Morphometric analysis of loading-induced changes in collagen-fibril populations in young tendons.

This study was designed to gain more detailed morphological information on skeletal tendons in the course of adaptation to physical loading. The effect on collagen fibrils was investigated in 6-week-old mice by means of electron microscopy. Physical loading was performed on a treadmill 5 days a week for 1, 3, 5, 7 and 10 weeks. Morphometric analysis of collagen fibrils revealed the mean diameter, the diameter distribution, the number and the cross-sectional area. The principal observations included: 1. After one week of physical loading an increase in mean fibril diameter (30%, p less than or equal to 0.01), in number (15%, p less than or equal to 0.05), and in cross-sectional area (15%, p less than or equal to 0.05), as well as a change in mean fibril diameter distribution. 2. From the third to the seventh week a fall under the level of the controls in mean diameter (26%, p less than or equal to 0.01), in number (26%, p less than or equal to 0.01), and a reduced cross-sectional area (17%, p less than or equal to 0.01), accompanied by signs of splitting of individual collagen fibrils. 3. In the long-term study an increase in fibril number (29%, p less than or equal to 0.01), a fall in mean diameter from 189 nm in the controls to 179 nm (p less than or equal to 0.05) but no statistically significant change in the relative cross-sectional area (32%) per unit in comparison to unloaded tendons. The possible physiological implications of the findings are discussed in the light of several regulatory mechanisms known to appear during the course of physical loading in connective tissues.