Projection neurons are added to the high vocal center (HVC) of adult songbirds. Here we report on events associated with their initial arrival in HVC. Neurons formed in adult canaries were labeled with [(3)H]-thymidine and examined 8, 15, 22, and 31 days later. By 8 days, some [(3)H]-labeled cells with the nuclear profile of postmigratory neurons were already present in HVC but could not be retrogradely labeled by Fluoro-Gold injections in the robust nucleus of the archistriatum (RA); 7 days later, a few such cells could be backfilled from RA. Thus, new neurons may arrive in HVC as much as 1 week prior to establishing connections with RA. By 31 days, 43% of the [(3)H]-labeled neurons could be backfilled from RA. In no case were new neurons backfilled by tracer injections into Area X, suggesting that newly formed HVC cells do not establish a transient connection with this region. At all survival times, the somata of new neurons were often clustered tightly together with other HVC neurons that differed in age and projection. Between days 15 and 25 after their birth, half of the new HVC neurons disappeared. We conclude: (1) that neurons arrive in HVC earlier than previously thought, (2) that soon after their arrival they become part of cell clusters in HVC, and (3) that in addition to the previously described death of new neurons that occurs over a period of months, there is an early wave of death that occurs soon after new neurons adopt a postmigratory phenotype.
Copyright 1999 Wiley-Liss, Inc.