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Abstract:
Information processing requires morphological structures, which develop in mammals not only pre- but also postnatally. Especially in altricial species, the brain growth continues far beyond birth. The American mink (Neovision vison var. atratus) is born with eyes and ears closed and depends primarily on its sense of smell for nutrition and neonatal survival. As reported before (Weiler Bennegger, 2015) the olfactory bulb, the main station for olfactory information processing, follows its own growth pattern, different to the rest of the brain.
Information processing however depends on the composition of the neurons and neuronal layers. Taken the fact, that the olfactory sense is functional already from birth on, the question arises: Is the olfactory bulb growing uniformely or do the layers also show signs of allometric growth? Therefore we analyzed the absolute volume of each olfactory bulb layer in a total of 36 female minks at
different postnatal ages (newborns (postnatal day 0 =P0) up to 7 months = P210) using histological sections and a morphometric system. A continuous increase in volume was observed in the external plexiform layer (P0: 0.04±0.01 mm3; P60: 13.62±0.38 mm3; P210: 24.42±1.56 mm3) and granule cell layer (P0: 0.30±0.02 mm3; P60: 12.98±0.47 mm3; P210: 30.85±1.43 mm3) following the continuous increase of the whole olfactory bulb volume. In contrast, volume maxima at P60 with subsequent significant reduction (p<0.01) were reached in the internal plexiform layer (P60: 3.70±0.30 mm3; P90-120: 2.02±0.47 mm3), the internal medullar layer (stratum album: P60: 13.95±0.54 mm3; P90-120: 11.32±0.79 mm3; P210: 9.71±2.27 mm3), and the subependymal layer (P60: 4.69±0.13 mm3; P90-120: 2.64±0.14 mm3; P210: 1.12±0.34 mm3). The mitral cell layer showed a significant maximum at P90-150 (4.50±0.27 mm3; P210: 3.78±0.37 mm3). These results indicate that layer specific growth pattern exist in the olfactory bulb, even with overshoot phenomena. Absolute layer volume reduction in a continuously growing olfactory bulb even maximizes differences in the composition and indicates differential olfactory information processing during specific developmental phases.
The specific absolute volume reductions can be caused by different factors. While the overshoot and concomitantly following reduction in the mitral cell layer might be a result of passing cells on their way from the center of the bulb to the outer layers, the absolute reduction of the subependymal layer might be related to the retraction of the ventricle to a more posterior region, out of the bulb, leaving the migratory stream for renewal of neurons. Reduction in the stratum album indicates a retraction of
centrifugal fibers from the brain during juvenile brain reduction (reported earlier, Weiler Bennegger, 2015). All of this concentrates information-processing neurons within the bulb.
These results indicate a rearrangement of neurons and underlying networks, by establishing a filtering system, according to the necessity of increasing olfactory challenges during biological phases: as neonates to identify milk and social cues; as juveniles detect family members, enemies, prey, predators; as adults additionally identify sexual partners; suggesting that, although the olfactory system is functional at birth, the information processing changes during postnatal life.
