Melanin in the inner ear: micromorphological and microanalytical investigations

Abstract

Melanin-pigments are found in various parts of the inner ear, especially in the neighbourhood of those epithelia that are believed to be involved in the secretion and/or absorption of endolymphatic fluid. Microprobe analysis (LAMMA, X-ray) measurements were performed in different parts of the inner ear in tissues containing melanin. The tissues were shock-frozen, freeze-dried and embedded in Earle's medium (Spurr). The semi-thin sections used for microprobe analysis were cut dry in a conventional ultramicrotome and mounted on copper grids. Experimental manipulation of the endolymph ionic composition (increased Na+) stimulated the migration of melanosomes from the perinuclear region into the dendritic processes and the rearrangement of the dendritic processes in a close vicinity of the presumably transporting epithelia. The intracellular ion measurements showed the affinity of divalent ions for melanin (Mg++, Ca++, Sr++ and Ba++). It is proposed that melanin represents a physiologically important "reservoir" for essential trace elements and by its binding/release may play a key role in the enzymatically controlled processes of ionic pumps.