Maturation and death of adult-born olfactory bulb granule neurons: role of olfaction

Abstract

Young neurons born in the subventricular zone (SVZ) of adult mice migrate to the olfactory bulb (OB) where they differentiate into granule cells (GCs) and periglomerular interneurons. Using retroviral labeling of precursors in the SVZ, we describe five stages and the timing for the maturation of newly formed GCs: (1) tangentially migrating neuroblasts (days 2-7); (2) radially migrating young neurons (days 5-7); (3) GCs with a simple unbranched dendrite that does not extend beyond the mitral cell layer (days 9-13); (4) GCs with a nonspiny branched dendrite in the external plexiform layer (days 11-22); and (5) mature GCs (days 15-30). Using [3H]thymidine, we show that the maximum number of labeled GCs is observed around day 15 after injection. Interestingly, between days 15 and 45 after birth, soon after the cells developed spines, the number of [3H]thymidine-labeled GCs declined by 50%. Using anosmic mice, we found that sensory input was critical for the survival of GCs from day 15 to 45 after labeling. However, the number and morphology of 15-d-old cells in the granule cell layer was similar in anosmic and wild-type mice. We infer that the lack of activity did not have an effect on the generation, migration, and early differentiation of granule cells. Soon after young GCs matured, and presumably became synaptically connected, their survival depended on the level of activity that they received. This selection mechanism might allow the construction of specific OB circuits based on olfactory experience and suggests possible functions of OB cell replacement.

Fig. 1.

Alkaline phosphatase staining of virus-infected newly generated cells in the GCL of the olfactory bulb. Photomicrographs (left and center columns) and camera lucida drawings (right column) show examples of stained developing granule neurons at different maturation stages as defined in Results: class 1 (A–C); class 2 (D–F); class 3 (G–I); class 4 (J–L); class 5 (M–O). Scale bars, 25 μm.

Fig. 2.

Alkaline phosphatase staining of the dendritic arbors of virus-infected newly generated cells in EPL of the olfactory bulb. Representative photomicrographs show the dendritic arborization of developing granule neurons at early stages of maturation (A) and at later stages, after developing dendritic spines (B). Scale bars, 25 μm.

Fig. 3.

Time course of the development of adult-generated olfactory bulb granule neurons. Percentage of alkaline phosphatase-labeled cells in the GCL of the olfactory bulb belonging to each of the developing stages defined in Results (Fig. 1) at different survival times. Top diagram indicates an example of each morphological class. EPL, External plexiform layer; MCL, mitral cell layer;GCL, granule cell layer; RMS, rostral migratory stream.

Fig. 4.

Labeled cells in the GCL of the olfactory bulb at different survival points after [³H]thymidine injection. A, Photomicrograph of a [³H]thymidine-positive, NeuN-positive cell in the GCL. Scale bar, 20 μm. B, The number of [³H]thymidine-labeled cells in the GCL at different survival times. C, The volume of the GCL at different survival times. Values indicate mean ± SE.

Fig. 5.

Morphology and cell death in the olfactory bulbs of anosmic mice. Photomicrographs of a Nissl-stained coronal section of the olfactory bulb of an anosmic mouse (A) or a control littermate (B). Dotted lines outline the contour of the RMS. Scale bars, 0.5 mm. C, Volume of the GCL in anosmic (n = 3) and control mice (n = 4). Representative photomicrographs show TUNEL-stained apoptotic cells in the GCL of anosmic (D) and control mice (E). Scale bars, 50 μm. E, Quantification of TUNEL staining in the GCL of anosmic (KO) (n = 3) and control mice (wt) (n = 4). Values indicate mean ± SE. Statistical analysis was conducted by unpaired Student's t test. Significance was established at **p < 0.01.

Fig. 6.

Morphology and cell survival of newly generated cells in the olfactory bulb of anosmic mice. A, Photomicrograph of a class 4 cell in the GCL of an anosmic mouse. Several focal planes were combined in Photoshop to reconstruct the cell. Scale bar, 50 μm. B, Photomicrograph of the dendritic tree of a class 5 cell in the EPL of an anosmic mouse. Scale bar, 20 μm. C, The percentage of cells belonging to each of the five morphological classes in anosmic (KO) and control littermates (wt). D, Total number of [³H]thymidine- and BrdU-labeled cells in the GCL in control (n = 4) or anosmic mice (n = 3). Statistical analysis was conducted by unpaired Student's t test. Significance was established at *p < 0.05. Time line represents the injection protocol for the experiment. Arrowsindicate injection of either BrdU or [³H]thymidine. X indicates time of perfusion. Values indicate mean ± SE.