Early expression of axon guidance molecules in the embryonic chick mesencephalon and pretectum

Abstract

Early axon tracts in the developing vertebrate brain are established along precise paths. Yet, little is known about axon guidance processes at early stages of rostral brain development. Using whole mount in situ hybridisation in combination with immunohistochemistry, we have analysed the expression patterns of Slits, Netrins, Semaphorins and the respective receptors during the formation of the early axon scaffold, particularly focusing on the pretectalmesencephalic boundary. Many of these guidance molecules are expressed in close correlation with the growing tracts, and the nuclei of the corresponding neurons often express the respective receptors. The expression patterns of Slits and Netrins implicate them with the positioning of the longitudinal tracts along the dorsoventral axis, while Semaphorins could provide guidance at specific choice points. Our study provides a catalogue of gene expression for future studies on axon guidance mechanisms in the early brain.

Development of the early axon scaffold in the rostral brain of the chick embryo

“…2004; Devine & Key, 2008; Farmer et al. 2008; Riley et al. 2010), rather than the specification and differentiation of the neurones.…”

“…Most molecular studies have focussed on the guidance of the early axons (e.g. Anderson & Key, 1999;Hjorth & Key, 2001;Molle et al 2004;Devine & Key, 2008;Farmer et al 2008;Riley et al 2010), rather than the specification and differentiation of the neurones. Signalling from the prechordal plate influences the size of the vcc in zebrafish (Tallafuss et al 2003), while in chick the homeobox gene Sax1 has been implicated with the development of the MLF (Schubert & Lumsden, 2005).…”

Signals from the caudal diencephalon are required for the projection of the Interstitial Nuclei of Cajal

“…There is evidence of the expression of chemotropic molecules in neighboring regions to the projection of the INC. A recent study shows that netrin‐1 is expressed at the floor plate and in the ventral basal plate, a place where is located the INC projection (Kennedy et al, 1994). In addition, the Unc5H3 receptor is expressed at the INC nucleus region (Riley et al, 2010). Netrin‐1 participates at the spinal cord, as an attracting molecule in the commissural neurons projection (Kaprielain et al, 2000; Stoeckli, 1998) and as a repellent molecule for the trochlear motor axons (Parra and Zou, 2010).…”

“…Recently, it has been recognized that semaphorin 3 A expression patterns in the chick embryo include different regions of the ventral MFB, and the rostral and ventral pretectum adjacent to the INC. Neuropilin‐1, a known receptor for this semaphorin, is expressed in a small area in the ventral MFB, in the same location of the INC nucleus. Similarly, Plexin A4 is also located in the INC nucleus region (Riley et al, 2010).…”

“…Therefore, a variety of molecules could participate in the INC initial projection; semaphorin 3 A would exert a repelling effect forcing the projection in a descending direction, while the Sonic hedgehog and netrin molecules would be attracting the projection to the floor plate. Slit‐1 and Slit‐2 expression in the isthmus and through the longitudinal axis, ventral to the INC projection, would contribute to the dorso‐ventral location of this projection (Riley et al, 2010; Molle et al, 2004).…”