Spongiform change--an electron microscopic view.

Spongiform change is a hallmark of transmissible spongiform encephalopathies (TSEs) in prion diseases. They are defined as small round or oval empty spaces in the neuropil. When confluent, they merge to form "morula-like" structures. Their neuroanatomical distribution and grading within each of defined neuroanatomical areas underlie the lesion profile method used to discriminate strains in rodents and in cattle. Ultrastructurally, vacuoles develop within neuronal elements. They are membrane-bound and contain secondary vacuoles and "curled" membrane fragments. Separate type of vacuoles are those develop within myelin sheath. Those vacuoles develop through complicated opening at the major-dense and intraperiod lines. The histogenesis of spongiform vacuoles is unclear and the only hypothesis that they develop through the formation of abnormal configuration of plasma membranes (ACPMs) has never been substantiated. We suggested that vacuoles may develop through process of autophagy.