Abstract
Newly generated neurons are continuously added to the olfactory epithelium and olfactory bulbs of adult mammals. Studies also report newly generated neurons in the piriform cortex, the primary cortical projection site of the olfactory bulbs. The current study used BrdU-injection paradigms, and in vivo and in vitro DiI tracing methods to address three fundamental issues of these cells: their origin, migratory route and fate. The results show that 1 day after a BrdU-injection, BrdU/DCX double-labeled cells appear deep to the ventricular subependyma, within the white matter. Such cells appear further ventral and caudal in the ensuing days, first appearing in the rostral piriform cortex of mice at 2 days after the BrdU-injection, and at 4 days in the rat. In the caudal piriform cortex, BrdU/DCX labeled cells first appear at 4 days after the injection in mice and 7 days in rats. The time it takes for these cells to appear in the piriform cortex and the temporal distribution pattern suggest that they migrate from outside this region. DiI tracing methods confirmed a migratory route to the piriform cortex from the ventricular subependyma. The presence of BrdU/NeuN labeled cells as early as 7 days after a BrdU injection in mice and 10 days in the rat and lasting as long as 41 days indicates that some of these cells have extended survival durations in the adult piriform cortex.
Highlights
The olfactory system is one of the oldest and most well preserved mammalian sensory systems
The results of this study suggest that the newly generated neurons of the adult piriform cortex arise from the subventricular zone of the lateral ventricle, migrate through subcortical white and gray matter structures to reach the piriform cortex, and some of these cells differentiate and survive in the piriform cortex for the 41-day duration of the study
BrdU/DCX double-labeled cells were first detected in the caudal piriform cortex at 7 days after a single BrdU injection
Summary
The olfactory system is one of the oldest and most well preserved mammalian sensory systems. A third region of the olfactory system, the piriform cortex, displays newly generated neurons in the adult brain of several mammalian species, including rodents, rabbits and non-human primates (Bernier et al 2002; Luzzati et al 2003; Pekcec et al 2006; Shapiro et al 2007b). Adult neurogenesis in the olfactory bulbs is important in odor discrimination, indicating that the newborn neurons become integrated into functional circuitry (Gheusi et al 2002) It is not known whether functional integration occurs for the newly generated neurons in the adult piriform cortex, it is known that in adult mice, they can survive for at least 21 days and display a mature neuronal phenotype (Shapiro et al 2007b)
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