This report provides a description of the normal developing inner ear of the zebrafish, Danio rerio, with special focus on the pars inferior. Zebrafish specimens, ranging in age from 3 to 30 days postfertilization (dpf), were processed for standard histologic sections or with a paint-fill method to show three-dimensional morphogenesis of the membranous labyrinth. Adult zebrafish (age 2 years) were also processed for inner ear paint-fills. Although development of the semicircular canals occurs rapidly (by 3 dpf), the pars inferior develops more gradually during days 5-20 postfertilization. A rudimentary endolymphatic duct emerges by 8 dpf. Differentiated hair cells of the lagenar macula are evident by 15 dpf, in a chamber located lateral and posterior to the saccule. By 20 dpf, the saccule itself is separated from the utricle, but remains connected by means of the utriculosaccular foramen. The maculae neglectae, each with differentiated hair cells, lie on the floor of the utricle near this foramen. A medial connection between the sacculi of right and left ears, the transverse canal, is also complete by 20 dpf. A ridge of mesenchyme, previously undescribed, bisects the saccule in zebrafish fry at 20 -30 dpf. The images in the paint-fill atlas should provide a baseline for future studies of mutant zebrafish ears.

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Chaperone proteins are considered to be fairly ubiquitous proteins that promote the correct folding and assembly of multiple newly synthesized proteins. While performing an embryonic screen in zebrafish using morpholino phosphorodiamidate oligonucleotides (MPOs), we identified a role for an endoplasmic reticulum chaperone protein family member, zebrafish GP96. Knockdown of GP96 resulted in a specific otolith formation defect during early ear development. Otolith precursor particles did not adhere to the kinocilia of the tether cells in the GP96-MPO-injected embryos, aggregating instead into a single clump. Although otolith development was abnormal, the patterning of the ear and the differentiation of tether cells and macular sensory and support cells was not affected. We have isolated and sequenced the full open reading frame of zebrafish GP96 and characterized its expression pattern. GP96 is expressed both maternally and zygotically. GP96 RNA is localized within the floorplate, hatching gland, and in the cells of the otic placode and otic vesicle, consistent with the function of GP96 in ear development. We conclude that the GP96 chaperone protein is involved in the otolith formation during normal ear development. This is the first report of a specific function during organism development being attributed to a chaperone class molecule.

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Mystery snails (Family Ampullariidae) are aquatic prosobranchs which possess structurally complex eyes at the tip of a cephalic eyestalk. No other sensory organs are found in association with this stalk. These snails possess the ability to regenerate the eye completely after amputation through the mid-eyestalk. Amputation induces gross changes in the cellular character of the entire eyestalk; in particular, an invagination of integumentary epithelium at the apex of the eyestalk stump produces a shallow cleft or "eyecup." Differentiation of all components of the eye apparently occurs by transdetermination of these epithelial cells. Retinal differentiation and the appearance of a new lens is observed as soon as 14 days postamputation. Complete eyes (by external observation), although smaller than the originals, have regenerated by 25 days postamputation. We compare this regeneration to the reconstruction of other animal tissues, in particular the regeneration of amphibian limbs.