Enthesis

The enthesis (plural entheses) is the connective tissue between tendon or ligament and bone.

There are two types of entheses: Fibrous entheses and fibrocartilaginous entheses.

In a fibrous enthesis, the collagenous tendon or ligament directly attaches to the bone.

In a fibrocartilaginous enthesis, the interface presents a gradient that crosses four transition zones:

1. Tendinous area displaying longitudinally oriented fibroblasts and a parallel arrangement of collagen fibres
2. Fibrocartilaginous region of variable thickness where the structure of the cells changes to chondrocytes
3. Abrupt transition from cartilaginous to calcified fibrocartilage—often called 'tidemark' or 'blue line'
4. Bone