At seven days of age, inbred strains such as SEC/Re 1J and C57BL/6 present clear variations of a number of behavioral and electrophysiological parameters. The present investigation was carried out in order to provide anatomical evidence of the interstrain variability in maturation during the first postnatal week by analysing the morphological characteristics of cortical areas in seven day old SEC/RelJ and C57BL/6 mice. The somatosensory cortex was chosen on the basis of its easy localization in histological preparations, its postnatal plasticity and its functional importance during ontogeny. Different measures were performed in areas 1, 3 and 40: cortical depth, depth of layers, grey/cell coefficient, and cellular volumes. The main interstrain differences were observed in areas 1 and 40 and interpreted either in terms of ontogenic variability or, alternatively, in terms of phenotypic character! sties of the brain cortex in inbred mice. Conversely, the level of maturation in area 3 was found remarkably similar in both strains. The relationship between morphological parameters and other measures of the postnatal development is discussed.
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