The axon reaction was studied using immature rats aged 4 or 14 days at the time of unilateral removal of the cingulate cortex. After survival times of one to 5 days they were sacrificed and the anterior thalamic nucleus was examined using ultrastructural methods. The degree of degenerative change was much more intense in the 4-day group where the center of the reactive area was characterized by pronounced lysis of the neuropil and the loss of most neurons. Dark neurons were observed in both groups but mitochondrial proliferation was restricted to the 14-day group. Evidence for microglial formation from pericytes was observed in the 4-day group while reactive astrocytes were observed in both groups. It was concluded that a rapid maturation of the thalamic injury response occurs between the 4th and the 14th day in the rat so that by the 14th day the response is similar in many respects to that of mature animals.
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