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First published online August 23, 2004
Journal of Experimental Biology 207, 3317-3327 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01143

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Evidence that thyroid hormone induces olfactory cellular proliferation in salmon during a sensitive period for imprinting

Sean C. Lema^* and Gabrielle A. Nevitt^*

Center for Animal Behavior and Section of Neurobiology, Physiology and Behavior, One Shields Avenue, University of California at Davis, Davis, CA 95616, USA

View larger version (135K): [in a new window]   Fig. 1. Section of the olfactory epithelium showing BrdU-ir in single and clustered cells. Positive staining appears as a black precipitate. (A) Typical round morphology of a BrdU-ir cell in the basal cell layer. (B) Elongated morphology typical of a BrdU-ir cell in the mid-apical cell layer. (C) An example of a BrdU-labeled cluster of four cells situated in the basal region of the epithelium. Labeled cells are black and appear to be in close juxtaposition to each other. Length bars (10 µm) distinguish the extent of the basal region in A, B and C. Scale bar in B, 10 µm.

View larger version (132K): [in a new window]   Fig. 2. Representative BrdU labeling in the olfactory epithelium of placebo fish (A-C) and T3-implanted fish (D-F). Photomicrographs are shown from six different individual fish. Scale bar, 40 µm.

View larger version (15K): [in a new window]   Fig. 3. Counts of BrdU-ir cells in the basal (A) and mid-apical (B) cell layers of the olfactory epithelium of T3-implanted (black bars) and placebo (gray bars) fish. Clustered and individual cells are considered separately in the first two columns, and together in the third column (`total cells'). Values are expressed as means ± S.E.M. Mann-Whitney U-tests: *P<0.05; **P<0.025.

View larger version (14K): [in a new window]   Fig. 4. The distribution of cluster sizes for each treatment group compared with a Poisson (random) distribution. The frequency of cluster sizes for the (A) T3-implanted group (black bars) and the (B) placebo group (black bars) compared with a frequency distribution generated by a truncated Poisson process (gray bars in A and B). Both distributions are significantly different from random (T3 implant, 2=12.6697, P=0.0004; placebo, 2=7.0978, P=0.008).

View larger version (12K): [in a new window]   Fig. 5. The distribution of cluster sizes from T3-implanted (black bars) and placebo (gray bars) groups. Distributions are not significantly different from each other (2=0.8289, P=0.9345).

View larger version (21K): [in a new window]   Fig. 6. (A) Profiles of body mass (triangles) and standard length (circles) plotted against sampling date. Data are given as means ± S.E.M. Numbers of fish sampled are shown in parentheses. (B) Condition factor (see Materials and methods) plotted against sampling date. Data are plotted as means ± S.E.M. Sample sizes are identical to those shown in A. Condition factor significantly decreased during the sampling period, which is a trend associated with smolting (asterisks indicate significant difference at overall P<0.05 between dates, Tukey HSD test).

View larger version (16K): [in a new window]   Fig. 7. Profiles of plasma T4 (A) and T3 (B) across the parr-smolt transformation. Data are plotted as means ± S.E.M. Sample sizes for each date are shown in parentheses in A.

View larger version (23K): [in a new window]   Fig. 8. Mean number of BrdU-ir cells (per 100 µm lamella length) plotted against sampling date. Data are given as means ± S.E.M. Basal cell counts are indicated by red triangles; mid-apical cell counts are indicated by blue triangles. Data are superimposed on a plot of mean plasma T4 levels for each sampling date (black squares). Sample sizes for BrdU-ir cell data for each date are shown in parentheses.

View larger version (18K): [in a new window]   Fig. 9. Mean number of BrdU-ir cells (per 100 µm lamella length) plotted against mean plasma T4 levels for each sampling date. (A,B) Single and clustered BrdU-ir cells considered together. The data indicate a significant positive correlation in the basal cell layer (A) but not in the mid-apical cell layer (B), suggesting that T4-induced proliferation is restricted to the basal cell layer. (C,D) Single BrdU-ir cells only. The data show a significant positive correlation in the basal cell layer (C) but not in the mid-apical cell layer (D).