Figure 3.

Breeding conditions promote maintenance of an initial cohort of new neurons and non-neuronal cells, but do not alter subsequent neuronal turnover. (A) Number of thymidine analog positive neurons in one hemisphere of HVC from cohort one across all survival times in nonbreeding and breeding condition. Depicted results include both BrdU and EdU positive cell data pooled within experimental group (see Table 2 for lack of thymidine analog order effect), with data from individual birds represented as grey diamonds and the group mean as a black bar with S.E.M.. The number of new HVC neurons maintained from the initial cohort of thymidine analog label are significantly higher under breeding conditions, suggesting that LDT conditions promote greater maintenance of an initial cohort of new neurons than in SD. Because in both nonbreeding and breeding conditions neurons from the first cohort are maintained, these data further indicate that some new neurons replace older neurons, while other new neurons die (see Figure 1C, right panel). (B) Number of new neurons maintained in HVC on one side of the brain from the second thymidine analog label in all experimental groups. Physiological condition and interval between injection of the two thymidine analogs did not significantly affect the number of new neurons from the second cohort maintained in HVC. These data suggest that during stable conditions, new neurons enter HVC to replace older neurons in equal amounts regardless of condition (Tables 2 and 3). (C-D) The number of new non-neuronal HVC cells maintained from the cohort labeled with the first thymidine analog (C) and the second thymidine analog (D) across all survival times in nonbreeding and breeding condition. LDT conditions lead to greater maintenance of new non-neuronal cells from the first thymidine analog-labeled cohort but not the second when compared to SD conditions (Table 3). These data suggest that breeding conditions not only promote the increased maintenance of an initial population of new neurons but also new non-neuronal cells, which together contribute to the growth of HVC.