The membranous labyrinth during larval development in lamprey (Lampetra planeri, Bloch, 1784)

SEM and CLSM studies were performed on the membranous labyrinth of Lampetra planeri, a threatened species of brook lamprey, spanning from the 1st to the 4th year of ammocoetes larval stages and on the adults. In all the examined stages, the entire membranous labyrinth does not show any morphologic differences, but only a progressive increase in size. SEM and CLSM observations show that the ciliated chamber is lined with numerous unsensorial multiciliated cells. In the early stages, the ciliary bundles were approximately 15 μm long, while in the late stages they reached 30 μm. In the crista sensory area, we observed two populations of hair cells. “Type II” cells are peculiar for this species and show both long stereocilia decreasing in length and a long kinocilium (10-12 μm). Two other types of ciliary bundles have been found on the sensory hair cells of the Macula communis: the first one has both kinocilium and stereocilia about 4-5 μm long; the second shows a long kinocilium (7-10 μm in length) and short stereocilia bundles with a gradual increase in length. In the early stages of development, the three macular areas show few and sparsely distributed hair cells. In the late developmental stages, hair cells become more numerous and densely populated.