Herpes simplex virus L particles contain spherical membrane-enclosed inclusion vesicles.

The fine structure of light (L) particles of herpes simplex virus type 1 was examined by cryo-electron microscopy and compared to that of virions. The L particles appeared to be spherical entities with significant variation in size, on average smaller in diameter than virions (140 nm compared to 180 nm). The technique confirmed that L particles are composed of an outer envelope, i.e. a bilaminar membrane with protruding glycoprotein spikes, and a uniformly granular tegument, but lack any nucleocapsid. In addition it revealed the presence of one or occasionally more spherical objects, termed inclusion vesicles (IVs), embedded in the tegument of a large proportion of L particles but not observed in virions, suggesting that presence of IVs is unique to the L particles. The IVs vary in size and appear to be composed of a bilaminar membrane without surface projections and filled with material of relatively low electron density, suggesting that the composition of IVs is distinct from that of the envelope and tegument of the L particles.