List of abbreviations
of micros-
specialist terms
explained in
English +

Every attempt was made to provide correct information and labelling, however any liability for eventual errors or incompleteness is rejected!

dieser Seite

Dr. med.
H. Jastrow

of use
Overview Actin filaments (Filamenta actinia):
Pages with explanations are linked to the text below the images if available! (Labelling is in German)
area densa + actin filaments
of a smooth muscle cell 1 (rat)
actin filaments & area
densa, smooth muscle cell 2 (rat)
detail of the previous image area densa + actin filaments
of a smooth muscle cell 3 (rat)
actin filaments , synapse en pass-
ent smooth muscle cell (rat)
cross-section of striated
muscle, actin filaments (rat)
longitudinal section of a
smooth muscle cell (monkey)
smooth muscle cell,
actin filaments (monkey)
organelles + actin filaments
smooth muscle cell (monkey)
smooth muscle cell in
cross-section (monkey)
I-band: actin filaments
skeletal muscle cell (monkey)
sarcomer showin parallel actin
filaments in striated muscle(rat)
Detail: I-band & 
Z-disc (rat)
Actin filaments (Terminologia histologica: Filamenta actinia) are the microfilaments of the cytoskeleton. They are present in all cells and comprised of two fibrils helical winding around each other. These fibrils form by polymerisation of free actin molecules, which have a molecular wheight of 42 000 Dalton (Da).
About 10% of all proteins in a cell are actin in muscle cells, in other cells the actin proportion ranges from ~1 to ~5%. Roughly parallel actin filaments are responsible for stability of microvilli. Regarding actin molecules, there are 3 different alpha-actins, one of which is exclusively present in one kind of muscle. In all other (non-muscle) cells beta- or gamma-actin is seen. One kind of gamma acin is found in smooth muscle cells of the gut besides the alpha actin typical for smooth muscle cells. Actin may be present as globular monomere (G-actin) or as polymerised, filamentous F-actin. Microfilaments are formed by this F-actin with linked proteins. All actin molecules have a magnesium ion binding adenosin-di-phosphate (ADP) or adenosin-tri-phosphate (ATP). Plate-like beta-actin monomeres are about 5.5 x 5.5 x 3 nanometers (nm) in size and have 4 subdomains. The helical structure of actin filaments may be seen under ultra-high resolution in the electron microscope. The diameter of a single filament is 7 nm in thinner and 9 nm in thick portions, i.e. the mean diameter is 8 nm. The sequence of the 28 subunits of an actin filament helix which consists of two F-actin fibrils helical winding around each other is repeated all 72 nm. Actin-cross-linking proteins connect actin filaments in all bundles or meshworks formed by actin in the cytoskeleton. These coss-linking proteins are classified according to the character of their actin binding domains in 3 groups:
group 1: 30 KD protein (33.000 Da; pseudopods, stress-fibres), EF-1a (50.000 Da; pseudopods), Fascin (55.000 Da; pseudopods, stress-fibres, microvilli, acrosomal processes), Scruin (102.000 Da; acrosomal processes).
group 2: Villin (92.000 Da; microvilli of enterocytes in gut and renal tubules), Dematin (48.000 Da; red blood cells).
group 3: Fimbrin (68.000 Da; microvilli, stereovilli, adhesive strips), alpha-Actinin (102.000 Da; smooth muscle cells, stress-fibres, pseudopods), Spectrin (alpha subunit: 280.000 + beta subunit: 246.000-275.000 Da; terminal web), Dystrophin (427.000 Da; cell membrane close meshwork in striated muscle cells), ABP 120 (92.000 Da; pseudopods), Filamin (280.000; pseudopods, stress-fibres).
The term contractile filaments is used for both, actin and myosin filaments together, in muscle cells. These contractile filaments cause contraction of these cells which finally bases on a knicking of myosin heads. Almost perfect parallel order of actin filaments is seen in the I-band of striated muscle cells (images above). These actin filaments are linked to each other at the Z-stripe with help of the capZ-protein. In the A-band myosin filaments are located between actin filaments.
In general, the three dimensional shape of any cell mainly is caused by the organisation of cell membrane-close actin filament bundles and meshworks and their connection to the cell membrane which is realised by the previously mentioned linking proteins. The latter also have connections to cell membrane proteins. Thus these connections also stabilise the cell membrane.

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--> Electron microscopic atlas Overview
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Five images were kindly provided by Prof. H. Wartenberg; other images, page & copyright H. Jastrow.