represent standard deviation. p < 0.05.

FIGURE 6

Representative H&E stained slides from each age examined demonstrating little change in fiber organization throughout post-natal development in the Achilles tendon. All images were taken at ×20.

FIGURE 7

(a) Representative samples of collagen fibrils throughout post-natal development. Scale bar = 200 µm. (b) Histogram of fibril distribution demonstrating increased fibril diameter mean and spread throughout post-natal Achilles tendon development. a: significantly different from 28 days; b: significantly different from 21 days; c: significantly different from 14 day; d: significantly different from 10 days. Significance set at p ≤ 0.05.