Effect of postnatal age on the ultrastructure of six anatomical areas of canine flexor digitorum profundus tendon

Abstract

We report findings of a transmission electron microscopic study comparing the morphological appearance of cells and extracellular matrix of two fibrocartilaginous regions of canine flexor digitorum profoundus (FDP) tendon with that for typical tendinous regions. In addition, we determined the size distribution of collagen fibrils in six anatomical areas of the tendon from animals of three different ages. Average collagen fibril diameters for each of the six anatomical sites of 11‐week‐old FDP tendon were consistently different from that for older tissue. As growth proceeds, fibrils in tendinous regions almost double in size and take on a broad bimodal distribution. Collagen diameters in fibrocartilaginous areas do not increase, but rather decrease in size with age. Finally, the cells and associated pericellular matrix of fibrocartilaginous areas of adolescent and mature FDP tendon are ultrastructurally distinct from those of typical tendinous regions. On the contrary, the cellular morphology of 11‐week‐old tendon was invariant regardless of the anatomical region examined. In summary, fibrocartilage of canine FDP tendon, although not evident at 11 weeks of age, is well established by 6–12 months after birth and is the result of cellular and extracellular matrix specialization.