A variety of cellu­lar processes and pathways occur in the organelles of eukaryotic cells in the cytosol, which is the region of the cytoplasm between and sur­rounding the organelles. Until a few decades ago, the cy­tosol of eukaryotic cells was regarded as a generally uninteresting gel-like substance in which the nucleus and other organelles were suspended. Cell biologists knew that proteins made up about 20-30 per cent of the cytosol, but these proteins were thought to be soluble and able to move freely. Except for those of known enzy­matic activity, little was understood about the structural or functional significance of cytosolic proteins.

Advances in microscopy and other investigative tech­niques have revealed that the interior of a eukaryotic cell is highly structured. Part of this structure is provided by the cytoskeleton: a complex network of interconnected filaments and tubules that extends throughout the cytosol, from the nucleus to the inner surface of the plasma membrane.

The term cytoskeleton accurately expresses the role of this polymer network in providing an architectural fra­me­­work for eukaryotic cells. It confers a high level of internal organisation on cells and enables them to assume and maintain complex shapes that would not otherwise be possible. The name does not, however, convey the dy­namic, changeable nature of the cytoskeleton and its criti­cal involvement in a great variety of cellular processes.

The cytoskeleton plays important roles in cell move­ment and division, and it positions and actively moves membrane-bounded organelles within the cytosol. It also plays a similar role for messenger RNA and other cellular components. Many enzymes in the cytosol are, in fact, probably not soluble at all but are physically clustered and attached to the cytoskeleton in close proximity to other enzymes involved in the same pathway, thereby facilitating the channeling of intermediates within each pathway. The cytoskeleton is also involved in many forms of cell move­ment and is intimately related to other processes such as cell signalling and cell-cell adhesion. The cytoskeleton is altered by events at the cell surface and, at the same time, appears to participate in and modulate these events.

It is a structural feature of eukaryotic cells revealed especially well by digital video microscopy, electron microscopy and immunofluorescence microscopy. It con­sists of an extensive three-dimensional network of micro­tubules, microfilaments and intermediate filaments that determines cell shape and allows a variety of movements.

Microtubules (MTs) are hollow tubes with walls consisting of heterodimers of a and b-tubulin polymerised linearly into protofilaments. MTs are polar structures and elongate preferentially from one end, known as the plus end. First identified as components of the axonemal structures of cilia and flagella and the mitotic spindle of dividing cells, microtubules are now recog­nised as a general cytoplasmic constituent of most eukaryotic cells. Microtubules can undergo cycles of catastrophic shortening or elongation, a phenomenon known as dynamic instability. Within cells, MT dynamics and growth are organised by microtubule-organising centres (MTOCs). The centrosome is a major MTOC that contains nucleation sites rich in g-tubulin that are used to nucleate MT growth. Microtubules are stabilised along their length and at their plus ends by microtubule-associated proteins.

Microfilaments (MFs) are double-stranded polymers of actin that were initially discovered because of their role in the contractile fibrils of muscle cells; they are now recognised as a component of vir­tually all eukaryotic cells. Microfilaments are required for many processes within cells, including locomotion and maintenance of cell shape. Like micro­tubules, MFs are polar structures, with actin monomers preferentially added to one end and removed from the other. Microfilament assembly within cells is reg­ulated by the small G proteins Rho, Rac, and Cdc42, derivatives of phosphatidyl inositol known as polyphosphoinositides, and capping proteins. Other actin cross-linking, severing and anchoring proteins regulate the organisation of MFs within cells, which range from the parallel arrays of actin in micro­villi to branched actin networks.

Intermediate filaments (IFs) are the most stable and least soluble constituents of the cytoskeleton. They appear to play a structural or tension-bearing role. IFs are tissue-specific and can be used to identify cell type. Such typing is useful in the diag­nosis of cancer, as tumor cells are known to retain the IF proteins of their tissue of ori­gin. All IF proteins have a highly conserved central-domain flanked by terminal regions that differ in size and sequence, presumably accounting for the functional diversity of IF proteins.

IFs, MTs, and MFs are intercon­nected within cells to form cytoskeletal networks that can withstand tension and compression, providing mechanical strength and rigidity to cells.

The writer is Associate Professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at tapanmaitra59@yahoo.co.in