Abstract
The inner ear is a fluid-filled closed-epithelial structure whose function requires maintenance of an internal hydrostatic pressure and fluid composition. The endolymphatic sac (ES) is a dead-end epithelial tube connected to the inner ear whose function is unclear. ES defects can cause distended ear tissue, a pathology often seen in hearing and balance disorders. Using live imaging of zebrafish larvae, we reveal that the ES undergoes cycles of slow pressure-driven inflation followed by rapid deflation. Absence of these cycles in lmx1bb mutants leads to distended ear tissue. Using serial-section electron microscopy and adaptive optics lattice light-sheet microscopy, we find a pressure relief valve in the ES comprised of partially separated apical junctions and dynamic overlapping basal lamellae that separate under pressure to release fluid. We propose that this lmx1-dependent pressure relief valve is required to maintain fluid homeostasis in the inner ear and other fluid-filled cavities.
Highlights
IntroductionUnderstanding the mechanisms by which organs use water-filled cavities to compartmentalize biochemical and biophysical environments is a fundamental problem
We saw that endolymphatic sac (ES) morphogenesis begins at 36 hpf as an evagination in the dorsal epithelial wall of the otic vesicle (Figure 1—figure supplement 1A, Video 1, dorsal region or interest highlighted with blue box, Figure 1B)
Between 36-60 hpf, the ES grows, elongates, and moves into a more central and medial position due to morphogenesis of the otic vesicle epithelium. This position is consistent with the prior in situ patterns of ES markers. These findings established that the zebrafish ES is optically accessible during embryonic and larval stages and that ES morphogenesis begins at 36 hpf
Summary
IntroductionUnderstanding the mechanisms by which organs use water-filled cavities to compartmentalize biochemical and biophysical environments is a fundamental problem. Unstable hydrostatic pressure in brain ventricles is correlated with hydrocephaly and mental disorders (Hardan, Minshew, Mallikarjuhn, & Keshavan, 2001; Kurokawa et al, 2000). We have recently shown that hydrostatic pressure inflates and controls the size of the developing zebrafish ear (Mosaliganti et al, under review). Unstable pressure in the ear can cause deafness and balance disorders like Pendred syndrome and Meniere’s disease (Belal & Antunez, 1980; Schuknecht & Gulya, 1983). This theme of harnessing hydrostatic pressure for normal development and controlling pressure for healthy physiology raises the question of how tissues regulate pressure. Not much is known about how these tissues manage fluctuating pressures because they have not been observed in vivo
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