Abstract
The olfactory bulb is a phylogenetical old structure, comprising originally a major part of the brain, and decreasing proportionally during evolution. Ontogeny usually resembles phylogeny, so we were interested, if the portion of the olfactory bulb (OB) decreases during development. Therefore we investigated the OB during postnatal development in the American mink (Neovison vison var. atratus), a species born very altricial, eyes and ears
closed, with a body weight less than the adult brain weight, thus a major part of development occurs postnatally. A total of 65 males ranging from newborn (postnatal day 0, P0) to 1.5 years were morphometrically analyzed. The volume of one olfactory bulb in newborns is 1.85 ± 0.04 mm3, increasing continuously (P15-30: 42.27 ± 3.01 mm3; P60-90: 94.64 ± 3.84 mm3) more than 80-fold to adult values (152.00 ± 9.14 mm3). In contrast, the brain weight increases postnatally not even 40-fold from P0 (0.33 ± 0.06 g) up to only P70, when maximal values of 13.01 ± 0.95 g are reached, and decreasing afterwards (adults: 11.30 ± 0.52 g). Thus, the
proportion of both olfactory bulbs on the total brain (bulb/brain ratio) increases postnatally dramatically from an initial value of 1.08 ± 0.09 % in newborns, to about 1.6 % during brain overshoot and further to 2.64 ± 0.11 % in adults (p<0.001). Thus the OB shows a different developmental pattern compared to ROB, with no neuronal overshoot however with a continuous increase in absolute size and of portion on the total brain, indicating the increasing importance during postnatal life (in newborns: nutrition and social odors; in juveniles: additionally
prey and predator odors; in adults: rivals, area marking and sexual cues) requiring increasing information processing and structure, resulting in a gain of olfactory function.
