Epithelial-Mesenchymal Transition as a Possible Mechanism of Semicircular Canal Morphogenesis in Chick Inner Ear

Abstract

Semicircular canals are sensory organs for balance, consisting of fluid-filled tubules that are arranged perpendicularly to each other in inner ear. The precise mechanism of the morphogenesis of this unique organ is still under investigation. Semicircular canals arise from the flattened pouch of epithelium. The centers of two apposing wall of the pouch approach each other and form a fusion plate. The clearing of the fusion plate makes a hole and leaves the remaining tissue as semicircular canals. Three mechanisms have been proposed for this clearing: programmed cell death, epithelial-mesenchymal transition, and retraction of the cells in the fusion plate to surrounding semicircular canals. Previous studies have revealed programmed cell death in the fusion plate, although other two hypotheses were not disproved. Here we examined the contribution of epithelial-mesenchymal transition and epithelial retraction to the morphogenesis of semicircular canals. We analyzed immunohistochemically the structural change in the epithelium of the developing fusion plate using molecular markers, basal lamina component laminin, cytoskeletal F-actin, and cellular junctional marker β -catenin. Our observation revealed that fusion plate epithelium lost its apico-basal polarity and intermingled with facing fusion plate cells, associated with the disruption of basal lamina. Moreover, there were several cells with mesenchymal appearance adjacent to the torn basal lamina. We also found the merging of apposing basal laminae at the border between forming canal and breaking fusion plate. These observations suggest that the epithelial-mesenchymal transition, rather than the epithelial retraction, may be responsible for clearing fusion plate cells. laminin; β -catenin; F-actin; basal lamina; epithelial-to-mesenchymal transition. Tohoku J. Exp. Med., 2008, 215 (3), 207-217. © 2008 Tohoku University Medical PressThe inner ear arises from the otic placode which is a patch of thickening epithelial tissue in the head ectoderm adjacent to the hindbrain. The otic placode invaginates to become the otic vesicle which is a simple fluid-filled sphere of epithelium. The otic vesicle changes its shape to a complex inner ear that has the cochlea, the vestibule and the three semicircular canals (Bissonnette and Fekete 1996;Torres and Giraldez 1998). Mechanisms of placode induction and regionalization within the otic vesicle have been focused recently in various model organisms (Ozaki et al.

Semicircular canal morphogenesis in the zebrafish inner ear requires the function ofgpr126(lauscher), an adhesion class G protein-coupled receptor gene

“…In the chick and mouse, flattened pouches are formed from the otocyst, rather than finger-like projections: it is the sides of these pouches, rather than the tips of the fingers, that recognize and fuse with each other (Martin and Swanson, 1993; Bissonnette and Fekete, 1996). The resultant fusion plate is thus much larger than that in Xenopus or zebrafish, and cell death is thought to be the primary mechanism for clearance of cells in this region, at least in the chick (Fekete et al, 1997; Lang et al, 2000; Cecconi et al, 2004; Kobayashi et al, 2008). In addition, the dfna5 / ugdh /HA pathway, which is implicated in semicircular canal projection outgrowth in the fish, does not appear to be conserved in the mouse (Busch-Nentwich et al, 2004; Van Laer et al, 2005).…”

“…Establishment of the zones of fusion and non-fusion in the mouse ear involves an antagonistic interaction between Lrig3 and Netrin1 (Salminen et al, 2000; Abraira et al, 2008), and regulation by Wnt signalling (Noda et al, 2012; Rakowiecki and Epstein, 2013). Resolution at the fusion plate in mouse, chick and Xenopus involves breakdown of the basal lamina (Haddon and Lewis, 1991; Salminen et al, 2000; Abraira et al, 2008), epithelial-to-mesenchymal transition (Salminen et al, 2000; Kobayashi et al, 2008) and cell death (Haddon and Lewis, 1991; Fekete et al, 1997; Cecconi et al, 2004). In the zebrafish, however, cell death does not appear to play a major role (Waterman and Bell, 1984; Fekete et al, 1997).…”

Embryonic developmental study on inner ear of Columba livia domestica (the early, intermediate stages)

“…In the present work, the formation of the three semicircular ducts initiates from paired outpocketings of otic epithelium (33mm embryo stage) then the two epithelial layers on the opposing walls become thinner and approach each other to form a fusion plate. 2000 ) and Kobayashi, ( 2008) on the Gallus , fowl and mouse . They described the same steps of the formation of the ducts but in the process of removing the fusion plate cells mechanism, there was a difference.…”

“…Comparative data on hearing-organ structure are available from the different vertebrates (Alsina et al, 2009). (Manley, 1990;Fay & Popper, 2002;Cantos et al, 2004;and kobayashi, 2008) worked on the inner ear chick, and reptiles. Inner ear lineages are also of interest in the context of evolutionary homologies.…”

Signaling regulating inner ear development: Cell fate determination, patterning, morphogenesis, and defects

“…The dorsal pouch gives rise to two superior (vertical) ducts, the anterior and posterior semicircular ducts, while the lateral pouch gives rise to the lateral (horizontal) semicircular duct. At the onset of semicircular duct formation, the opposing epithelia in the center of each pouch fuse to form the epithelial fusion plate, which is thereafter resorbed by epithelial-mesenchymal transition and apoptosis, resulting in the formation of tube-like semicircular ducts (Kobayashi et al 2008). Each semicircular duct connects with the utricle, and the two superior (anterior and posterior) ducts possess a common duct, the crus membranous commune.…”