The Role of the Cytoskeleton in the Responses of Target Cells to Hormones
To make the best use of its organelles and macromolecules, the cell requires a cytoskeleton. This term is applied to three systems of tubules found in most if not all cells, namely microtubules (diameter, 25 nm), intermediate filaments (diameter, 10 mn) and microfilaments (diameter 6 nm). The basic component of microtubules is the protein tubulin; intermediate filaments contain a variety of proteins (keratin, desmin, and vimentin) (Lazarides, 1980) and microfilaments are composed of actin. The term cytoskeleton is intended to evoke analogies with the skeletal system of vertebrate organ- isms which provides a rigid support for the attachment of the contractile machinery of the body. As a result of this relationship, the muscles can both shorten and provide force at a fixed length. The analogy has some virtue but inevi- tably comparisons between the mechanics of one cell and those of a whole organism cannot be taken too far. Since little is known about the functions of intermediate filaments (Lazari- des, 1980), this chapter will be confined to the consideration of microtubules and microfilaments. With respect to microfil- aments , the widespread occurrence of extramuscular myosin suggests that analogies with muscle may extend to myosin ATPase. However, at present this idea cannot be accepted at face value because too little is known about the functions of extramuscular myosin.
KeywordsLeydig Cell Intermediate Filament Open Triangle Adrenal Cell Steroid Synthesis
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