Epithelial fusion during early semicircular canal formation in the embryonic zebrafish, Brachydanio rerio

Abstract

The developing inner ear of the teleost, Brachydanio rerio, provides an opportunity for observing an epithelial fusion between the apical surfaces of apposed epithelia in a vertebrate embryo in vivo. The developing otocyst was filmed for periods up to 4 days in unanesthetized embryos, and specimens were fixed at intervals and processed for light microscopy, TEM, and SEM. The semicircular canals are formed as a consequence of the union between the tips of three cylindrical projections from the wall of the otocyst, which grow toward corresponding bulges of a projection from the lateral wall. The epithelial cells covering the projections contain extensive rough endopasmic reticulum, exhibit apical junctional complexes, and are not underlain by a basal lamina. The core of each projection contains large amounts of flocculent and fibrillar extracellular material. After a period of growth and elongation, the tip of each projection contacts, and adheres to, the appropriate bulge to create a circular, flattened, bilayered, epithelial plate. Small, focal junctions form between the apposed apical cell surfaces within the plate during this period, but they are not numerous. Junctional complexes do develop, however, between apposed cells at the periphery of the plate. After 1-2 hours, the basal surface of the plate exhibit considerable alteration in contour. Adjacent cells within the plate then separate to allow continuity of the connective tissue components of the two structures. The observations of this study indicate that following an initial period of contact and adhesion, cellular reorientation and changes in junctional contacts between adjacent cells within the epithelial plate, rather than cell degeneration, are responsible for perforation of the plate.

Three‐dimensional morphology of inner ear development in Xenopus laevis

“…3G,H, lateral and medial views). The process of canal formation is roughly similar in other vertebrates, with pillars (zebrafish) or plates (birds and mammals) joining centrally to demarcate the canals (Knowlton, 1967;Waterman and Bell, 1984;Martin and Swanson, 1993;Haddon and Lewis, 1996). In the early stage 47 paintfilled specimens, the canals are distinct from the utricle but still tightly apposed to its surface.…”

Early Development of Hearing in Zebrafish

“…Compared with mammals (see a review by Rübsamen and Lippe 1998), the zebrafish is a unique model for the study of early development of hearing and balance in vertebrates because the inner ear of zebrafish embryos is easily accessible for live imaging, physiological recording, and genetic manipulation (Waterman and Bell 1984;Haddon and Lewis 1996;Nicolson et al 1998;Bever and Fekete 2002;Whitfield 2002;Nicolson 2005;Zeddies and Fay 2005;Tanimoto et al 2011). Previous studies have reported on the development of auditory sensitivity in a few juvenile and adult teleosts using various physiological methods (Corwin 1983;Kenyon 1996;Higgs et al 2003;Sisneros and Bass 2005;Vasconcelos and Ladich 2008;Alderks and Sisneros 2011;Lechner et al 2012).…”

Development of the zebrafish inner ear

“…Semicircular canal formation involves a complex reshaping of the ear epithelium, and has been described in detail in the zebrafish by Waterman and Bell (1984). No initial flattened pouch of presumptive semicircular canal tissue is seen, in contrast to the amniote ear .…”

“…The lateral projection later forms three bulges, pointing rostrally, caudally, and ventrally; these eventually contact the rostral, caudal, and ventral projections, respectively, forming fusion plates at the points of contact. Rearrangement of the epithelium here generates a continuous tube or pillar; cell death does not seem to be prevalent at the fusion plate (Waterman and Bell, 1984). The pillar can then be thought of as the hub of the developing semicircular canal, the canal itself being the fluid-filled toroidal space surrounding each hub.…”