Astrocytes build blood vessel scaffolds for long distance neuron migrations

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http://www.guardian.co.uk/science/neurophilosophy/2012/feb/02/1

The journey undertaken by newborn neurons in the adult mouse brain is like the cellular equivalent of the arduous upstream migration of salmon returning to their hatching river. Soon after being born in the subventricular zone near the back of the brain, these cells embark on a long-distance migration to the front-most tip of the brain. Their final destination – the olfactory bulb – is the furthest point from their birth place, and they travel two-thirds of the length of the brain to get there.

Several years ago, a team of researchers from Canada showed that the pathway for this migration – called the rostral migratory stream – is lined with a scaffold of capillaries, and that the young cells crawl along the blood vessels during their journey. In a follow-up study, they now report that the construction and organization of the blood vessel scaffold is orchestrated by star-shaped cells called astrocytes.

Cell migration is a key feature of the developing brain, and the mechanisms underlying these cell movements are well characterised. As a general rule, migrating cells rely on the combined activity of chemical signals to find their way. They set off on their migration in response to a repulsive signal that pushes them away from their birth place and stay on track because their migratory pathway is flanked by a non-permissive signal which prevents them from deviating from the correct route. Finally, as they approach the end of their journey, attractive cues pull them in the right direction. Upon arriving at their destination, the cells turn and migrate into the bulb, then integrate into the existing neural circuits and participate in the processing of smell information.

The migrations that occur in the adult brain involve the same mechanisms. The tissues flanking the migratory pathway secrete a repulsive signalling molecule prevents migrating cells from veering in the wrong direction, and the olfactory bulb itself secretes attractive cues which keep them on track. But the migrations still occur when the bulb itself is surgically removed from the brains of mice, so the chemical signals within it are clearly not essential. Armen Saghatelyan and his colleagues therefore reasoned that the cells might also depend upon a physical substrate to find their way.