Development of cranial placodes: Insights from studies in chick

Abstract

This review focuses on how research, using chick as a model system, has contributed to our knowledge regarding the development of cranial placodes. This review highlights when and how molecular signaling events regulate early specification of placodal progenitor cells, as well as the development of individual placodes including morphological movements. In addition, we briefly describe various techniques used in chick that are important for studies in cell and developmental biology.Key words: border, chick, invagination, placodes, signaling. Chick as a model system to unravel the development of cranial placodesCranial placodes are transient thickenings of the embryonic head ectoderm at specific positions that can be detected during vertebrate development. Cranial placodes found in chick are the hypophyseal, olfactory, lens, trigeminal, otic and epibranchial ( Fig. 1). All of these placodes, except the hypophyseal, will contribute to sensory components of the peripheral nervous system (PNS), and are usually referred to as sensory placodes. The hypophyseal placode will eventually form the endocrine part of the pituitary. Among the sensory placodes, the lens is the only placode that will not give rise to any neuronal derivatives. The individual placodes will give rise to specific cell types, and together the placodes form a variety of cell types including sensory receptor cells, neurons, endocrine cells and supporting cells.Cranial placodes derive from the neural plate border (border), a region of the embryonic ectoderm located between the prospective neural and the prospective epidermal domains. Recent studies have shown when and how extracellular signals result in the specification of border cells and the individual specification of various placodes. Although specification of placodal progenitors appears to be partly regulated by common molecular mechanisms, the development and patterning of individual cranial placodes are controlled by unique molecular codes. A key question during development of any tissue or organ is how a cell adopts one fate over another. Equally important is to understand how morphological movements occur, forming tissues and organs in specific shapes. The development of placodes includes both of these developmental processes, and these events must be strictly coordinated to form functional mature structures. To analyze the acquisition of cell identity and morphological movements, various molecular and developmental biology techniques in chick can be used, which are briefly mentioned in Box 1. This review, which focus on results from chick, can be used as a complement to existing reviews concerning placode development (Baker & Bronner-Fraser 2001;Streit 2004Streit , 2007Brugmann & Moody 2005;Schlosser 2006Schlosser , 2008Schlosser , 2010Ohyama et al. 2007;Sauka-Spengler & Bronner-Fraser 2008;McCabe & Bronner-Fraser 2009;Ladher et al. 2010;Gunhaga 2011;Patthey & Gunhaga 2011). In the following sections, we discuss recent progress concerning early development of cranial placodes using chick as ...

Tissue growth constrained by extracellular matrix drives invagination during optic cup morphogenesis

“…Fibronectin is a primary matrix constituent during eye development (Kwan 2014). These results suggest that placode formation is a prerequisite for invagination in the eye, as has been found in other morphogenetic problems (Huang et al 2011; Jidigam and Gunhaga 2013), and that retinal and lens placode formation requires the ECM. However, they do not prove that the ECM is necessary for the invagination process itself.…”

The molecular basis of craniofacial placode development

“…The regionalization and specification of individual cranial placode is a multistep process that is regulated by a number of signaling molecules and transcription factors (Figure ). We will briefly discuss these steps in this section and direct readers to the following reviews for a more detailed discussion …”

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

“…Pre-placodal ectoderm is induced by a combination of FGF signaling and the attenuation of Wnt and BMP signals (Bailey and Streit, 2006; Grocott et al, 2012; Litsiou et al, 2005; Streit, 2007). Once induced, a second set of signals direct different regions of pre-placodal ectoderm to differentiate into distinct cranial placodes along the anterior-posterior axis of the head (Baker and Bronner-Fraser, 2001; Jidigam and Gunhaga, 2013; Patthey and Gunhaga, 2011, 2013; Schlosser, 2006, 2010). These cranial placodes give rise to paired sensory organs of the vertebrate head, including the entire inner ear and the VIIIth cranial ganglion that innervates the sensory regions of the ear.…”