3H-thymidine-radiographic studies of neurogenesis in the rat olfactory bulb

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Abstract
Neurogenesis in the rat olfactory bulb was examined with 3H-thymidine-radiography. For the animals in the prenatal groups, the initial 3H-thy-midine exposures were separated by 24 h; they were the offspring of pregnant females given two injections on consecutive embryonic (E) days (E12–E13, E13–E14, ... E21–E22). For the animals in the postnatal (P) groups, the initial 3H-thymidine injections were separated by 48 h, each group receiving either four (P0–P3, P2–P4, ... P6–P9) or two (P8–P9, P10–P11, ... P20–P21) consecutive daily injections. On P60, the percentage of labeled cells and the proportion of cells added during either 24 h or 48 h periods were quantified at several anatomical levels for each neuronal population in the main olfactory bulb (mitral cells, tufted cells, granule cells, interneurons in the external plexiform layer, periglomerular granule cells) and accessory olfactory bulb (output neurons, granule cells, periglomerular granule cells). The total time span of neurogenesis extends from E12 to beyond P20. Output neurons are prenatally generated over 5–9 day periods (with most neurogenesis occurring over 2–4 days) in a strict sequential order beginning with the accessory bulb output neurons (E13–E14) and ending with the interstitial tufted cells lying between the glomeruli in the main bulb (E20–E22): These data are correlated with the main and accessory bulb projection fields in the amygdala and with the chronology of amygdala neurogenesis. With the exception of the granule cells in the accessory bulb (88% generated between E15–E22), the rest of the interneuronal populations are generated postnatally and nearly simultaneously. While most neurons (75–80%) originate during the first three weeks of life, all interneuronal populations, including accessory bulb granule cells, show some neurogenesis beyond P20. Injections of 3H-thymidine in juvenile and adult rats indicates neurogenesis up to P60 in the accessory bulb and up to P180 in the main bulb, especially in the main bulb granule cell population. There is circumstantial evidence for turnover of main bulb granule cells during adult life.