The subcommissural organ and the development of the posterior commissure in chick embryos

Abstract

The subcommissural organ (SCO) is an ependymal differentiation located in the diencephalon under the posterior commissure (PC). SCO-spondin, a glycoprotein released by the SCO, belongs to the thrombospondin superfamily and shares molecular domains with axonal pathfinding molecules. Several lines of evidence suggest a relationship between the SCO and the development of the PC in the chick: (1) their close location to each other, (2) their differentiation at the same developmental stage in the chick, (3) the abnormal PC found in null mutants lacking an SCO and (4) the release by the SCO of SCO-spondin. By application of DiI crystals in the PC of chick embryos, we have identified the neurons that give rise to the PC. Labelling is confined to the magnocellular nucleus of the PC (MNPC). To gain insight into the role of the SCO in PC development, coculture experiments of explants of the MNPC region (MNPCr) from embryos at embryonic day 4 (E4) with SCO explants from E4 or E13 embryos have been performed and the neurite outgrowth from the MNPCr explants has been analysed. In the case of coculture of E4 MNPCr with E4 SCO, the number of neurites growing from the MNPCr is higher at the side facing the SCO. However, when E4 MNPCr and E13 SCO are cocultured, the neurites grow mostly at the side opposite to the SCO. These data suggest that, at early stages of development, the SCO releases some attractive or permissive molecule(s) for the growing of the PC, whereas at later stages, the SCO has a repulsive effect over neurites arising from MNPCr.

Subdivisions of chick diencephalic roof plate: Implication in the formation of the posterior commissure

“…Nevertheless, it has been proposed that the cells of the SCO play an important role in this process (Gobron et al, 2000;Meiniel, 2001;Fernandez-Llebrez et al, 2001Lehmann and Naumann, 2005;Caprile et al, 2009;Hoyo-Becerra et al, 2010). This notion is supported by the following evidence: (i) SCO-spondin shares similarities with proteins involved in axonal guidance, (ii) this protein is expressed by the SCO concomitantly with PC formation, (iii) SCO-spondin is secreted toward the extracellular matrix in close contact with axons of the PC, (iv) SCO-spondin promotes neuritic outgrowth in vitro, and (v) defective SCO differentiation correlates with impaired PC development.…”

“…The results revealed an effect on neuronal aggregation and neurite outgrowth, but the bidimensionality of the culture did not permit to evaluate an effect on axonal fasciculation. (2) The experiments reported by Hoyo-Becerra et al (2010) were designed to see the effect of SCO explants on explants of the magnocellular nucleus of the PC by the inclusion of both types of explants in a collagen gel matrix. The results revealed an effect of SCO on neurite length but did not show an effect on axonal fasciculation, probably because the SCO-spondin secreted by SCO cells remains aggregated on the cell surface (Caprile et al, 2009), so it did not spread in the collagen gel matrix.…”

“…In the chick, pioneer axons that form the PC cross the midline at stage HH18 and, as development proceeds, fibers from diverse pretectal nuclei join these intrepid axons and the process of fasciculation occurs. (Puelles and Rubinstein, 2003;Caprile et al, 2009;Hoyo-Becerra et al, 2010).…”

Extracellular Matrix: Functions in the Nervous System

“…SCO-spondin is secreted by ependymal cells of the subcommissural organ (SCO) in the developing vertebrate brain (Gobron et al 1996;Goncalves-Mendes et al 2003;Meiniel et al 2008). TSR motifs of SCO-spondin induce neurite extension in neuronal cell lines in a b1-integrin-dependent fashion; immunohistochemical evidence suggests it may control axonal development in vivo (Bamdad et al 2004;Caprile et al 2009;Hoyo-Becerra et al 2010). …”

The anatomy of the posterior commissure

“…Some fibers are believed to be derived from the posterior part of the thalamus and from the superior colliculus and to continue directly to the medial longitudinal fasciculus. Fibers from the thalamic, pretectal, tectal region, fibers from the superior colliculus and the habenular nuclei are known to connect with the posterior commissure, but they have not been shown anatomically (3)(4)(5)12).…”