The structure and development of the 'plana semilunata' of the labyrinth of the trout.

Five plana semilunata consisting of tall epithelial cells were found in the semicircular canal ampullae of the trout labyrinth. All these areas began to develop after hatching. Their cells were cuboid at fry stages and prismatic in juveniles and adults. Cell outlines were smooth, with no basal infoldings of the plasma membrane or apical microvilli. The cell nucleus occupied basal locations in juveniles but migrated centrally in adults. The most conspicuous organelle, the smooth endoplasmic reticulum, was continuous with a well-developed vacuolar system. These cells stored numerous glycogen granules. The ultrastructural features of the plana semilunata strongly suggest their involvement in endolymph secretion.