The tympanic membrane: a biochemical updating of structural components.

A cross-section of the tympanic membrane is discussed at the molecular level. Actually, the tympanic membrane can be described as a continuation of a vast number of structural proteins and molecular interactions. The epidermal layer is subject to soft keratinization, a differentiation process implying the genesis of a chemically resistant cell matrix (cytokeratins, filaggrin) and cornified membrane (involucrin, keratolinin, loricrin, transglutaminase). The epidermis is anchored in the connective tissue by means of hemidesmosomes whose biochemical nature begun to be elucidated rapidly in recent years. Of the first importance are the HD1 inner plaque protein, supposed to be associated with intermediate filaments, and the transmembrane components in the dense plaque (BPAG2 and alpha 6 beta 4 integrin), whose extracellular domains directly interact with basement membrane components. Though the complete characterization of the basement membrane has not yet been achieved, the molecular catalogue of its three distinct layers is really impressive (laminin, fibronectin, nidogen, kalinin, K-laminin, type IV collagen, heparan sulphate proteoglycan, type III and VII collagen, ...). Besides numerous mutual interactions, it appears that mainly laminin and kalinin (anchoring filaments) promote binding of epidermal cells. Furthermore, a continuation may exist between anchoring fibrils in the deepest layer and anchoring filaments in the upper layer of the basement membrane. Finally, the lamina propria, a specific type of extracellular matrix, is perhaps the most difficult to dissect, though substantial progress has been made in the last few years (fibronectin, type I, III, V and VI collagen, fibrillin, ...).