Fine-structural characterization of the somatic innervation of the tympanic membrane in normal, sympathectomized, and neurotoxin-denervated rats.

The pars tensa of the rat tympanic membrane (TM) consists largely of a lamina propria of specialized unbanded collagen bounded on the outer surface by an unusually thin epidermal layer and on the inner surface by a flat, single-cell mucosal layer. The mucosal layer is innervated solely by unmyelinated (C) axons, whereas the cutaneous layer is supplied by both myelinated and C axons. The outer surface differs from general body skin, lacking dermal papillae, hairs, sweat glands, and distinctive dermal corpuscular structures. Epidermal innervation includes distinctive terminals in the basal layer, unassociated with Merkel cells, and deeper intraepidermal smaller endings containing accumulations of mitochondria and vesicles. The sensory nature of these endings can be inferred by their extensive, but not total, elimination following neonatal capsaicin treatment (a potent neurotoxin for thin sensory fibers) and their preservation following surgical or neurotoxin sympathectomy. The thin mucosal epithelium displays capillaries and beaded axons close to the free surface of the middle ear. The unmyelinated terminals contain predominantly large, dense-core vesicles (LDCVs). Capsaicin treatment results in extensive elimination of terminals containing LDCVs in surface epithelia. A possible small trophic influence of sensory thin-fiber supply was noted on the development of the epidermal layers. The sensory modalities elicited by natural stimulation of the TM is considered in relation to the pattern of innervation.