Comparative Morphology of Avian and Murine Leukemia Viruses2

Abstract

Avian leukosis virus (ALV) and murine leukemia virus (MuLV) differed in a number of ultrastructural features. The most prominent difference between virions of the two groups was in the structure of the nucleoid which resulted from variations in the maturation process. The nucleoid of ALV formed by condensation of the inner shell of the immature form so that it was small (ca. 36 mµ) and was separated from the intermediate membrane by a lucent space. In contrast, the nucleoid of MuLV formed by dispersion of the inner shell of the immature form to fill completely the volume enclosed by the intermediate membrane. Thus the MuLV nucleoid was large (ca. 65 mµ) and no lucent space was visible between the nucleoid and intermediate membrane, so that the latter was no longer seen as a separate structure. The length of surface projections also differed. The surface projections of ALV were longer (ca. 13 m#x00B5;) than those of MuLV (ca. 8 m#x00B5;). The apparent structure of virions may have been markedly altered by the preparative technique used and the nature of the host tissue. Osmium Fixation best preserved virions in tissue cultures. Glutaraldehyde fixation was the desirable method with viruses fixed in animal tissue; however, in cell culture, it was less desirable because internal structures were obscured. Treatment of fixed virus with uranyl acetate before embedment markedly disrupted viral morphology.