Experimental transmission of human subacute spongiform encephalopathy to small rodents. II. Ultrastructural study of spongy state in the gray and white matter.

Light and electron microscopic findings of spongy state in four species of small rodents, viz, mice, rats, Mongolian gerbils, and guinea pigs, are described. The spongy state existed in both gray and white matter; its intensity varied in each species, and in the gray matter corresponded to vacuoles within the neuropil. They were of two types; one was the true vacuoles within neurites, and the other was markedly swollen cell processes, some of which were also identified as neurites. In the white matter, the spongy state corresponded mainly to distension of the myelin sheaths, due to splitting of the major dense line or swelling of the inner loop, and partly to intra-axonal vacuoles. In mice before appearance of clinical symptoms, the vacuolation occurred first in the cerebral white matter 5 weeks after inoculation and in the cerebral cortex at 7 weeks. The occurrence and development of the vacuoles are discussed.