Postnatal development of the mouse cerebral neocortex. III. Some dynamical aspects

In this paper we analyse our data from the quantitative cytoarchitectonic study of motor and sensory areas 10--4--3 and 2(Leuba and coll., 1977) and of visual and auditory area 5--17--18a--18--41 and 20 (Heumann and coll., 1977). The analysis of neuronal densities and cortical depths shows the following facts during the postnatal maturation: 1. Generally the sequence of maturation of the cortical layers is the same in the 9 studied areas: first layers I. and V, then VIb and VIa followed by III and IV and finally II. 2. This sequence is the same between 5 and 10 days as between 10 and 30 days. Still the maturation is much faster between 5 and 10 days. After 30 day differences between the layers are not very important. 3. For layers I, V and VI our 9 areas show an approximately similar degree of development from 5 days on. 4. Layers II, III and IV are in a faster period of development between 5 and 10 days. At this moment and for these layers, areas 17, 2, 3, 4 and 10 are less well developed than areas 41, 20, 18 and 18a. 5. One can thus consider that in mice the maturation of the areas seems to be more correlated with their localization than with their function. Effectively areas 41, 20 and 18a are in the vicinity of the rhinal fissure and a maturational gradient may start in this fissure. 6. In terms of neuronal densities and cortical depths an adult aspect of the layers and of the areas seems to be reached around 30 days. But some other phenomena of maturation exist after 30 days.