Postnatal development of the mouse cerebral neocortex. I. Quantitative cytoarchitectonics of some motor and sensory areas.

Quantitative data have been obtained (numbers of neurons and glial cells in a cube of 0.000 25 mm3, thicknesses of cortical layers, nuclear diameters of neurons) on Cresyl Violet stained sections from the six cortical layers of the Swiss Albinos mouse brain through the postnatal development at 5, 10, 30, 60 and 180 days. 1) Normative data of the cortical cell population are now available from motor areas 4 and 10 and sensory areas 3 and 2 according to KRIEG (1946). The evolution of the neuronal densities through time was obtained and was taken as a criterion of maturation for the different layers in the studied cortical areas. 2) Areas 4, 10, 3 and 2 have been described histologically at all the mentioned ages. 3) The nuclear diameter of the neurons increases in all the layers between 5 and 10 days and less between 10 and 30 days. It remains stable from 30 days on. 4) At adult age the sensory areas 3 and 2 show a greater thickness of the group of supragranular layers II, III and IV than of the infragranular layers V and VI together. Layer V shows the same thickness in areas 2, 3 and 4. The same is true for layer VI in the same areas. In area 10 layers V and VI are thicker than in areas 2, 3 and 4. At younger ages (5 and 10 days) the relative thicknesses of the different layers are of the same order but each layer is thinner. 5) The neuronal density shows the same type of development in the four studied areas. Maturation in terms of decrease of neuronal densities is very rapid between 5 and 10 days and slows down till 30 days with speeds depending on the layers. Layers I, V and VI show faster and earlier maturation than layers II, III and IV. 6) The glial density is mostly stable through the development. This does not mean that there is no glial proliferation. 7) The delineation between the different areas according to KRIEG's (1946) atals and/or CAVINESS's (1975) publication are discussed taking into account our quantitative data.