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Journal of the Anatomical Society of India

Morphological Asymmetry In Olfactory Bulbs In Rat

Author(s): Khanna, J. and Sengupta P.

Vol. 50, No. 1 (2001-01 - 2001-06)

Department of Anatomy, University College of Medical Sciences, Shahdara, Delhi-INDIA

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Abstract

The concept of functionally and morphologically symmetrical brain has been generally abandoned during the last decade.The right hemisphere of the rat brain participates preferentially in spatial and emotional behaviour and sense of smell has been linked withboth of them. However, little is known of the anatomical basis of the asymmetrical behaviour. Stimulated by these results of functionalasymmetry between the olfactory bulbs of the right left sides of the albino rats, the present study of measuring the volume of olfactory bulbshas been undertaken.

The study was conducted on 20 albino rats of both sexes. The weight and volume of whole brain was recorded. Based on cytoarchitecture, myeloarchitecture and physiological observations in literature two concentric strata are identified in olfactory bulbs of albino rats. Volume of each stratum was measurd separately on both sides.

The findings suggested the volume asymmetry of olfactory bulbs as well as in one of its subdivisions. The total volume of olfactory bulb and volume of outer stratum is greater on the right side. The asymmetry did not depend on sex. The morphological asymmetry suggests a difference in the asymmetrical organization of olfactory pathways and these may be useful indicator in explaining the different behaviour.

Key words : Olfactory Bulbs, Asymmetry

Introduction :

Although cerebral asymmetry was long thought to be a distinctive feature of human brain there is an increasing evidence to suggest that right hemispheric lesions do not produce similar behavioural effects as that of left hemispheric lesions. Dennenberg et al (1978) and Sherman et al (1980) studied the effects of lateralized brain lesions on various behavioural responses & concluded that the right hemisphere of the rat brain participates preferentially in spatial and emotional behaviour and the sense of the smell has been linked with both of them. However, little is known of anatomical basis of these asymmetrical behaviour. Leonard et al (1981) found that removal of right or left olfactory bulb had asymmetrical effects on hudding behaviour in rats. Stimulated by these results of functional asymmetry between the olfactory bulb of right and left side, the present study has been undertaken to measure the volume of olfactory bulbs in rats.

Material and Methods :

The study was conducted on 20 albino rats of both the sexes. The brains were dissected out after anaesthetizing the animals with ether and perfusing with 10% formal saline. The weight and volume of whole brain was recorded. 10 micron thick, paraffin sections of olfactory bulbs of the rat brain have been divided into two concentric strata according to the cytoarchitecture, myeloarchitecture and knowledge of physiological observations (Sidman et al, 1971), Sharp et al, 1977) The outer stratum of the bulb consisted of the lamina glomerulosa, granularis externa, plexiformis externa, cellularum mitralium and plexiformis interna. The inner stratum consisted of lamina granulosa interna and medullaris interna (Fig. I & II). Camera lucida drawings of the both of strata were taken on the graph paper of every 10th section (Fig. III).The total volume of each stratum was calculated.

Results :

The total volume of right and left bulbs was

8.13 + 1.8mm3 and 6.99 + 1.7mm3 respectively. The difference between the right and left bulb was found to be statistically significant (P < 0.01). The overall asymmetry was determined by asymmetry in the outer stratum. The outer stratum was significantly larger on the right side (mean value right side 5.30 ± 1.2 and left side 4.23 + 1.2 respectively). It was found to be statistically significant (P < 0.001) whereas the inner stratum was not significantly asymmeterical [mean value right side 2.83 + 0.63 and left side 2.75cm + 0.62 respectively. (p > 0.05)]. Asymmetry is present in both the sexes but magnitude of asymmetry tends to be greater in males.

Discussion :

The findings suggest the presence of volume asymmetry in the rat olfactory bulb as well as in one of its subdivisions. The total olfactory bulb and outer stratum are larger on right side and asymmetry did not depend on sex although magnitude of asymmetry tended to be greater in males. No significant difference was observed in the volume of inner stratum of right and left olfactory bulb. The morphological asymmetery may suggest a difference in the functional asymmetrical organisation of olfactory pathways and these may be useful to consider in explaining the asymmetrical behavioural symptoms.

The studies of Scalia and Winans (1975) and Skeen and Hall (1976) provide evidence that in several species the cells of superficial stratum give rise to most of central olfactory projections whereas inner granular layer receives afferents from ipsilateral and contralateral olfactory nuclei. The afferents from the outer stratum make up the main portion in lateral olfactory tract. Therefore asymmetry in the outer stratum shows that this system is connected more discretely with the central olfactory regions within the same hemisphere. The morphological asymmetry suggests a difference in this asymmetrical organization of olfactory pathways and may be responsible for different behaviour. The absence of asymmetry in the inner stratum suggests averaging affect of a commissural system.

