AN IMMUNOHISTOCHEMICAL STUDY OF ENTHESIS DEVELOPMENT IN THE MEDIAL COLLATERAL LIGAMENT OF THE RAT KNEE-JOINT

The changing distributions of collagens and glycosaminoglycans have be en studied at the attachments of the medial collateral ligament during postnatal development The ligament is of particular interest because it has a fibrocartilaginous attachment to the femeoral epiphysis, but a fibrous one to the tibial metaphysis. Ligaments were examined in rat s killed at birth and at 2, 4, 6, 8, 10, 20, 30, 45, 60, 90 and 120 da ys after birth. Cryosections were immunolabelled with monoclonal and p olyclonal antibodies against types I and II collagen, chondroitin 4 an d 6 sulfate, dermatan and keratan sulfate. Although the ligament is at tached at both ends to bones that develop from cartilage, there was a striking difference in collagen labelling. Type II collagen was only f ound in spicules of calcified cartilage in bone beneath the tibial ent hesis after ossification had commenced, but there was a continuous ban d of labelling at all stages of development at the femoral enthesis. I nitially, the cartilage at the femoral attachment lacked type I collag en, but hv 45 days labelling was continuous from ligament to bone. Con tinuity of labelling was seen much earlier at the tibial enthesis, as soon as bone had formed. There were also marked changes in glycosamino glycan distribution. Keratan sulfate was present at both entheses up t o 45 days, but only at the femoral enthesis thereafter. Both attachmen ts labelled throughout life for dermatan sulfate, but chondroitin 4 an d 6 sulfate were only found at the femoral end The results suggest tha t enthesial cartilage at the femoral attachment was initially derived from the cartilaginous bone rudiment but was quickly eroded on its dee p surface by endochondral ossification as bone formed at the attachmen t site. It was replaced by fibrocartilage developing in the ligament. This mechanism allows enthesis cartilage/fibrocartilage to contribute to the growth of a bone at a secondary centre of ossification in addit ion to dissipating stress at the ligament-bone junction.