A-type virus particles A term used originally by electron microscopists to designate a morphologically defined group of RNA virus particles, often found in tumor cells. They are double-shelled spherical particles, appearing in thin sections as two concentric rings, the outer with a diameter of 65-75nm and the inner with a diameter of approximately 50nm. The inner ring usually appears more dense. The center is electron-lucent but contains some amorphous material. They are always intracellular and morphologically similar but there are at least two groups. (1) Intracytoplasmic particles within the ground substance of the cytoplasm where they may form large paranuclear masses within or close to the Golgi area. They are typically seen in mouse mammary tumor cells but are also present in cells of lymphomas in mice with the mammary tumor virus. They are intermediates in the assembly of B-type or D-type virions. (2) Intracisternal A particles (IAP) which appear in uninfected rodent-derived cells, budding from the inner surface of the cisternae of the endo-plasmic reticulum. They are more variable in size than the first group of A-type particles and are not considered to be related to other morphological types such as Band C-type particles. They are believed to represent expression of endogenous proviral genetic elements, present in high copy number in rodent cells.
Bernhard W (1960) Cancer Res 20, 712 Dalton AJ (1972) J Natl Cancer Inst 49, 323
atypical measles A syndrome reported in children exposed to wild-type measles virus who had received inactivated measles vaccine 2-4 years previously. The symptoms were severe and included an extensive maculopapular rash and interstitial pneumonia.
Frey HM and Krugman S (1981) Am J Med Sci 281, 51
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