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embryos demonstrates that early BMP signaling during gastrulation induces four
transcription factors (Tfap2a, Tfap2c, Foxi1, and Gata3) that subsequently enable
PPR formation within the NNE (Saint-Jeannet and Moody 2014 ).
4.2.1 Formation of the Preplacodal Region
The PPR is a horseshoe-shaped portion of ectoderm around the anterior neural plate
and neural crest (Schlosser 2014 ). The PPR is named for the various sensory plac-
odes residing within it. These placodes give rise to a myriad of cell types and neu-
rons that form the cranial sensory ganglia and tissues (i.e., the lens, inner ear, and
olfactory epithelium) (Bailey and Streit 2005 ; Streit 2007 ). The PPR emerges
through attenuation of both BMP and Wnt signaling, in conjunction with increased
fibroblast growth factor (FGF) signaling, at the interface of the neuroectoderm and
NNE (Ahrens and Schlosser 2005 ; Litsiou et al. 2005 ; Esterberg and Fritz 2009 ;
Saint-Jeannet and Moody 2014 ). The role of reduced BMP signaling is supported
by studies which demonstrate that local decreases in BMP signaling expand the
PPR in both chick and Xenopus (Glavic et al. 2004 ; Ahrens and Schlosser 2005 ;
Litsiou et al. 2005 ). In zebrafish, this dissipation of BMP signaling may be facili-
tated by Dlx3-mediated expression of BMP antagonists, cv2 and Bambi-b (Esterberg
and Fritz 2009 ; Reichert et al. 2013 ). This BMP antagonism with concurrent FGF
signaling is sufficient for PPR emergence in zebrafish, chick, and frog (Ahrens and
Schlosser 2005 ; Esterberg and Fritz 2009 ; Saint-Jeannet and Moody 2014 ).
The changes in BMP and FGF signaling lead to the induction of the transcription
factor Six1 and its cofactor Eya1, the well-established markers for PPR formation
(Schlosser and Ahrens 2004 ; Ahrens and Schlosser 2005 ; Sato et al. 2010 ; Schlosser
2014 ). Confirming this embryologic role, overexpression of Six1 in Xenopus
increases the PPR at the expense of the neural crest and epidermis (Brugmann et al.
2004 ). In humans, mutations in Six1 and Eya1 cause the branchio-oto-renal syn-
drome, clinical features of which include ear anomalies and sensorineural hearing
loss (Ruf et al. 2004 ). Aside from members of the Six family, studies in zebrafish
and Xenopus implicate a network of additional transcription factors involved in PPR
specification including Tfap2a/c, Foxi1/3, and Gata2/3 (Litsiou et al. 2005 ; Pieper
et al. 2012 ; Bhat et al. 2013 ; Saint-Jeannet and Moody 2014 ).
4.2.2 Formation of the Otic Epibranchial Placode Domain
The PPR becomes further subdivided into thickened patches of ectoderm which
represent the various cranial placodes (Pieper et al. 2012 ). It is from these cranial
placodes that various cranial sensory organs, such as the inner ear, emerge (Litsiou
A.N. Elghouche et al.