Morphological and physiological development of olfactory receptor cells in rainbow trout (Salmo gairdneri) embryos.

The morphological and functional differentiation of the olfactory receptor cells were investigated in developing rainbow trout (Salmo gairdneri) embryos by means of light and electron (transmission and scanning) microscopy and electrophysiology. Ciliated receptor cells first appeared when the olfactory placode was folded to form a groovelike structure rostrad to the eye at stage 24 (day 18; 18 days postfertilization). Ciliated receptor cells predominated until immature microvillar receptor cells developed in stage 28 (day 26) embryos. At stage 29, the day of hatching, the anterior edge of the olfactory epithelium contained only ciliated receptor cells, and the midregion contained both ciliated and microvillar receptor cells. Spontaneous neural firing activity was recorded from the olfactory mucosa as early as stage 25. The neural responses to amino acids were initially recorded from stage 26 embryos, containing sparse ciliated receptor cells with a few short cilia. The D-enantiomers of amino acids were less effective. From these results we concluded that in rainbow trout the olfactory receptor cell has two separate morphological forms, ciliated and microvillar. These are ontogenetically distinct; the ciliated receptor cells preceded the microvillar. The ciliated receptor cells respond to amino acid stimulation.