Migration of LHRH neurons into the spinal cord: evidence for axon‐dependent migration from the transplanted chick olfactory placode

Abstract

In the chick embryo, luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and migrate along the olfactory nerve to the forebrain. In previous studies, we demonstrated that LHRH neurons followed the trigeminal nerve when the olfactory nerve was physically interrupted. To examine whether LHRH neurons possess the capacity to migrate along the different type of axons, the olfactory placode was transplanted into the base of the forelimb. Three to five days after the transplantation, LHRH neurons were detectable in the spinal nerve, the dorsal root ganglion, the sympathetic ganglion and the spinal cord. Double or triple labelling studies for LHRH, somatostatin and/or axonin-1 showed that LHRH neurons entered the spinal nerve in contact with the olfactory axons, which are specifically immunoreactive to somatostatin. Migrating LHRH neurons continued to associate closely with the olfactory axons in the spinal nerve. However, some LHRH neurons often migrated along with the axonin-1 positive spinal sensory axons, maintaining a distance from the olfactory axons. Furthermore, a few LHRH neurons were observed in the ventral root and the ventral funiculus independent of olfactory axons. As LHRH neurons were observed in the motor component of the spinal nerve, it is probable that LHRH neurons also invaded the spinal cord using the motor axons as a guiding substrate for their migration. These results suggest that the migration mode of LHRH neurons is axon dependent in the peripheral region, however, chemical identity with regard to axonal substrate choice for migration was not specified in the present study.

Netrin 1 provides a chemoattractive cue for the ventral migration of GnRH neurons in the chick forebrain

“…The antibody recognizes a single band of about 140 kD on Western blotting of axonin-1 from E14 chick brain membrane and ocular vitreous fluid (Ruegg et al, 1989;Wolfer et al, 1994). This antibody labels developing fiber tracts as previously reported in mice and chicks (Ruegg et al, 1989b;Wolfer et al, 1994;Murakami and Arai, 2002).…”

“…The antibody also binds precursor peptides of GnRH, but had less affinity for chicken GnRH-II in a competitive binding experiment (Park and Wakabayashi, 1986). Immunostaining was eliminated by preincubation of the diluted antiserum (1:2,000) with 1 lg/mL of synthetic LHRH (Murakami and Arai, 2002). Staining with this antibody coincides with the mRNA expression pattern of the chicken GnRH-I (Kanaho et al, 2009).…”

“…Immunohistochemical staining was performed as described previously (Murakami et al, 2000;Murakami and Arai, 2002). Tissues were incubated with primary antibody for 1-4 nights, and subsequently with a speciesspecific secondary antibody conjugated to fluorescent dyes for immunofluorescent staining, or to biotin for the avidin-biotin method.…”

“…GnRH neurons relocate along axons outside the brain (Murakami et al, 1995;Murakami and Arai, 2002). A subset of olfactory fibers that express neural cell adhesion molecule (NCAM), PSA, or somatostatin extend along the medial surface in the chick telencephalon (Norgren and Brackenbury, 1993;Murakami and Arai, 1994b;Murakami et al, 2000).…”

“…In various vertebrates, septohypothalamic GnRH neurons originate from the olfactory placode and migrate along the olfactory, terminal, and/or vomeronasal nerves to reach the forebrain, and then finally, the hypothalamus during embryogenesis Wray et al, 1989;Muske and Moore, 1990;Ronnekleiv and Resko, 1990;Murakami et al, 1991;Sullivan and Silverman, 1993;Parhar et al, 1995). Experimental studies (Murakami et al, 1995;Murakami and Arai, 2002) and a report on Kallmann syndrome showed that the migrational pattern of GnRH neurons is principally axondependent outside the brain. Once GnRH neurons enter the forebrain, most of them migrate tangentially along a subset of olfactory fibers that take a caudal trajectory (Norgren and Brakenbury, 1993;Murakami and Arai, 1994b;Wray et al, 1994;Yoshida et al, 1995), suggesting a type of axophilic migration.…”