Structure of the Hagfish Inner Ear

Summary

The inner ear of the Atlantic hagfish Myxine glutinosa was examined by light microscopical serial sections as well as transmission and scanning electron microscopy. For comparison, ears from some Japanese and Pacific species have also been examined.

In all species, the labyrinth contains a large macula communis and two cristae. The macula is covered with numerous round statoconia, of different diameters. Various stages in the formation of statoconia can be seen in cells surrounding the macula.

The sensory epithelium of the macula may be divided in an anterior horizontal part, a middle vertical and a posterior horizontally positioned part. It consists of sensory hair cells and supporting cells. The hair cells are apically equipped with a bundle consisting of a kinocilium and 10–25 stereovilli, forming an organ pipe configuration. The ciliary axoneme consists of a ring of nine outer double microtubules, with no central microtubules.

Most of the apical cytoplasm of the hair cell is occupied by a cuticular plate. In some macular hair cells a filamentous structure can be seen in various parts of the cell. In the hair cell a roundish synaptic body is often seen, surrounded by vesicles, opposite to the afferent nerve endings. Cholinergic nerve fibres, supposed to be efferent nerve fibres, however, can be visualized in the sensory epithelia from cholinesterasestaining.

The cristae are ring-shaped and lack a proper cupula. Apically, the crista hair cells have very long kinocilia, up to 35 μm, and a bundle of stereovilli, arranged as on the macular hair cells. The presence of mitoses and apoptotic cells indicate a continuous turnover of the hair cells.