Vesicular bodies in fish maculae are artifacts not contributing to otolithgrowth

The presence, morphology and possible origin of vesicle-like bodies (VBs) w ithin the inner ear macular otolithic membrane of developmental stages of c ichlid fish Oreochromis mossambicus and neonate (i.e. functionally fully de veloped except the reproductive organs) swordtail fish Xiphophorus helleri were analyzed by means of transmission and scanning electron microscopy (TE M and SEM, respectively) employing various fixation procedures. Some author s believe that these VBs are involved in the formation of the organic phase of inner ear otoliths (or statoliths in birds and mammals). Decreasing the osmolarity of the fixation medium from a value rather close to that of nat ive fresh water fish tissue (i.e. 250 mOsm and 290-300 mOsm, respectively) to a value of fixatives mostly employed in TEM studies (ca. 190 mOsm), the amount of VBs increased and the components of sensory inner ear tissue incr easingly dilated. Whilst a conventional prefixation with aldehydes followed by osmium tetroxide postfixation yielded numerous VBs, only few of them we re observed when the tissue was fixed with aldehydes and osmium tetroxide s imultaneously. Therefore, the results demonstrate that inner ear sensory ep ithelia are extremely sensitive to altering fixation media. On this backgro und it must be concluded that VBs are fixative (i.e. glutaraldehyde) induce d artificial structures, so-called membrane blisters. Thus, the protein mat rix of otoliths (and possibly that of statoliths in higher vertebrates) is rather provided by secretion processes than by the release of vesicles. (C) 2001 Elsevier Science B.V. All rights reserved.