Long bones develop first from embryonic mesenchymal stem cells that coalesce to form a "blastema," with a scant but uniform type I collagen matrix. 1 The blastema transforms at early fetal stages into a cartilaginous structure, or cartilage "anlage," with collagen type II as the main extracellular component. 2-6 The cartilage "anlage" contains a mixture of fusiform and round chondroblast cells, 7 that upon terminal differentiation will hypertrophy (become unusually large), and begin to express proteins that attract blood vessels and facilitate biomineralization. 8 Mineralized bone begins to form when the fetal cartilage undergoes focal hypertrophy which launches a process of endochondral ossification (EO). The very first cartilage-bone interfaces to form in the body are at the primary ossification centers in the shaft of developing long bones. These areas are Keywords ► endochondral ossification ► tidemark ► calcified cartilage ► articular cartilage ► collagen type II ► collagen type I ► bone ► blood vessels ► growth plate ► glycosaminoglycan AbstractIn the knee joint, the purpose of the cartilage-bone interface is to maintain structural integrity of the osteochondral unit during walking, kneeling, pivoting, and jumpingduring which tensile, compressive, and shear forces are transmitted from the viscoelastic articular cartilage layer to the much stiffer mineralized end of the long bone. Mature articular cartilage is integrated with subchondral bone through a $20 to $250 µm thick layer of calcified cartilage. Inside the calcified cartilage layer, perpendicular chondrocyte-derived collagen type II fibers become structurally cemented to collagen type I osteoid deposited by osteoblasts. The mature mineralization front is delineated by a thin $5 µm undulating tidemark structure that forms at the base of articular cartilage. Growth plate cartilage is anchored to epiphyseal bone, sometimes via a thin layer of calcified cartilage and tidemark, while the hypertrophic edge does not form a tidemark and undergoes continual vascular invasion and endochondral ossification (EO) until skeletal maturity upon which the growth plates are fully resorbed and replaced by bone. In this review, the formation of the cartilage-bone interface during skeletal development and cartilage repair, and its structure and composition are presented. Animal models and human anatomical studies show that the tidemark is a dynamic structure that forms within a purely collagen type II-positive and collagen type I-negative hyaline cartilage matrix. Cartilage repair strategies that elicit fibrocartilage, a mixture of collagen type I and type II, are predicted to show little tidemark/calcified cartilage regeneration and to develop a less stable repair tissue-bone interface. The tidemark can be regenerated through a bone marrow-driven growth process of EO near the articular surface.

BackgroundIn this study we evaluated a novel approach to guide the bone marrow-driven articular cartilage repair response in skeletally aged rabbits. We hypothesized that dispersed chitosan particles implanted close to the bone marrow degrade in situ in a molecular mass-dependent manner, and attract more stromal cells to the site in aged rabbits compared to the blood clot in untreated controls.MethodsThree microdrill hole defects, 1.4 mm diameter and 2 mm deep, were created in both knee trochlea of 30 month-old New Zealand White rabbits. Each of 3 isotonic chitosan solutions (150, 40, 10 kDa, 80% degree of deaceylation, with fluorescent chitosan tracer) was mixed with autologous rabbit whole blood, clotted with Tissue Factor to form cylindrical implants, and press-fit in drill holes in the left knee while contralateral holes received Tissue Factor or no treatment. At day 1 or day 21 post-operative, defects were analyzed by micro-computed tomography, histomorphometry and stereology for bone and soft tissue repair.ResultsAll 3 implants filled the top of defects at day 1 and were partly degraded in situ at 21 days post-operative. All implants attracted neutrophils, osteoclasts and abundant bone marrow-derived stromal cells, stimulated bone resorption followed by new woven bone repair (bone remodeling) and promoted repair tissue-bone integration. 150 kDa chitosan implant was less degraded, and elicited more apoptotic neutrophils and bone resorption than 10 kDa chitosan implant. Drilled controls elicited a poorly integrated fibrous or fibrocartilaginous tissue.ConclusionsPre-solidified implants elicit stromal cells and vigorous bone plate remodeling through a phase involving neutrophil chemotaxis. Pre-solidified chitosan implants are tunable by molecular mass, and could be beneficial for augmented marrow stimulation therapy if the recruited stromal cells can progress to bone and cartilage repair.

In a challenging aged rabbit model, bone marrow-derived hyaline cartilage repair can be promoted by treating acute drill holes with a biodegradable subchondral implant that elicits bone plate resorption followed by anabolic WB repair within a 70-day repair period.

 Microparticle-depleted blood plasma failed to coagulate in plastic cups  Polyethylene-carboxylate L-PPE:O nanocoatings were created by glow-discharge plasma  Hydroxyapatite particles, glass microbeads, and L-PPE:O coatings had anionic surfaces  Anionic surfaces induced burst coagulation of microparticle-depleted plasma via FXII  Microparticles and anionic surfaces can activate thrombin without platelet activation

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