The calcified-noncalcified cartilage interface: the tidemark.

Tidemark is an interface which may better be defined by biochemical methods than by morphology. It originates, by chondrocyte activity, between calcified and noncalcified cartilage layers of any kind, hyaline or fibrous, in areas exposed to either loading (joint) or pulling (insertion). In the articular cartilage it appears with skeletal maturation, in other localizations it is age-independent. It should be regarded as a special instance of a broader phenomenon of the calcification/mineralization front. Inside the joint cartilage its changes reflect the slow remodelling of the calcified layer and its inapparent shift towards the surface of the articular cartilage. In the marginal transitional zone of the joint, tidemark smoothly passes into the periosteum. Chondrocytes on both sides of the tidemark are positive for alkaline phosphatase and the positive reaction continuously goes on to the periosteum.