Ultrastructural characterization of novel intracisternal virus-like particles associated with a spontaneous subcutaneous tumor from a japanese wild mouse, mus musculus molossinus (mon/nm)

Ultrastructure of a new type of intracisternal virus-like particles (Mol-P) found in a subcutaneous tumor from a Japanese wild mouse, Mar musculus molossinus, was described. MoI-Ps were observed in two forms: as budding particles at the membrane of the rough endoplasmic reticulum (r-er) and as free particles, 120 nm In average diameter, In the cisternae. The envelopes of budding forms were continuous with the r-er membrane and each contained a small dense core, 45 nm in average diameter. The free forms were round, oval, or pear- shaped, and were bounded by an envelope with a unit membrane structure. Different from the coexisting intracisternal A particles Identical in profile to those reported previously, the free forms were characterized by the presence of an eccentrically located nucleold. The nucleoid consisted of a small dense core surrounded by an intermediate membranous structure (IMS) about 5 nm thick. The IMS displayed either a closed-circular structure or a whirled, open-circular profile. In the latter IMS, one end was terminated at the envelope and the other at the core. Possible origins of Mol-P were discussed In relation to known retroviruses.