Dark hair cells in the inner ear of the rainbow trout. A study of the influence of different fixation methods

The occurrence of dark staining cells in different tissues has been suggest ed to be artefactual and caused during the fixation process. In inner ear s ensory epithelia, dark hair cells (DHC) have been suggested to be apoptotic cells. We have examined whether dark cells represent dying cells or whethe r they are the results of fixation artefacts. The effects of buffer osmolar ity and different fixation methods on the incidence of dark hair cells in t he inner ear macula sacculi of the rainbow trout (Oncorhynchus mykiss) were investigated by light and electron microscopy. Glutaraldehyde in phosphate buffer with osmolarities of 0, 135, 225, 425, and 560 mosmol were used for fixation by immersion. For comparison, fixation by vascular perfusion as w ell as the effects of mechanical injury and delayed fixation were studied. DHC were found in all examined saccular maculae except for the delayed fixa tion protocol where almost all the sensory cells were lost. The number of D HC accounted for 2.5-12.9 parts per thousand of the sensory cells. Neither the buffer osmolarity nor the fixation method had significant effects on th e frequencies of DHC. Mitotic cell division events were seen exclusively in the apical cell strata of the sensory epithelium. The DHC are suggested to be associated with apoptosis rather than fixation artefacts.