Adenoviruses are non-enveloped icosahedral particles.
The capsid is built up from 252 capsomers (T=25), of which 240 are hexavalent and 12 (situated at the apices) are pentavalent.
A "penton fibre"projects from each apex.
The penton fibres consist of a slender shaft with a globular head.
They are involved in the process of attachment of the virus particle to the host cell.
Fibres easily become detached during preparation for electron microscopy.