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Prominent Postnatal Streams of Interneurons in the Ferret Cortex


Recent work shows that human brain development continues extensively into the postnatal period as specific cortical circuits continue to receive streams of late migrating interneurons. Rodent models of this phenomenon either fail to identify similar contributions or reveal much smaller migrating populations of cells. I asked whether the ferret, a gyrencephalic mammal with large increases in brain size after birth, possesses postnatal streams of young interneurons like in the human. I sectioned the brains of ferret kits at P20 (equivalent to human term), P40, P65 and P90 and stained for Doublecortin (DCX), a marker expressed by young neurons. In addition to the rostral migratory stream to the olfactory bulb, three additional streams were discovered in the white matter at P20: one oriented rostrally towards the prefrontal cortex, one oriented dorsally towards the posterior sigmoid gyrus, and one oriented caudally towards the occipital cortex. Each stream diminished at a unique rate before disappearing by P90. The DCX+ cells in each stream expressed GAD67, as well as a molecular signature consistent with cells born in the caudal ganglionic eminence. Young interneurons were observed transitioning from the white matter to the cortex over time, and only small percentages appeared to undergo apoptosis or remain in the white matter until adulthood. My work shows that the ferret brain possesses robust postnatal streams of young interneurons oriented towards cortical destinations, perhaps suggesting that this phenomenon is a common method used to build large gyrencephalic brains.

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