What's the Matter? White Matter?

HVC→RA neurons. For some birds, song deterioration This article offers a rare glimpse into the regulation occurred during the first week after induced death. This of new cell recruitment into a fully mature brain. Disdeterioration was then followed by variable recovery to covering the mechanisms governing the type of neuron normal song production, during a time concurrent with added and its placement within the song circuit will recruitment of new neurons. How the detailed time provide important information regarding restrictions on courses of cell death and new neuron incorporation neurogenesis in the adult brain. Furthermore, if new neucompare to that of song deterioration and recovery is ron recruitment is essential for song recovery, then trainunclear; however, this result raises the possibility that ing a new neuron for its new job is likely to involve a new neurons can restore components of song lost after complex interaction of different signals, including mocell death. lecular cues, contacts with neighboring neurons, and The mechanism for this increased and restricted neuexperience. The song system is a promising arena for ronal recruitment in HVC is unknown and may involve sorting these questions out, given its provocative mix control at the levels of neurogenesis, of migration into of neurogenesis and vocal behavior. HVC, or of neuron survival. Particularly, why aren’t HVC→X neurons replaced? Are progenitor cells incapable of making this type of neuron, or is the molecular Michele M. Solis environment in HVC unable to attract or sustain this type Department of Psychology of neuron? The developing nervous system has a wealth University of Washington of mechanisms controlling cell fate determination Seattle, Washington 98195 (McConnell, 1995), and it will be important to compare these to the mechanisms available to adult brains. The variability in both song deterioration and song Selected Reading recovery is curious: only some of the birds exhibited song degradation, and among those, their songs did Alvarez-Buylla, A., and Kirn, J.R. (1997). J. Neurobiol. 33, 585–601. not always return to the original song. The authors rule Eriksson, P.S., Perfilieva, E., Bjork-Eriksson, T., Alborn, A.M., Nordout differences in lesion placement and in song comborg, C., Peterson, D.A., and Gage, F.H. (1998). Nat. Med. 4, 1313–