Cell migration in the developing chick diencephalon

Abstract

We previously reported that retrovirally marked clones in the mature chick diencephalon were widely dispersed in the mediolateral, dorsoventral and rostrocaudal planes. The current study was undertaken to define the migration routes that led to the dispersion. Embryos were infected between stages 10 and 14 with a retroviral stock encoding alkaline phosphatase and a library of molecular tags. Embryos were harvested 2.5-5.5 days later and the brains were fixed and serially sectioned. Sibling relationships were determined following PCR amplification and sequencing of the molecular tag. On embryonic day 4, all clones were organized in radial columns spanning the neuroepithelium, which was composed primarily of a ventricular zone at this age. No tangential migration was seen in the ventricular zone. On embryonic day 5, most clones remained radial with many cells located in the ventricular zone; however, a few clones had cells migrating perpendicular to the radial column, in either a rostrocaudal or dorsoventral direction. The tangential migration began just beyond the basal limit of the ventricular zone. On embryonic days 6 and 7, many clones had cells migrating perpendicular to the radial column, which spanned from the ventricular to the pial surface. The migrating cells appeared to be aligned along axes that were perpendicular to the radial column. Using a combination of DiI tracing, immunohistochemistry and electron microscopy, we have determined that axonal tracts are present and are aligned with the migrating cells, suggesting that they support the non-radial cell migration. These data indicate that migration along pathways independent of radial glia occur outside of the ventricular zone in more than 50% of the clones in the chick diencephalon.

Melanin-concentrating hormone (MCH) neurons in the developing chick brain

“…Tangential migration is not uncommon in the developing chicken hypothalamus, especially after E10 (Arnol-Aldea and Cepko, 1996;Golden et al, 1997). It was previously reported to occur along existing axons (Golden et al, 1997). The presence of MCH axons around the 3rd ventricle starting from early ages (see Section 2.6) suggests that these axons could contribute to support the tangential migration of MCH neurons in cranial direction.…”

“…This small group of more anterior MCH neurons becomes evident by E10 and appears to migrate tangentially, along the anteroposterior axis. Tangential migration is not uncommon in the developing chicken hypothalamus, especially after E10 (Arnol-Aldea and Cepko, 1996;Golden et al, 1997). It was previously reported to occur along existing axons (Golden et al, 1997).…”

“…The hypothesis that the HPO is the source of MCH neurons in chickens is strongly supported by the facts that MCH neurons are present inside the HPO and those closer to the HPO are smaller, more roundish and less strongly stained than those located more laterally (as in Cardot et al, 1999). Based on early detection in medial positions, staining gradients and orientation of labeled neurites (cells generally migrate in the direction of their cellular processes; Golden et al, 1997), MCH neurons appear to migrate from the HPO in 3 directions: anteriorly (in small numbers, remaining in medial locations), dorsolaterally, and most of them, ventrolaterally. Their distribution did not change substantially with age.…”