Histogenesis of the mesocortical area of the mouse telencephalon.

The histogenesis of the mesocortex of the mouse telencephalon was studied by plotting the progress of neuron release on reconstructions of the medial pallial wall. Histological changes were correlated with cell birth date using data obtained from autoradiographs of mice pulse-labelled with tritiated thymidine during the prenatal period of neuron birth. It was found that neuron release for this area began rostrally at about E12-E13 and spread rapidly from this origin in a caudal direction across the medial wall. Neurons accumulating in the intermediate layer were at first more or less equally spaced. About E14, neurons in the outer intermediate layer began to line up to form a 'mesocortical plate'. This plate was formed from older nuclei and therefore overlay a deeper intermediate layer composed of younger cells. The mesocortex continued to develop by progressive withdrawal of younger cells from the deep intermediate layer into more superficial layers of the definitive cortex. In most of the mesocortical area, however, this original pattern was superseded by release of neuron generations which migrated directly to the outer intermediate layer to form a plate of densely packed immature neurons. This population was continuous with a similar population forming the isocortical plate of the lateral telencephalic wall. It was postulated that the wave of neuron birth and release which gave rise to the isocortical plate was propagated beyond the isocortical boundary into mesocortical territory as far as the boundaries of the subiculum, indusium griseum and anterior hippocampal rudiment.