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The brains of 10 spider monkeys inoculated intracerebrally with brain suspension from kuru patients have been studied histologically and ultrastructurally. The animals were killed by perfusion of fixative from four to forty-one weeks after inoculation, when healthy and free of neurological signs. Definite histopathological changes had occurred as early as four weeks after inoculation, when moderate numbers of bi-nucleated neurons were found within the limbic cortex, striatum, the hypothalamus and amongst the Purkinje cells of the cerebellum. At later stages of incubation a moderate loss of neurons in the cerebral and cerebellar cortex and a mild to moderate proliferation of fibrous astrocytes here and also in the hypothalamus were the most striking features. None of our cases showed either status spongiosus or the generalized astrocytic proliferation and hypertrophy, characteristic of fully developed experimental kuru, in any region of the brain. The principal ultrastructural abnormalities consisted of the formation of membrane-bound intracytoplasmic vacuoles, predominantly within dendrites, and of concentric laminar arrays derived from the endoplasmic reticulum. The former were seen in all regions of the brain examined and at all stages of incubation. Concentric laminar arrays were confined to the cerebellar nodulus, where they were most numerous in dendrites and neuronal perikarya four weeks after inoculation. Both changes are interpreted as an indication that the kuru agent acts upon the plasma membrane from an early stage onwards and, by stimulating its growth, leads to the formation of complex, membrane-bounded vacuoles and to hyperplasia of the endoplasmic reticulum. The formation of vacuoles is further regarded as the first sign of status spongiosus on an ultrastructural level. Attention is drawn to the great similarities between the changes observed in the present material and those described in the brains of patients dying from kuru and of primates with fully developed experimental kuru. The significance of the relatively rapid spread of the kuru agent throughout the brain is discussed in relation to the concept of "slow virus" diseases.

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