Fig. 1.

Stages of HCMV assembly. HCMV assembly was documented in human embryonic lung or foreskin fibroblast cells fixed in 2% formaldehyde/1.5% glutaraldehyde and postfixed with reduced OsO₄ and tannic acid. (A–E) Human embryonic lung cells. (A) The nucleus contains empty icosahedral capsids (e.g., white arrow) or nucleocapsids enclosing a darker ring composed of the viral DNA (black arrow). (B) The capsids bud into the lumen of the nuclear envelope, acquiring a first membrane from the inner nuclear envelope (black arrowheads). This membrane is lost during fusion with the outer membrane of the nuclear envelope, so that naked capsids are released into the cytoplasm. (C) A cytoplasmic “virus factory” composed of electron‐dense accumulations of viral tegument proteins (asterisk) and various membrane vesicles. HCMV capsid particles acquire a layer of tegument and then proceed to wrap into the membrane vesicles. (D) Detail of the envelopment of tegumented capsids at cytoplasmic membranes near the Golgi complex (Go). The membrane vesicles are deformed into characteristic crescents (black arrowheads). This leads to the production of mature enveloped HCMV particles within cytoplasmic vacuoles (white arrow). (E) Fusion of these vesicles with the plasma membrane releases HCMV particles into the extracellular space. (F and G) Human foreskin fibroblasts. (F) Three types of enveloping particles in the cytoplasm. The white and black arrows indicate envelopment of an empty capsid and a DNA‐filled nucleocapsid, respectively, while the asterisk marks an enveloped tegument aggregate or “dense body.” Note also the empty capsid (white arrowhead) in the nucleus (N), which lacks the tegument layer. (G) Two multivesicular late endosomes (1, 2) and a lysosome containing multilamellar membranes (L). Endosome 2 contains several assembled HCMV virions. Images were taken from cells infected for 72 h (A and B), 96 h (E, F, and G), or 120 h (C and D). Scale bars = 200 nm.