The morphometric analysis of changes occurring in the rat and rabbit ventroposterolateral (VPL) nucleus of the thalamus during the postnatal development was performed using unbiased stereological methods. The materials used in the study included 30 Wistar rats and 32 New Zealand rabbits aged from P0 to P180 (P-postnatal day), which were divided into six and eight age groups, respectively. The following stereological parameters of VPL nucleus on the cresyl violet stained sections were determined: volume of the nucleus, numerical density and total number of neurons. The total number of neurons indicated that the development of VPL nucleus in both species ended within the third week of postnatal life. The volume of VPL nucleus increased gradually (by about 2.2 and 5 times in rats and rabbits, respectively) in comparison with the volume of the cerebral hemisphere during the development from P0 to adulthood. The numerical density of VPL neurons decreased rapidly at the beginning of postnatal life and stabilized by the end of the third week. In both species, the gradual increase in the volume of VPL nucleus and the simultaneous decrease in the neuronal density in the first week of postnatal life were mainly caused by changes in the neuropil volume. The total number of cells did not change remarkably during the first postnatal week. However, it decreased significantly during the second week. This decrease was probably due to the naturaly occuring cell death. These results show that the most prominent qualitative and quantitative changes in VPL nucleus and its neurons occur during the first two weeks of postnatal life of rats and rabbits. Also, because the thalamocortical relay neurons completely acquire their physiological features, this the most critical period of time for their morphological maturation.
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