The ectodermal placodes: a dysfunctional family

Abstract

The ectodermal placodes are focal thickenings of the cranial embryonic ectoderm that contribute extensively to the cranial sensory systems of the vertebrates. The ectodermal placodes have long been thought of as representing a coherent group, which share a developmental and evolutionary history. However, it is now becoming clear that there are substantial differences between the placodes with respect to their early development, their induction and their evolution. Indeed, it is now hard to consider the ectodermal placodes as a single entity. Rather, they fall into a number of distinct classes and it is within each of these that the members share a common development and evolution.

Early Development of the Pituitary Gland: Induction and Shaping of Rathke’s Pouch

“…They are involved in the formation of sense organs (eye, nose, ear) and cranial sensory ganglia. Despite being grouped under the same terminology and developing from the same territory, namely the ectoderm at its border with the neural plate (the latter including the region that gives rise to neural crest), they differ in their induction, development and derivatives (for review see [3,4]). Six types of cranial placode exist in the mouse ( Fig.…”

Induction and specification of the vertebrate ectodermal placodes: precursors of the cranial sensory organs

“…All of the individual placodes mentioned above have been hypothesized to originate from the PPE (Knouff, 1935; Schlosser and Northcutt, 2000; Baker and Bronner‐Fraser, 2001). Although it has been argued that individual placodes are molecularly specified from the onset of their development (Graham and Begbie, 2000; Begbie and Graham, 2001), a recent study of gene expression in the PPE supports the notion that this broad area of undifferentiated ectoderm is molecularly biased to express a common placodal fate (Schlosser and Ahrens, 2004). These authors propose that the biased state is determined by the expression of certain transcription factors, such as six1 and eya1 , because their expression initially is restricted to the PPE in a characteristic ‘horseshoe’ shape surrounding the anterior neural plate described in the classical literature (Figure 1), prior to the thickening of the epithelium.…”

Early regionalization of the otic placode and its regulation by the Notch signaling pathway

“…Some placodes such as the epibranchial and trigeminal give rise only to sensory neurons, while others like the otic and olfactory produce other cell types in addition to sensory neurons (D'Amico-Martel, 1982;Baker and Bronner-Fraser, 2001;Begbie and Graham, 2001;Schlosser, 2005). The inner ear originates from the otic placode, an oval thickening of the ectoderm adjacent to the hindbrain.…”

“…The neurogenic placodes are specialized ectodermal regions found in the embryonic vertebrate head, which contribute extensively to the cranial sense organs (Begbie and Graham, 2001). Some placodes such as the epibranchial and trigeminal give rise only to sensory neurons, while others like the otic and olfactory produce other cell types in addition to sensory neurons (D'Amico-Martel, 1982;Baker and Bronner-Fraser, 2001;Begbie and Graham, 2001;Schlosser, 2005).…”