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

Abstract

The subcommissural organ (SCO) is a roof plate differentiation located in the caudal diencephalon under the posterior commissure (PC). A role for SCO and its secretory product, SCO-spondin, in the formation of the PC has been proposed. Here, we provide immunohistochemical evidence to suggest that SCO is anatomically divided in a bilateral region positive for SCO-spondin that surrounds a negative medial region. Remarkably, axons contacting the lateral region are highly fasciculated, in sharp contrast with the defasciculated axons of the medial region. In addition, lateral axon fascicles run toward the midline inside of tunnels limited by the basal prolongations of SCO cells and extracellular SCO-spondin. Our in vitro data in collagen gel matrices show that SCO-spondin induces axonal growth and fasciculation of pretectal explants. Together, our findings support the idea that SCO-spondin participates in the guidance and fasciculation of axons of the PC. Developmental Dynamics 239:2584-2593,

Complementary expression of EphA7 and SCO-spondin during posterior commissure development

“…In the LDRP, these axons became fasciculated and advanced towards the MDRP, where some turned to the ipsilateral side (Stanic et al, 2010 and present work Figure 3 ). The trajectory of these axons can be divided into the following three stages ( Figure 6 ): (1) advance from the ventral region to the lateral RP; (2) fasciculation in the lateral RP; and (3) midline decision to turn towards the ipsilateral side or continue to the opposite side.…”

“…It was previously shown that SCO-spondin is secreted by the lateral diencephalic RP from the early stages of development, but expression was missing in the few midline RP cells (Stanic et al, 2010). In vivo and in vitro evidence shows that SCO-spondin is important for axon fasciculation, since axons located on the lateral RP are highly fasciculated in contrast to the defasciculated axons located on the midline, where some turn to the ipsilateral side (Stanic et al, 2010; discontinuous arrow in Figure 3B ). Immunohistochemical analysis of EphA7 expression at the HH30 stage revealed that this protein was expressed at the midline, specifically in cells lacking SCO-spondin (continuous arrows in Figures 3C,D ; double arrow in Figure 3E ).…”

“…Immunohistochemistry was performed as described previously (Stanic et al, 2010). Antibodies were diluted in Tris–HCl buffer containing 1% bovine serum albumin (Tris–BSA).The antibodies used were rabbit anti-EphA7 (Ab5411-100, Abcam, dilution 1/200), mouse anti-βIII tubulin antibody (clone Tuj1, Promega, Madison, WI, USA; dilution 1/1000), 3A10 (Developmental Studies Hybridoma Bank, dilution 1/2), and the anti-Reissner’s fiber glycoproteins antibody that recognizes SCO-spondin made in rabbit or rat (dilution 1/1000; kindly donated by E. Rodriguez; Caprile et al, 2009).…”

“…The PC axons principally come from neurons of the magnocellular nucleus of the posterior commissure (MCPC) located at the ventrolateral pretectum that run dorsally to the midline and from neurons of the parvocellular nucleus of the posterior commissure (PCPC) that project ventrally in order to reach and cross the midline RP (Ferran et al, 2007, 2009; Merchán et al, 2011; Ware and Schubert, 2011). The molecules involved in guiding the axons of the PC are not yet fully described, but there is some evidence suggesting the participation of the underlying RP (Bach et al, 2003; Hoyo-Becerra et al, 2010; Stanic et al, 2010; Grondona et al, 2012; Vera et al, 2013). …”

“…The RP of the pretectum, or embryonic SCO, is divided into two regions, a bilateral diencephalic RP (LDRP), which secretes the SCO-spondin that is responsible for fasciculation and guiding the axons towards the midline, and a medial diencephalic RP (MDRP), where some axons turn to the ipsilateral side in response to unknown guidance cues (Stanic et al, 2010). Given this lack of knowledge, the aim of the present work was to elucidate the guidance cues secreted by the midline cells that are responsible for turning ipsilateral axons.…”