Abstract
The neural crest (NC) cells and cranial placodes are two ectoderm-derived innovations in vertebrates that led to the acquisition of a complex head structure required for a predatory lifestyle. They both originate from the neural border (NB), a portion of the ectoderm located between the neural plate (NP), and the lateral non-neural ectoderm. The NC gives rise to a vast array of tissues and cell types such as peripheral neurons and glial cells, melanocytes, secretory cells, and cranial skeletal and connective cells. Together with cells derived from the cranial placodes, which contribute to sensory organs in the head, the NC also forms the cranial sensory ganglia. Multiple in vivo studies in different model systems have uncovered the signaling pathways and genetic factors that govern the positioning, development, and differentiation of these tissues. In this literature review, we give an overview of NC and placode development, focusing on the early gene regulatory network that controls the formation of the NB during early embryonic stages, and later dictates the choice between the NC and placode progenitor fates.
Highlights
The “New Head” hypothesis (Gans and Northcutt, 1983; Northcutt, 2005) suggests that the presence of a complex head is a significant evolutionary difference between vertebrates and other chordates
The vertebrate head has appeared concomitantly with two unique tissues, which are not present in earlier-derived organisms: the neural crest (NC) and the sensory placodes. These tissues are formed at the border of the neural fold on the dorsal side of the embryo: placode progenitors (PP) are present rostrally and NC precursors are located more posteriorly (Figure 1A)
In this brief literature review, we focus on the regulation of the early stages of neural border (NB) development, followed by its specification into NC and PP
Summary
The “New Head” hypothesis (Gans and Northcutt, 1983; Northcutt, 2005) suggests that the presence of a complex head is a significant evolutionary difference between vertebrates and other chordates. The vertebrate head has appeared concomitantly with two unique tissues, which are not present (or present in rudimentary form) in earlier-derived organisms: the neural crest (NC) and the sensory placodes. These tissues are formed at the border of the neural fold on the dorsal side of the embryo: placode progenitors (PP) are present rostrally and NC precursors are located more posteriorly (Figure 1A). In order to understand the development of these tissues and uncover the molecular basis of human pathologies, functional studies have been conducted using various vertebrate animal models In this brief literature review, we focus on the regulation of the early stages of NB development, followed by its specification into NC and PP. We emphasize the common and specific pathways and the gene regulatory network (GRN) controlling the balanced emergence of both cell types around the NP
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