These studies correspond with those of Sherman et al, 1982 and Kolb et al, 1983 who found various regions of right hemisphere to be significantly larger than the homologus region of the left hemisphere. Sherman and Galaburda (1984) found the total right isocortex of albino rat significantly larger than left. Diamond et al (1982) were able to show that in male rats right hippocampus is thicker than the left. De Olmos et al (1978) stressed the existence of a close relationship between the hippocampus and sense of smell. The side difference we ascribe here might reflect this close relationship.

VOLUMETRIC MEASUREMENT OF DIFFERENT STRATA OF OLFACTORY BULBS

No. of animal Weight of Animal (gm) Sex of Animal Outer Stratum Inner Stratum Total Volume
Rt Lt Dif. Rt Lt Dif. Rt Lt Dif.
1 210 M 4.90 3.70 1.20 2.70 2.80 -0.10 7.60 6.50 1.10
2 240 M 5.12 3.52 1.60 3.38 3.30 0.08 8.50 6.82 1.68
3 180 M 5.41 3.45 1.96 2.38 2.27 0.11 7.79 5.72 2.07
4 160 M 4.80 3.57 1.23 2.79 2.63 0.16 7.59 6.20 1.39
5 150 M 3.43 2.42 1.01 2.11 2.08 0.03 5.54 4.50 1.04
6 170 M 3.84 3.54 0.30 1.68 1.58 0.10 5.52 5.12 0.40
7 260 F 7.14 6.97 0.17 3.52 3.66 -0.14 10.66 10.63 0.03
8 185 F 3.43 3.42 0.01 2.11 2.28 0.03 5.74 5.70 0.04
9 135 M 5.15 4.06 1.09 2.67 2.50 0.17 7.82 6.56 1.26
10 310 M 7.45 6.66 0.79 3.63 3.48 0.15 11.08 10.14 0.94
11 250 M 5.00 3.85 1.15 3.50 3.43 0.07 8.50 7.28 1.22
12 265 M 7.20 5.50 1.70 3.62 3.50 .012 10.82 9.00 1.82
13 230 M 5.28 3.85 1.43 2.21 2.20 0.01 7.49 6.05 1.44
14 190 F 3.69 2.79 0.90 1.90 1.83 0.07 5.59 4.62 0.97
15 200 M 4.95 3.85 1.10 2.35 2.21 0.14 7.30 6.06 1.24
16 285 M 7.35 6.30 1.05 3.63 3.52 0.11 10.98 9.82 1.16
17 210 M 5.10 4.03 1.07 2.83 2.70 0.13 7.93 6.73 1.20
18 290 M 6.90 5.69 1.21 3.57 3.42 0.15 10.47 9.11 1.36
19 235 M 5.10 3.77 1.33 3.03 2.90 0.13 8.13 6.67 1.46
20 195 F 4.80 3.83 0.97 2.94 2.86 0.08 7.74 6.69 1.05

References :

  1. Denenberg, V. H.; Garbanati, J., Sherman, G., Yatzey, D. A. and Kaplan R. (1978): Infantile stimulation induces brain lateralization in rats Science 201 : 1150-1152.
  2. Kolb, B., Sutherland, R.J. Nonnemann, A. J. and Whishaw I.
  3. W. (1983): Asymmetry in the cerebral hemispheres of the rat,mouse, rabbit, and cat : the right hemisphere is larger. Experimental Neurology 78 : 348-359.
  4. Leonard, C.M., Williamson, M. and Freund G. (1981): Asymmetrical effects on early olfactory bulb lesions: is the hamster brain lateralized ? Society of Neurosciences Abstract 7 : 399.
  5. Scalia, F., and Winnans S. (1975) : The differential projections of the olfactory bulb and assessory olfactory bulb in mammals. Journal of Comparative Neurology 161: 31-56.
  6. Sharp, F.R., Kauer, J. S and Shephered, G. M. (1977) : Laminar analysis of 2-deoxyglucose uptake in the olfactory cortex of rabbit and rat. Journal of Neurophysiology 40 : 800 - 813.
  7. Sherman, G.F., and Galaburda A. M. (1984) : Neocortical asymmetry and open-field behaviour in the rat. Experimental Neurology 86 : 473-482.
  8. Sherman, G. F., Galaburda, A. M. and Geschwid, N. (1982). Neuroanatomical asymmetries in non-human species. Trends in Neurosciences 12 : 429-431.
  9. Sherman, G. F., Garbanati, J.A., Rosen, G.D., Yatzey,D. A. and Denengerb, V. H. (1980): Brain and behavioural asymmetries for special preference in rats Brain Research 192 : 61-67.
  10. Sidman, R. L., Angeine, J. B., and piercce E. T. Atlas of the Mouse Brain and Spinal Cord. Harvard University Press, Cambridge, M. A. (1971).
  11. Skeen, L.C., and Hall W. C. (1976) : Efferent projections of the man and accessory olfactory bulb in the tree shrew (Tupaia glis) Journal of Comparative Neurology 172 : 1-36.

J. Anat. Soc. India 50(1) 31-33 (2001) Lamina Medularis Interna (g)

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Fig. 1 & 2. Olfactory Bulb Showing different layers

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Fig. 3 Camera Lucida Drawings Showing both the Olfactory Bulbs.

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