Postnatal development of the visual cortex of the mouse after enucleation at birth.

Quantitative data in the neocortex up to the age of 180 days (neuronal densities, number of neurones, glial cells, dendritic intersections and spines) were compared in normal mice and mice enucleated at birth. Bilateral enucleation induced an increase of neuronal density in all cortical layers of areas 17, 18a, and 41, the supragranular layers II-III being more affected than layers IV-VI. This was noticed in layer II 10 days after the operation and was maximal in all layers between 30 and 60 days; at 180 days there was some return to normal of the neuronal density in all layers. The total number of neurones and glial cells were the same in the bilaterally enucleated mice as in the controls. No reaction in dendritic branching was evident for pyramids of layers III and V in areas 17 and 41 after bilateral enucleation. In contrast the number of spines was reduced on the apical dendrites of pyramids from layers II and V in area 17, but not in area 41. After unilateral enucleation the reaction was less severe and delayed compared with bilateral enucleation, the first signs of increase of neuronal density appearing 30 days after the lesion in the contralateral hemisphere. The contralateral areas 17 and 18a were more affected than the ipsilateral ones and area 41 showed no change compared to the control. As after bilateral enucleation, layers IV and V were least affected by unilateral enucleation in both ipsi- and contralateral cortices. These results suggest that deafferentation in an immature system affects the development of all cortical layers but with a greatest intensity in supragranular layers, which are not the main direct targets of thalamo-cortical input.