Energy dispersive x-ray analysis of inner ear fluids and tissues during the ontogeny of cochlear function.

Energy dispersive x-ray analysis (EDXA) was used to characterize freeze-dried, microdissected samples of inner ear fluids and tissues from neonatal gerbils throughout the period of cochlear functional development, between 10 and 18 days after birth. EDXA spectra from endolymph residue demonstrated that the adult ionic composition of this fluid is established before functional onset, and before the appearance of the endocochlear potential. However, stria vascularis spectra showed substantial increases in relative phosphorus and sodium content during the period in which auditory thresholds improve by approximately 100 dB, in which the endocochlear potential appears and increases to its adult value, and in which the level of metabolism of the stria increases dramatically. The increase in relative phosphorus content of stria in particular occurred with a time course which was very similar to, though slightly earlier than, that of the developmental increase in the endocochlear potential. It is concluded that increases in the relative phosphorus content of stria may represent increases in inorganic phosphate associated with strial metabolism. Increases in relative sodium content may reflect the action of an ion transport system which directs this element into strial cells during generation of the endocochlear potential.