In addition to the actin microfilament and microtuble cytoskeletal systems found in all mammalian cells, there is a third cytoskeletal network comprised of filaments which are intermediate in size between the other two systems and which are therefore termed intermediate filaments (Figure 3.1 & 3.2). Lazarides described five classes of intermediate filament within higher eukaryotic cells, distinguishable biochemically and immunologically: keratin, desmin, vimentin, neurofilaments and glial filaments (Lazarides, 1980). The current classification of the different types of intermediate filament is shown in Table 3.1, below.