They are fibrous or fine tubular structure which constitute the supportive structures of the cell. Cytoskelatal structures are of three types:
(i) microtubuls.
(ii) microfilaments.
(iii) intermediate fibres.
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(i) Microtubules:
(Robertis & Franchi, 1953). They are unbranched tubules of indefinite length, 25 nm of thickness with 15 nm core and formed of 13 helically arranged protofilaments of a and p, tubulins. Lateral projections occur for establishing cross- bridges. Microtubules grow from nucleating regions.
Their tips can grow and shorten quickly. They are basic structures of spindle apparatus, centrioles, basal bodies, cilia and flagella. They play important role in cell division, and transport of ions, water and small organic and inorganic molecules.
(b) Microfilaments:
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(Paleviz et al, 1974). They are cylindrical rods or filaments of actin-like material with a diameter of 5-6 nm and that of indefinite length. Microfilaments can form hexagonal bundles and three dimensional microtrabecular lattice, take part in cytoplasmic streaming, membrane undulations, microvilli cleavage.
(c) Intermediate Fibres:
They are solid, unbranched protein fibrils of about 10 nm thikness which are made up of noncontrac- tile protein like keratin, desmin, vimetin etc.
Centrioles:
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They are minute, submicroscopic subcylindrical structures of 0.3-0.5 |am length and 0.15 (im diameter which usually occur in pair (diplosome) inside a specialised cytoplasm called centrosphere or kinplasm. The complex is called centrosome (Boveri, 1888) or central apparatus.
Each centriole has whorl of nine triplet fibrils of tubulin with interconnections amongst adjacent triplets (C- A linkers) as well as with a central hub through spokes. Centrioles are surrounded by massules or pericentriolar satellites for formation of new centrioles in G2 phase. (Typical replication is absent).
ATPase activity is present. Centrioles are required to form basal bodies, cilia, flagella and astral spindle poles. They occur in most animal cells except some protozoan protists.
Amongst others they are found in flagellate forms or forms having flagellate stages (e.g. many green algae, bryophytes, pteridophytes and cycads). Many visual receptor cells and retinal rod cells are originated from centrioles and cilia like bodies. Centrioles in some way can initiate and control the formation of fibrous protein structures.