Periods of development of brain structure in the ontogenesis of rats (review).

Abstract

The mass of the brain of the newborn rat is only 13% of its final value [7]. During the first i0 days of postnatal development, the rat brain grows vigorously in the oral-caudal direction, and at a slower rate in height and width. The thickness of the cerebral cortex of the newborn rat is equal to 0.74 [8] or 0.59-0.63 [9] mm. In i0 days the width of the cortex more than doubles, but not uniformly over the entire hemisphere. The increase in the width of the cortex occurs both on account of growth of the cel] bodies of the neurons~ forexample~ in the somatosensory zone the volume of the neurons increases by a factor of more than 1.5 in the first five days of development alone) and on account of a decrease in the density of cells of the cortex per unit area. The latter index undergoes a threefold decrease in the first I0 days of development [i0]. The cytoarchitectonics of the cerebral cortex of newborn rats is extremely immature: the first, a complex of the second to sixth layers, and the seventh layer (the somato-sensory region of the neocortex) can be distinguished in it. The pyramidal neurons are still weakly differentiated; they have only apical dendrites, basal dendrites in the form of rudiments, and the axons of the pyramids do not yet have collaterals. The cell nuclei are light colored and