Morphogenesis of otoliths during larval development in brook lamprey, Lampetra planeri

Otolith morphogenesis of the brook lamprey, Lampetra planeri, was analysed from larval to adult stages. The brook lamprey remains juvenile for about 4 years, facilitating analysis of otoliths maturation that permits to identify relevant evolutionary traits in this primitive species and to compare our results with more evoluted species of vertebrate taxa. We combined histochemical, immunohistochemical, scanning electron microscopy, elemental analysis and X-ray diffraction of lamprey otoliths to establish possible relationships between otolithic mass, individual crystals, the otolithic organic substance that binds individual otoconia together and the inorganic elements that mineralize the lamprey otoliths. Histochemical analysis of the otoliths suggests that mineralization occurs gradually, beginning near the apex of the secretory epithelium. Then, the otoconia increase in size by deposition of layers of a dense crystalline substance. Immunohistochemical reactivity of calcium binding proteins indicates that calmodulin, calbindin, S-100 and parvalbumin are parts of the uncalcified organic mass that holds otoconia together. Imaging of the immunoreactivity of each protein by Confocal Laser Scanning Microscopy in ammocoete at the first year of the larval stage shows weak reaction products which, however, gradually increase in intensity, with peak value in ammocoete at the fourth year of the larval stage.