Postnatal maturation of tendon, cruciate ligament, meniscus and articular cartilage: a histological study in sheep.

Orthopaedic basic science data on immature skeletons are rare in the literature. Since the number of knee injuries in young humans is steadily increasing, studies on immature animals such as sheep, which can be used as model systems are becoming more and more important. However, no baseline data are available on physiologic and morphologic changes during growth in the relevant tissues. In the present study, histomorphometric changes in the tendon of the musculus flexor digitalis superficialis, the cranial cruciate ligament, the medial meniscus and the articular cartilage of the medial femoral condyle were identified in sheep between the ages of 1 and 40 weeks postnatally. Profound changes in tissue composition during growth could be observed. A high cellularity in the early postnatal period decreases to a constant lower level after 18 weeks. Similar changes during postnatal growth could be observed for blood vessel density. Also, staining of alpha-smooth muscle actin (SMA) and vascular endothelial growth factor (VEGF) steadily decreased. In contrast, the number of components of extracellular matrix steadily increased in all tissues. The age of 18 weeks seems to be a threshold after which the tissue composition of the observed structures remains constant in this species.