Mechanisms of cranial placode assembly

Abstract

Cranial placodes are transient ectodermal structures contributing to the paired sensory organs and ganglia of the vertebrate head. Placode progenitors are initially spread and intermixed within a continuous embryonic territory surrounding the anterior neural plate, the so-called panplacodal region, which progressively breaks into distinct and compact placodal structures. The mechanisms driving the formation of these discrete placodes from the initial scattered distribution of their progenitors are poorly understood, and the implication of cell fate changes, local sorting out or massive cell movements is still a matter of debate. Here, we discuss different models that could account for placode assembly and review recent studies unraveling novel cellular and molecular aspects of this key event in the construction of the vertebrate head.

Pioneer neurog1 expressing cells ingress into the otic epithelium and instruct neuronal specification

“…Precursors from the PPR segregate and coalesce into individual cranial placodes, which progressively acquire specific identities (Breau and Schneider-Maunoury, 2014; Streit, 2002; Bhat and Riley, 2011; Saint-Jeannet and Moody, 2014; McCarroll et al, 2012). Our data revealed that otic neurog1 is expressed before of what it was conceived and outside the epithelium by a group of cells that ingress during morphogenesis.…”

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

“…After the discovery of the basic molecular cascades which determine general and/or specific cell fates (for review, see Schlosser, 2006), brand new hypotheses on the roles of cell fate changes, local sorting-out processes, and massive cell movements await further examination (Breau and Schneider-Maunoury, 2014). Other important issues still in need of clarification are the molecular regulation and the very exact functions of large-scale apoptosis first observed during the morphogenesis of placodes in Tupaia belangeri.…”

Directional cell movements downstream of Gbx2 and Otx2 control the assembly of sensory placodes

“…Initially, placode precursors occupy a unique territory, the pre-placodal region (PPR), where cells of different fates are interspersed (Bhattacharyya et al, 2004; Kozlowski et al, 1997; McCarroll et al, 2012; Streit, 2002; Xu et al, 2008; Pieper et al, 2011). Although it is controversial whether cell sorting segregates placode progenitors (Pieper et al, 2011), at later stages placodal cells must somehow coalesce to form spatially distinct placodes along the anterior-posterior axis (Breau and Schneider-Maunoury, 2014). In chick, DiI labelling reveals some movement of cell groups during otic, olfactory and lens placode formation (Bhattacharyya et al, 2004; Streit, 2002); while in zebrafish, cells move directionally in an integrin-α5 dependent manner as they are recruited into the otic placode (Bhat and Riley, 2011).…”