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A quantitative neurohistological study of the long term effects in the rat brain of stimulation in infancy.

Infant rats were either handled daily during the first 10 days of life or left undisturbed. Brains were examined, using quantitative histological methods, at 22 days postnatum and at 6 months postnatum. At 22 days the handled animals showed a deficit in glial cells in both the anterior limb of the anterior commissure and in the indusium griseum. The deficit in the former was in the oligodendrocyte fraction, whereas in the latter the deficit was in astrocytes. Myelination was unaffected, the percentage of myelinated axons, the diameter of myelinated and unmyelinated axons and the mean number of myelin lamellae being identical in handled and control groups. The numbers of mitotic figures in the subependymal layer were also similar in both groups. At 6 months of age the handled group contained more glial cells in both the rostral part of the anterior limb of the anterior commissure and the indusium griseum. In the former, the increase in glial cells was due to an increase in the number of oligodendrocytes in the handled group compared with controls; in the latter, the main increase was in astrocytes. The percentage of myelinated axons and the number of subependymal mitotic figures were both significantly higher in handled animals. It was concluded that infantile handling leads to an initial growth retardation associated with a long term increase in glial cell number. It was suggested that the initial retardation might be the result of glial cells remaining for a longer than normal time in the proliferative subependymal pool before migrating to the periphery. It was noted that the timing of the handling coincided with the period of normal rapid proliferation of oligodendrocytes in the anterior commissure and of astrocytes in the indusium griseum.