Part I: Development and Physiology of the Temporomandibular Joint.

PURPOSE OF REVIEW: Investigate the developmental physiology of the temporomandibular joint (TMJ), a unique articulation between the cranium and the mandible. RECENT FINDINGS: Principal regulatory factors for TMJ and disc development are Indian hedgehog (IHH) and bone morphogenetic protein (BMP-2). The mechanism is closely associated with ear morphogenesis. Secondary condylar cartilage emerges as a subperiosteal blastema on the medial surface of the posterior mandible. The condylar articular surface is immunoreactive for tenascin-C, so it is a modified fibrous periosteum with an underlying proliferative zone (cambrium layer) that differentiates into fibrocartilage. The latter cushions high loads and subsequently produces endochondral bone. The TMJ is a heavily loaded joint with three cushioning layers of fibrocartilage in the disc, as well as in subarticular zones in the fossa and mandibular condyle. The periosteal articular surface produces fibrocartilage to resist heavy loads, and has unique healing and adaptive properties for maintaining life support functions under adverse environmental conditions.

Extracellular matrix bioscaffolds in tissue remodeling and morphogenesis.

During normal morphogenesis the extracellular matrix (ECM) influences cell motility, proliferation, apoptosis, and differentiation. Tissue engineers have attempted to harness the cell signaling potential of ECM to promote the functional reconstruction, if not regeneration, of injured or missing adult tissues that otherwise heal by the formation of scar tissue. ECM bioscaffolds, derived from decellularized tissues, have been used to promote the formation of site appropriate, functional tissues in many clinical applications including skeletal muscle, fibrocartilage, lower urinary tract, and esophageal reconstruction, among others. These scaffolds function by the release or exposure of growth factors and cryptic peptides, modulation of the immune response, and recruitment of progenitor cells. Herein, we describe this process of ECM induced constructive remodeling and examine similarities to normal tissue morphogenesis.

Sensitivity of notochordal disc cells to mechanical loading: an experimental animal study.

The immature disc nucleus pulposus (NP) consists of notochordal cells (NCs). With maturation NCs disappear in humans, to be replaced by chondrocyte-like mature NP cells (MNPCs); this change in cell phenotype coincidences with early signs of disc degeneration. The reasons for NC disappearance are important to understand disc degeneration, but remain unknown, yet. This study investigated, whether loading induced a change from a notochordal nucleus phenotype to a chondrocyte-like one. An in vivo disc compression model with fixateur externe was used in 36 mature rabbits. Discs were compressed for different time periods (1, 28, 56 days), and compared with uncompressed control discs (56 days without treatment), and discs with sham compression (28 days). Nucleus cell phenotype was determined by histology and immunohistochemistry. NCs, but not MNPCs highly expressed bone-morphogenetic-protein 2 and cytokeratin 8, thus NC and MNPC numbers could be determined. A histologic score was used to detect structural endplate changes after compression (28 days). Control and sham compressed discs contained around 70% NCs and 30% MNPCs, to be decreased to <10% NCs after 28-56 days of loading. NC density fell sharply by >50% after 28-56 days of compression (P < 0.05 vs. controls). Signs of decreased endplate cellularity and increased endplate sclerosis and fibrosis were found after loading. These experiments show that NCs were less resistant to mechanical stress than MNPCs suggesting that increased intradiscal pressures after loading, and limited nutrition through structurally altered endplates could instigate the disappearance of NCs.

Development of the human Achilles tendon enthesis organ.

The attachment of the Achilles tendon is part of an 'enthesis organ' that reduces stress concentration at the hard-soft tissue interface. The organ also includes opposing sesamoid and periosteal fibrocartilages, a bursa and Kager's fat pad. In addition, the deep crural and plantar fasciae contribute to Achilles stress dissipation and could also be regarded as components. Here we describe the sequence in which these various tissues differentiate. Serial sections of feet from spontaneously aborted foetuses (crown rump lengths 22-322 mm) were examined. All slides formed part of an existing collection of histologically sectioned embryological material, obtained under Spanish law and housed in the Universidad Complutense, Madrid. From the earliest stages, it was evident that the Achilles tendon and plantar fascia had a mutual attachment to the calcaneal perichondrium. The first components of the enthesis organ to appear (in the 45-mm foetus) were the retrocalcaneal bursa and the crural fascia. The former developed by cavitation within the mesenchyme that later gave rise to Kager's fat pad. The tip of the putative fat pad protruded into the developing bursa in the 110-mm foetus and fully differentiated adipocytes were apparent in the 17-mm foetus. All three fibrocartilages were first recognisable in the 332-mm foetus--at which time adipogenesis had commenced in the heel fat pad. The sequence in which the various elements became apparent suggests that bursal formation and the appearance of the crural fascia may be necessary to facilitate the foot movements that subsequently lead to fibrocartilage differentiation. The later commencement of adipogenesis in the heel than in Kager's pad probably reflects the non-weight environment in utero. The direct continuity between plantar fascia and Achilles tendon that is characteristic of the adult reflects the initial attachment of both structures to the calcaneal perichondrium rather than to the skeletal anlagen itself.