Tendons slow down as they age

Muscles are anchored to bones via fibrous structures called tendons, which are made from specialized cells and large proteins called collagens. Tendons in newborns have round cells that can easily divide to make new cells, allowing the tissue to grow. Compared to newborns, tendon cells in adults have a star-like shape and appear to have stopped dividing. Tendon cells in adults are less abundant than in newborns, with more of the tendon made up of collagen. Adult tendons also do not heal as well as young tendons following an injury. However, it was previously unknown when the characteristics of young tendons are lost. Additionally, it was unclear whether adult tendon cells completely stop dividing or simply do so more slowly, or if these changes in cell division are what causes adult tendons to heal less easily.

To investigate this, Grinstein et al. used microscopy and cellular and molecular tools to examine tendon cell division in mice of different ages. The results indicated that tendon cells continue to divide after birth, but they do so more slowly as mice age. The researchers saw the most significant changes in the mice after they reached two weeks of age, which is also when the structure of the tendons begins to transition to that of an adult.

Young mice, which are growing and learning to move, have tendons that produce new cells faster than adult tendons. This may be necessary for young tendons to grow and adapt to the strains of movement. As tendons age, most of their cells lose the ability to divide, which seems to prevent the tendons from healing fully after injury. Notably, Grinstein et al. found that some cells are still able to divide slowly in adult tendons, which may be important for healing.

Further research is needed to examine the mechanisms involved in these changes and the factors that drive them, but a deeper understanding of tendon biology could lead to new therapies for treating tendon injuries.