Establishing the pre-placodal region and breaking it into placodes with distinct identities

Abstract

Specialized sensory organs in the vertebrate head originate from thickenings in the embryonic ectoderm called cranial sensory placodes. These placodes, as well as the neural crest, arise from a zone of ectoderm that borders the neural plate. This zone separates into a precursor field for the neural crest that lies adjacent to the neural plate, and a precursor field for the placodes, called the pre-placodal region (PPR), that lies lateral to the neural crest. The neural crest domain and the PPR are established in response to signaling events mediated by BMPs, FGFs and Wnts, which differentially activate transcription factors in these territories. In the PPR, members of the Six and Eya families, act in part to repress neural crest specific transcription factors, thus solidifying a placode developmental program. Subsequently, in response to environmental cues the PPR is further subdivided into placodal territories with distinct characteristics, each expressing a specific repertoire of transcription factors that provides the necessary information for their progression to mature sensory organs. In this review we summarize recent advances in the characterization of the signaling molecules and transcriptional effectors that regulate PPR specification and its subdivision into placodal domains with distinct identities.

Sensory evolution in blind cavefish is driven by early embryonic events during gastrulation and neurulation

“…5F,G). This finding is distinct from the well-established early role of Bmp signaling in the specification of the neural plate border region/preplacodal region, which involves Bmp2/4/7 activity from the ventral side of the embryo at late blastula stages and the subsequent inhibition of Bmp signaling from the epidermis by Bmp inhibitors during gastrulation (Ahrens and Schlosser, 2005;Kwon et al, 2010;Nguyen et al, 1998;Reichert et al, 2013;Saint-Jeannet and Moody, 2014). In chick, a direct role for Bmp signaling in placodal precursor specification at late gastrula stage, as well as a role for the time of Bmp exposure in the decision to follow a lens versus an olfactory fate, have been described (Sjödal et al, 2007).…”

Early development of the nervous system of the eutherian <i>Tupaia belangeri</i>

“…(neurogenic) placodes which segregate from the U-shaped panplacodal primordium (Streit, 2007;Schlosser, 2010;Saint-Jeannet and Moody, 2014). This hypothesis could be fully confirmed on the example of the trigeminal placode (Obermayer, 2009;Knabe et al, 2009: Fig.…”

Vertebrate Cranial Placodes as Evolutionary Innovations—The Ancestor's Tale

“…The development of the PPE is intimately associated with the origin of other ectodermal territories such as the epidermis, neural plate, and neural crest (reviewed in Grocott et al, 2012;Groves & LaBonne, 2014;Ozair, Kintner, & Brivanlou, 2013;Saint-Jeannet & Moody, 2014;Schlosser, 2010Schlosser, , 2014. During gastrulation, signals including inhibitors of bone morphogenetic proteins (BMPs), inhibitors of wingless/integrated proteins (Wnts), and fibroblast growth factors (FGFs) from the organizer, a signaling Dorsally (green) and ventrally (yellow) restricted transcription factors overlap during gastrulation but subsequently resolve into mutually exclusive neural and nonneural competence territories, respectively.…”

“…4B). Wnt, FGF, and retinoic acid (RA) have all been identified as factors which promote expression of posterior and inhibit expression of anteriorly expressed TFs (reviewed in Grocott et al, 2012;Saint-Jeannet & Moody, 2014;Schlosser, 2010Schlosser, , 2014. For example, direct Wnt targets such as Gbx2 and Irx1 are induced in the posterior and their anterior expression boundary is subsequently sharpened by cross-repressive interactions with anterior TFs Otx2 and Fezf, respectively (Broccoli, Boncinelli, & Wurst, 1999;Martinez-Barbera et al, 2001;Millet et al, 1999;Rodriguez-Seguel, Alarcon, & Gomez-Skarmeta, 2009).…”