Classification and ultrastructural appearance

Retroviruses are enveloped viruses that contain two copies of a single-stranded RNA genome. Their characteristic feature is an RNA-dependent DNA polymerase, termed reverse transcriptase, which allows the virus to reverse transcribe its RNA genome 'backwards' into a DNA copy, hence the name retrovirus. This DNA copy becomes integrated into the host genome during retroviral infection and serves as the template for the transcription of viral genes and the production of viral proteins (see below).

Retroviral genetic information can be transmitted by two distinct mechanisms. 'Horizontal' or infectious transmission occurs when the virion fuses with uninfected cells. Retroviruses which utilize this mechanism of infection are known as exogenous retroviruses. 'Vertical' transmission occurs when germline cells become infected and the integrated viral genome is then transmitted to subsequent generations as an endogenous genome. The term 'vertical' transmission is also used to describe an exogenous infection of the fetus occurring in utero or perinat-ally. Both exogenous and endogenous retroviruses are found in many species (see Table 1 for some examples). Retroviruses are conventionally classified as oncoviruses (subdivided morphologically into B-type, C-type, D-type viruses), lentiviruses, and foamy (spuma) viruses. B- and D-type viruses bud from the cell membrane with their cores already assembled in the cytoplasm whereas C-type viruses, lentiviruses and foamy viruses assemble their cores during the budding process. Mature lentiviral particles differ from C-type viruses by the conical shape of their core (the C-type core has a more spherical shape).

A-type particles are intracellular, retroviral corelike particles which are occasionally seen in some tissues or cell lines. In cells infected with D- or B-type retroviruses they occur in the cytoplasm and represent intracellular B- or D-type particles. In contrast, intracisternal A-type particles are localized in the endoplasmic reticulum and have been described in a strain of diabetic mice, in some human cell lines, in preterm placenta of some primates, including humans, as well as in short-term cocultures of biopsies from patients with Sjogren's disease.

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