Bone Adaptability at the Tendon-Bone Insertion and Communication Pathways With Fibrocartilage

The functional integration of soft tissues into bone requires complex adaptation strategies [1]. Tendon and bone are both characterized by cells (tenocytes and osteocytes) that communicate extensively thanks to their dense networks: nanotubes in tendons and the osteocyte lacunocanalicular network (LCN) in bone. These networks serve multiple functions: nanotubes in tendon are believed to provide biomechanical stability [2] while the LCN is involved in bone mechanoresponsiveness [3] and mineralization [4]. At the interface between theses tissues lies the enthesis, a specialized insertion site often featuring fibrocartilage (FC), a fibrous form of cartilage reinforced with minerals before anchoring to bone. Our research focuses specifically on the Achilles tendon insertion, comparing enthesis with periosteal FC, two contiguous tissues sustaining different loading conditions: tension at enthesis and compression/shear at periosteal FC. We have previously revealed microstructural [5] and material heterogeneity [6] of mineralized FC. The aim of this study is to investigate the behavior of fibrochondrocytes (FC cells) and their physical interaction pathways with bone functional porosity. Rat samples were stained with rhodamine and then micro-computed tomography, quantitative backscattered electron imaging, second harmonic generation imaging and confocal laser scanning microscopy (CLSM) were combined on the same locations to highlight functional porosity at multiple length scales, as well as its link with mineral content and matrix organization. At the enthesis, we identified a connection path between trabecular bone marrow space and FC through the anisotropic subchondral channel network, perforating the bone-FC interface at several locations. Communication between bone and FC cells seems to be primarily indirect: osteocytes are highly connected to perforating channels through many canaliculi, these channels crossing the bone-FC interface and reaching the fibrochondrocyte rows. The striking absence of such connections at the adjacent periosteal region suggests that communication between bone and fibrocartilage may be needed for a proper enthesis biomechanical function.