CYTOSKELETON
Bundles of protein fibres in the cytoplasm form a reticulum. This reticulum determines the shape of a cell and allows cell movements. In animal cells the region near the nucleus containing a pair of centrioles arranges the cytoskeleton. The three main kinds of cytoskeleton fibres are: microtubules, actin filaments, intermediate filaments. The most important among them are actin filaments and microtubules. They participate in mechanisms of cell movements. Actin filaments permit muscle contraction. Microtubules are the main structural and power elements permitting movements of cilia and flagella. Microtubules in the form of the mitotic spindle are the most important part of the apparatus providing the right distribution of DNA between daughter cells during cell division.