Outside the striola, lies the extrastriola, which is populated primarily by type II hair cells in birds. In birds, a type I hair cell rich zone lies within a unique area, termed the “striola” ( Werner, 1933), which also differs in receptor cell density and otoconia formation ( Jörgensen and Andersen, 1973 Rosenhall, 1970 Si et al., 2003 Zakir et al., 2003). The different hair cell types are not heterogeneous throughout the maculae, but instead exhibit regional distributions that have been characterized in mammals ( Desai et al., 2005 Fernandez et al., 1990 Lysakowski and Goldberg, 2008), amphibians ( Baird and Schuff 1994), reptiles ( Xue and Peterson 2006 Severinsen et al., 2003) and birds ( Jørgensen and Anderson 1973 Rosenhall 1970 Si et al., 2003 Zakir et al., 2003).
Type I hair cells are innervated by an enclosing calyceal terminal while type II hair cells are innervated by bouton terminals ( Wersäll, 1956). Otolith receptors in amniotes contain two types of hair cells and three afferent types with distinct morphological innervation patterns. These studies were conducted in quail, a non-altricial species, where a rapid embryogenesis (16 days incubation) of a functional vestibular system must occur so that upon hatching, regulation of posture, gaze, and spatial orientation is intact. Here, we performed a comprehensive morphological examination of otolith receptor formation and maturation into adulthood. 2003 Sienknecht and Fekete 2008 Oh et al. 1993), or the molecular genetics underlying macular formation and hair cell fate ( Bever et al. 1984), otoconia formation ( Blasiole et al. Most studies have focused upon either stereocilia ( Denman-Johnson and Forge 1999, Kido et al. However, little is known about otolith system development. These are essential functions, about which all coordinated and reflexive movements, as well as spatial orientation and navigation through space depend.
The vestibular otolith receptors have evolved to detect linear motion and head position with respect to gravity. Bouton fibers had large innervation fields, with arborous branches and many terminal boutons. Calyx fibers were the least complex, followed by dimorph units. Calyx and dimorph afferents were primarily confined to the striolar regions, while bouton fibers were located in the extrastriola and type II band. Finally, the topographic organization of afferent macular innervation in the adult quail utricle was quantified. Calyx afferents innervating only type I hair cells did not develop until E14. Calyceal terminal formation began at E10, however no mature calyces were observed until E12, when all fibers appeared to be dimorphs. Initial innervation of the maculae was by small fibers with terminal growth cones at E6, followed by collateral branches with apparent bouton terminals at E8. Immunohistochemistry and neural tracing techniques were employed to examine the shape and location of the striolar regions. Less than half of all immature hair cells observed had non-polarized internal kinocilia with the remaining exhibiting planar polarity. Stereocilia polarization was initiated early, with defining reversal zones forming at E8. Increases in hair cell density were dependent upon macular location striolar hair cells developed first followed by hair cells in extrastriola regions. The otolith maculae epithelial areas increased exponentially throughout embryonic development reaching asymptotic values near post-hatch day P7. Here we describe epithelial growth, hair cell density, stereocilia polarization, and afferent nerve innervation during development. The present study examined the morphological development of the otolith vestibular receptors in quail.
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