Development of the olfactory system in turbot (Psetta maxima L.).

We have examined the histogenesis of the olfactory system during turbot development using histological and immunohistochemical methods. Proliferating cell nuclear antigen (PCNA) immunohistochemistry was used to detect dividing cells, whereas calretinin (CR) immunohistochemistry was used to distinguish some neuronal components of the olfactory system. Around hatching, the olfactory placode of embryos transforms into an olfactory pit, which enlarges progressively during development. In metamorphic turbots, the right olfactory organ moves to the tip of the head. Each olfactory chamber opens to the external medium by two nostrils and accessory nasal sacs develop during metamorphosis. The order of birth of olfactory receptor cells in the sensory epithelium follows the pattern of most teleosts: ciliated cells differentiate prior to microvillous cells in turbot larvae, and crypt cells are generated during metamorphosis. Axons of olfactory sensory neurons reach the rostral forebrain by hatching, and calretinin-immunoreactive (CR-ir) glomerular fields were apparent during the subsequent larval development. During metamorphosis olfactory bulbs become strongly distorted by head torsion and glomeruli acquire asymmetric organization. The spatio-temporal course of proliferation in the olfactory system reveals changes in the distribution of dividing cells in the sensory epithelium throughout the developmental period investigated. In the olfactory bulb, proliferative activity becomes restricted to the ventral periventricular zone in turbot larvae, as well as in metamorphic specimens.