The ability of cells to adopt a particular shape, maintain polarity, organize the relationship of intracellular organelles, and move about depends on the intracellular scaffolding of proteins called the cytoskeleton (Fig. 1). In eukaryotic cells, there are three major classes of cytoskeletal proteins:

• Actin microfilaments are 5- to 9-nm diameter fibrils formed from the globular protein actin (G-actin), the most abundant cytosolic protein in cells. The G-actin monomers noncovalently polymerize into long filaments (F-actin) that intertwine to form double-stranded helices with a defined polarity; new globular subunits are added (or lost) at the “positive” end of the strand. In muscle cells, the filamentous protein myosin binds to actin, and moves along it, driven by ATP hydrolysis (the basis of muscle contraction). In non-muscle cells, F-actin assembles via an assortment of actin-binding proteins into well-organized bundles and networks that control cell shape and movement.

• Intermediate filaments are 10-nm diameter fibrils that comprise a large and heterogeneous family. Individual types have characteristic tissue-specific patterns of expression that can be useful for assigning a cell of origin for poorly differentiated tumors.

• Lamin A, B, and C: nuclear lamina of all cells

• Vimentin: mesenchymal cells (fibroblasts, endothelium)

• Desmin: muscle cells, forming the scaffold on which actin and myosin contract

• Neurofilaments: axons of neurons, imparting strength and rigidity

• Glial fibrillary acidic protein: glial cells around neurons

• Cytokeratins: at least 30 distinct varieties, subdivided into acidic (type I) and neutral/basic (type II); different types present in different cells, hence can be used as cell markers

Fig1. Cytoskeletal elements and cell-cell interactions. Interepithelial adhesion involves several different surface protein interactions, including through tight junctions and desmosomes; adhesion to the extracellular matrix involves cellular integrins (and associated proteins) within hemidesmosomes. See text for details.

These ropelike intermediate filament fibers are found predominantly in a polymerized form within cells and do not usually actively reorganize like actin and microtubules. They impart tensile strength and allow cells to bear mechanical stress. The nuclear membrane lamins are important not only for maintaining nuclear morphology but also for regulating normal nuclear transcription. The importance of lamins is seen in rare but fascinating disorders caused by lamin mutations, which range from certain froms of muscular dystrophy to progeria, a disease of premature aging. Intermediate filaments also form the major structural proteins of skin and hair.

• Microtubules are 25-nm-thick fibrils composed of noncovalently polymerized dimers of α- and β-tubulin arrayed in constantly elongating or shrinking hollow tubes with a defined polarity; the ends are designated “+” or “−”. The “−” end is typically embedded in a microtubule organizing center (MTOC or centrosome) near the nucleus where it is associated with paired centrioles; the “+” end elongates or recedes in response to various stimuli by the addition or subtraction of tubulin dimers. Within cells, microtubules can serve as connecting cables for “molecular motor” proteins that use ATP to move vesicles, organelles, or other molecules around cells along microtubules. There are two varieties of these motor proteins: kinesins, for anterograde (− to +) transport, and dyneins, for retrograde (+ to −) transport; they also participate in sister chromatid separation during mitosis. Notably, microtubules (and their associated motors) have been adapted to form motile cilia (e.g., in bronchial epithelium) or flagella (in sperm).

Cell-Cell Interactions. Cells interact and communicate with one another by forming junctions that provide mechanical links and enable surface receptors to recognize ligands on other cells. Cell junctions are organized into three basic types (Fig. 1):

• Occluding junctions (tight junctions) seal adjacent cells together to create a continuous barrier that restricts the paracellular (between cells) movement of ions and other molecules. Viewed en face, occluding junctions forma tight meshlike network of macromolecular contacts between neighboring cells. The complexes that mediate the cell-cell interactions are composed of multiple transmembrane proteins, including occludin, claudin, zonulin, and catenin. Besides forming a high-resistance barrier to solute movement, this zone also represents the boundary that allows the segregation of apical and basolateral domains of cells, helping to maintain cellular polarity. Nevertheless, these junctions (as well as the desmosomes described later) are dynamic structures that can dissociate and reform as required to facilitate epithelial proliferation or inflammatory cell migration.

• Anchoring junctions (desmosomes) mechanically attach cells—and their intracellular cytoskeletons—to other cells or to the extracellular matrix (ECM). When the adhesion focus is between cells, and is small and rivet-like, it is designated a spot desmosome or macula adherens. When such a focus attaches the cell to the ECM, it is called a hemidesmosome. Similar adhesion domains can also occur as broad bands between cells, where they are denoted as belt desmosomes.

Cell-cell desmosomal junctions are formed by homotypic association of transmembrane glycoproteins called cadherins. In spot desmosomes, the cadherins are called desmogleins and desmocollins; they are linked to intracellular intermediate filaments and allow extracellular forces to be mechanically communicated (and dissipated) over multiple cells. In belt desmosomes, the transmembrane adhesion molecules are called E-cadherins and are associated with intracellular actin microfilaments, by which they can influence cell shape and/or motility. In hemidesmosomes, the transmembrane connector proteins are called integrins; like cadherins, these attach to intracellular intermediate filaments, and thus functionally link the cytoskeleton to the extracellular matrix. Focal adhesion complexes are large (>100 proteins) macromolecular complexes that can be localized at hemidesmosomes, and include proteins that can generate intracellular signals when cells are subjected to increased shear stress, such as endothelium in the bloodstream, or cardiac myocytes in a failing heart.

• Communicating junctions (gap junctions) mediate the passage of chemical or electrical signals from one cell to another. The junction consists of a dense planar array of 1.5- to 2-nm pores (called connexons) formed by hexamers of transmembrane proteins called connexins. These pores permit the passage of ions, nucleotides, sugars, amino acids, vitamins, and other small molecules; the permeability of the junction is rapidly reduced by lowered intracellular pH or increased intracellular calcium. Gap junctions play a critical role in cell-cell communication; in cardiac myocytes, for example, cellto-cell calcium fluxes through gap junctions allow the myocardium to behave like a functional syncytium capable of coordinated waves of contraction—the beating of the heart.

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مواضيع ذات صلة

Waste Disposal: Lysosomes and Proteasomes

Biosynthetic Machinery: Endoplasmic Reticulum and Golgi

Plasma Membrane: Protection and Nutrient Acquisition

Cytoplasmic Structures in Prokaryotic Cell

Eukaryotic Cell Structure

Cell Differentiation

إنقسام الخلية النباتية Plant Cell Division

الرايبوسومات Ribosomes

Ameboid Movement

The Mitochondria Extract Energy From Nutrients

Synthesis of Cellular Structures by Endoplasmic Reticulum and Golgi Apparatus

Ingestion by the Cell—Endocytosis