There was an error in Development 139, 245-257.On p. 252, in the second paragraph of the section 'Establishing gradients and boundaries in the developing cochlea', it should read 'In adults…from high (basal) to low (apical).'The authors apologise to readers for this mistake. Development 139, 826 (2012) so is not yet available in most cases. This review will summarize recent findings on the identity of these signaling and morphogen molecules and how they are coordinated in space and time to generate the tissue and cellular architecture of the ear.
From placode to ear -how graded signals turn ectoderm into the inner earThe entire inner ear and its associated sensory ganglia derive from the otic placode (see Glossary, Box 1). It is one of a series of cranial placodes that collectively give rise to all craniofacial sensory organs, the lens and a subset of cranial ganglia (Baker and Bronner-Fraser, 2001;Schlosser, 2010). The precursors of different placodes are distributed in an anterior-posterior (AP) sequence around the rostral neural plate (see Glossary, Box 1) termed the pre-placodal domain (PPD) (Schlosser, 2006;Streit, 2007). The PPD is induced by signals, such as FGFs, from both the neural plate and the underlying cranial mesoderm, and inhibition of both Wnt and BMP signaling is required to correctly position the PPD at the neural plate border and to prevent it from extending into the embryonic trunk (Kwon et al., 2010;Litsiou et al., 2005).It is now well established that FGF signals are both necessary and sufficient to induce early otic placode markers from competent pre-placodal ectoderm (Fig. 2) (Ohyama et al., 2007;Schimmang, 2007), although both the location of the FGF signals and the specific FGFs responsible for otic induction vary from species to species. The earliest markers of the future inner ear that are induced in response to FGF signaling are members of the paired homeoboxcontaining Pax2/5/8 transcription factor family -Pax8 in anamniotes and Pax2 in amniotes (Groves and Bronner-Fraser, 2000;Heller and Brandli, 1999;Ohyama and Groves, 2004;Pfeffer et al., 1998). Fate-mapping studies in mice and chick have suggested that although the Pax2-expressing domain is likely to contain all the progenitors of the inner ear, it also harbors cells that will give rise to the epibranchial placodes (see Glossary, Box 1) and epidermis (Ohyama and Groves, 2004;Ohyama et al., 2007;Streit, 2001). This Pax2-expressing progenitor domain has been termed the pre-otic field or the otic-epibranchial progenitor domain (OEPD) (Ladher et al., 2000;Freter et al., 2008).The refinement of the OEPD into distinct otic, epibranchial and epidermal territories is regulated by graded Wnt signals emanating from the midline (Fig. 2) (Ohyama et al., 2006). Examination of the cranial region of Wnt reporter mice revealed that OEPD cells closest to the neural plate, which are fated to form the otic placode, receive high levels of Wnt signaling, whereas more lateral OEPD cells, fated to form epidermis and the epibranchial placodes, receiv...
