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Origin and fate of neural macrophages in a stab wound of the brain of the young rat.

Colloidal carbon was injected intravenously into young rats to label circulating monocytes before making a stab wound in the brain. The rats were killed 3-16 days after the stab wound. Demonstration of non-specific esterase, thiamine pyrophosphatase and 5'-nucleotidase was carried out on the carbon-labelled macrophages at the site of lesion at various survival times. In rats killed 3-5 days after the injury numerous carbon-labelled macrophages were present in the needle passage as well as in the marginal area of the lesion and they showed a positive reaction for non-specific esterase. The reaction of the enzyme was found in some of the dense bodies in the form of punctate precipitates. The reaction for thiamine pyrophosphatase was seen in the Golgi saccules as well as on the plasma membrane, although in the latter the reaction was weaker. Intense reaction for 5'-nucleotidase was localised over the plasma membrane as well as over the dense bodies. The carbon-labelled macrophages displaying the activities of the above enzymes in the 3-5 days postoperative group were of the round type. However, in the 8-16 postoperative days animals, the cells were either oval or had assumed an elongated outline resembling the microglial cells seen in the tissue taken from the normal side. It is concluded that circulating monocytes are a main source of brain macrophage in traumatic brain lesions. In the healing process of the wound some of the cells regress to become microglial cells as shown by the presence of the carbon particles as well as non-specific esterase, thiamine pyrophosphatase and 5'-nucleotidase activity in the various stages of structural transformation.