Structure and function of the tympanic membrane: a review.

Advances made in the last three decades of research on morphological, cell biological, and immunobiological aspects of the tympanic membrane of laboratory animals and humans are reviewed. The tympanic membrane is composed of the pars flaccida and pars tensa with considerable variations in their size and thickness. Both pars flaccida and pars tensa consist of an epidermal layer, a lamina propria, and a mucosal epithelial layer. The fibrils of the fibrous layer contain a large amount of type II and type III collagen, and a small amount of type I collagen. It has been suggested that such an unusual collagen composition is the underlying reason for the unique physical feature of the pars tensa of the tympanic membrane. A large number of mast cells are found in the Shrapnell's membrane and these cells are suggested to be responsible for the production of middle ear effusion. The cellular basis for the epidermal migration and the role of epidermal and fibroblast growth factors in epidermal cell proliferation and in wound healing are also reviewed. Future research directions on tympanic membrane biology and pathobiology are discussed.