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Abstract:
The olfactory bulb is composed of neurons, arranged in typical layers: I) the outermost layer, the fila olfactoria layer (FILA) constitutes the axons of the olfactory sensory cells carrying the olfactory information input; II) in the glomerular layer (GLOM) the olfactory information is synaptically transmitted to the mitral cell apical dendrite within the glomeruli, surrounded by periglomerular interneurons; III) the external plexiform layer (EPL) is a processing layer, comprising the external dendrites of the granule cells synapsing to the mitral cell secondary dendrites; IV) in the mitral cell layer (MCL), the somata of these relais neurons are located; V) the internal plexiform layer (IPL), is a connecting layer, where mitral cell axons and granule cell dendrites pass; VI) the granule cell layer (GCL) contains the granule cell somata, the major interneurons for information processing; VII) in the stratum album (STR) many centrifugal fibers reach the bulb; VIII) the subependymal layer (SUB) surrounds the ventricle; IX) the ventricle (VEN) is seen prenatally. During the postnatal development, the bulb grows continuously until adulthood, thus we were interested if the layers expand proportionally indicating same functional activity from birth on, or if the layers develop asymmetrically, pointing to activity-dependent network compositions. Therefore, we investigated histologically the olfactory bulb of the American mink (Neovison vison var. atratus), a species born very altricial in which a major part of development takes place postnatally.
A total of 36 males (aged newborns, postnatal day 1, P1, up to seven months, P210) were analyzed using a computer-assisted graphic tablet to measure the layers in nissl-stained frozen sections. The total bulb volume increases in size continuously from 1.85 ±0.04 mm3 at P1 to 152.00±9.14 mm3 in adults, a 82-fold increase, however the composition pattern is highly significantly different. The inner
layers (SUB P1: 7.43±0.42%, P210: 0.92±0.72%; STR P1: 16.98±0.47%, P210: 8.61±1.26%) decrease in proportion possibly due to the retraction of the ventricle and reduction of centrifugal migration, whereas the outermost layer (FILA) increases significantly in proportion (P1: 15.92±1.08%, P210: 26.90±5.18%).
As shown earlier, the fila volume is rather correlated to the size of the animal than to the size of the brain; with the fact that the postnatal body size increase is unproportionally higher compared to brain size increase, the earlier findings are in line with our current result of postnatal increase in the FILAproportion. Furthermore, layers containing the cell bodies increase less in volume compared to the whole bulb (MCL 24-fold; GCL 70-fold), whereas the processing plexiform layers increase unproportionally more (EPL 361-fold; IPL 188-fold) so their proportion on the whole bulb increases postnatally (EPL: P1 4.57±0.32%, P210 19.16±2.74%, IPL: P1 1.45±0.17%; P210 3.74±0.54%). This indicates that the major changes are
less related to the number of cells but to the size of neurons especially the expansion and branching of their processes. This suggests that at birth a great number of cells are already present while the formation of the neuronal network connectivity develops predominantly postnatally. Thus, the olfactory bulb layers do not expand proportionally during postnatal development but the
composition of the layers change according to their information processing and functional activity.
