Changes in the surface epithelium of the developing olfactory organ in the sand sole (Psettichthys melanastictus) and the rex sole (Glyptocephalus zachirus).

The olfactory organ of the sand sole (Psettichthys melanastictus) and the rex sole (Glyptocephalus zachirus) begins its development as a primary olfactory groove containing a sensory epithelium with two or more cell types. The groove becomes dumbbell shaped as its lateral margins grow towards each other and eventually fuse, resulting in the formation of an olfactory chamber with openings for the anterior and posterior nares. Dimensions of the primary olfactory groove are different between these two species. Fusion of the olfactory bridge occurs about the time of metamorphosis in the sand sole, but before metamorphosis in the rex sole. The sensory epithelium of the sand sole larva contains microvillar cells, ciliated type I cells, and ciliated type II cells at hatching, while the rex sole larva has only microvillar and ciliated type II cells. Ciliated type I cells were observed in rex sole only after the olfactory bridge had fused. The olfactory epithelium in both species is directly exposed to the fish's environment for some length of time before it becomes enclosed within an olfactory chamber.