Sexual dimorphism in olfactory bulb structure

Sexual dimorphism in mammals is often observed in the accessory olfactory system, however, this system is rudimentary in species such as the American mink. Thus, we were interested, if sexdependent structural differences exist in the main olfactory system. Olfactory bulbs of adult American minks color-variety “standard” (Neovison vison var. atratus) were processed histologically and always the right bulb analyzed with a morphometric system using weight/volume correction factors. The olfactory bulb is significantly bigger in males (152mm3) compared to females (107mm3), however, females are much smaller than males and so is their absolute brain weight (m 11.51g; f 8.43g). Thus, the bulb-portion on the whole brain is similar (m 1.37\%; f 1.32\%). On the other hand, the brain relative to the body mass is much bigger in females (0.85\%) compared to males (0.55\%), resulting in significantly different bulb/body ratios (m 0.0076\%; f 0.0112\%). Sex-dependent differences exist also in the proportion of the neuronal layers: In males, the olfactory fila comprise the major portion of all layers (26.9\%) but only 17.7\% in females, where the majors are granule cell (GCL 28.8\%) and external plexiform layer (EPL 22.8\%) overwhelming the proportions in males (GCL 21.7\%; EPL 19.2\%) significantly. The mitral cell layer is significantly thicker in males (4.7\%) than females (3.5\%).This indicates that the fila layer is related more to overall body size (increase in the olfactory sheet / axon numbers) whereas the information processing GCL and EPL is related to the brain size. This suggests also, that information processing is much more complex in females and to sustain the extended GCL there might be a higher neurogenesis. Therefore, a significant sexual dimorphism in the main olfactory bulb exists, not reported before