ABSTRACT
Literature on the anatomy of the African Giant rats (AGR) is gradually building; there is paucity of information on the detailed morphometric, histologic analysis of the cerebrum and olfactory bulbs of the AGR. No stereologically study has been done on the olfactory bulb and cerebrum of the AGR. The quantification of cerebral layers and olfactory bulbs, their neuronal population and volume in AGRs, may shed light on neuroanatomical basis of their highly developed olfactory ability of the rodent and some of its behaviour. The general aim of the study was to investigate the cerebrum and olfactory bulbs of the AGR in order to elucidate the neuroanatomical basis for their special olfactory functions. Ten adult AGR (Cricetomys gambianus, Waterhouse-1840), consisting of five male and five females were used for this study and following standard procedures, the animals were captured alive around Zaria, using locally made rat traps, without any injury on them. The animals were anaesthetized with chloroform, sacrificed and transcardially perfused with a phosphate buffered solution of (pH 7.2, M = 0.12) 4% formaldehyde and 1% glutaraldehyde. The brain were removed and placed in the same fixative. The morphologic characteristics of the cerebrum and olfactory bulbs were observed with the naked eyes and electronic magnifying lens, and morphological features were recorded. Morphometric parameters of the cerebrum andolfactory bulb were taken, recorded and analyzed statistically using descriptive statistics and student's t- test. Histological sections were prepared after fixation and routine tissue processing, embedded in a low-gelling temperature agarose (6% agar), tissue sections of 60 um cut, perpendicular to the long axis (coronal section), were serially obtained with a calibrated vibratome and staining in thionin stain, stained tissues were viewed under the microscope and photomicrographs were taken. Designed-based stereological methods (optical fractionation and Cavalieri principle) were employed to estimate the neuronal number in olfactory bulb and volumes of the different layers in the olfactory bulb and cerebral cortex stained in 0.2% Thionin solution. The results showed that the adult AGR have lissencephalics cerebrum, a mean absolute cerebral weight of 3.44 � 0.07g, relative cerebral weight of 0.29 � 0.23, a globular shaped olfactory bulb, the olfactory bulb accounts for 8 % the total body weight in the AGR, cyto-architectural arrangement was similar to that of the olfactory bulb of dog, and other rodents, a mean neuronal number in the different layers of the olfactory bulbs irrespective of sex for the periglomerular cells, tufted cells, mitral cells, periglomerular cells and the granule cells account for 20.72%, 6.44%, 4.61%, 3.27% and 63.68% of the neurons in the five layers of the olfactory bulb in AGR respectively. Results of the mean volumes of the glomerular cell layers and the granule cell layer of the AGR, showed that Layers I, 11, 111, IV, V and VI accounted for 19.7%, 18.5%, 18.6%, 21% and 23.7% of the volume of the cerebral cortex of the AGR, respectively. This study for the first time reports on the volume of the different layers of the olfactory bulbs and cerebral cortex in the African giant rat (Cricetomys gambianus). The results obtained from this study can be used in comparative anatomy of rodents of similar species, comparative neurology of rodents and other mammals, and in elucidating the mechanism of olfaction and other brain functions in this macrosomatic rodent. The number and of neurons and the volumes of the individual layers in olfactory bulb and the cerebrum, may represent a more accurate indicator of the neural machinery and synaptic microcircuitry in these brain regions in rodents and other mammals.