The endolymphatic sac: a scanning and transmission electron microscopy study

Oxygenated fixation demonstrates novel and improved ultrastructural features of the human endolymphatic sac

OBJECTIVES/HYPOTHESIS

The purpose of the present study is to describe in detail the ultrastructure of the human endolymphatic sac using a new and improved method of fixation as well as a refined surgical approach in obtaining specimens.

STUDY DESIGN

Transmission electron microscopy of the human endolymphatic sac, employing an oxygenated fixative.

METHODS

Eighteen tissue samples of the human endolymphatic sac were obtained during surgery for vestibular schwannoma using the translabyrinthine approach. The specimens were fixed in 2% glutaraldehyde in an oxygenated fluorocarbon blood substitute vehicle before preparation by routine methods for transmission electron microscopy. We focused on the epithelial cell layer, subepithelial tissue, intraluminal content, and vascular tissue in both the intra- and extraosseous part of the endolymphatic sac.

RESULTS

We observed well-defined endolymphatic sac epithelial cell lining in all 18 specimens. In general, we found very well-preserved specimens with well-defined intracellular structures. In contrast to the results in former studies, a minimum of fixation artifacts was observed in the present study. Three different cell types were observed in the intraosseous part of the sac: mitochondria-rich cells, ribosome-rich cells, and nonclassifiable cells. A fourth cell type was found in the extraosseous part. Novel ultrastructural features of the epithelial lining and the subepithelial layer are described and discussed.

CONCLUSIONS

The results in the present study indicate an improvement in obtaining human tissue with optimal fixation for ultrastructural analysis and provide several novel morphologic observations. The potential functions of the endolymphatic sac are discussed with reference to former studies.

Lack of pendrin expression leads to deafness and expansion of the endolymphatic compartment in inner ears of Foxi1 null mutant mice

Mice that lack the winged helix/forkhead gene Foxi1 (also known as Fkh10) are deaf and display shaker/waltzer behavior, an indication of disturbed balance. While Foxi1 is expressed in the entire otic vesicle at E9.5, it becomes gradually restricted to the endolymphatic duct/sac epithelium and at E16.5 Foxi1 expression in the inner ear is confined to this epithelium. Histological sections, paintfill experiments and whole-mount hybridizations reveal no abnormality in inner ear development of Foxi1(-/-) mice before E13.5. Between E13.5 and E16.5 the membranous labyrinth of inner ears from null mutants starts to expand as can be seen in histological sections, paint-fill experiments and three-dimensional reconstruction. Postnatally, inner ears of Foxi1(-/-) mice are extremely expanded, and large irregular cavities, compressing the cerebellum and the otherwise normal middle ear, have replaced the delicate compartments of the wild-type inner ear. This phenotype resembles that of the human sensorineural deafness syndrome Pendred syndrome, caused by mutations in the PDS gene. In situ hybridization of Foxi1(-/-) endolymphatic duct/sac epithelium shows a complete lack of the transcript encoding the chloride/iodide transporter pendrin. Based on this, we would like to suggest that Foxi1 is an upstream regulator of pendrin and that the phenotype seen in Foxi1 null mice is, at least in part, due to defective pendrin-mediated chloride ion resorption in the endolymphatic duct/sac epithelium. We show that this regulation could be mediated by absence of a specific endolymphatic cell type--FORE (forkhead related) cells--expressing Foxi1, Pds, Coch and Jag1. Thus, mutations in FOXI1 could prove to cause a Pendred syndrome-like human deafness.