Microtubules

The microtubules are slender tubular structures composed of globular proteins called tubulin. Microtubules function in many ways, including development and maintenance of cell form. They participate in intracellular transport mechanisms, including axoplasmic transport in neurons and melanin dispersion in pigment cells of the skin. Other functions include formation of the basic structure for sev­eral complex cytoplasmic organelles, including the centri-oles, basal bodies, cilia, and flagella. Abnormalities of the cytoskeleton may contribute to alterations in cell mobility and function. For example, proper functioning of the micro­tubules is essential for various stages of leukocyte (white blood cell) migration. In certain disease conditions, such as diabetes mellitus, alterations in leukocyte mobility and migration may interfere with the chemotaxis and phagocy­tosis of the inflammatory response and predispose toward the development of bacterial infection.

Cells can rapidly assemble and disassemble micro­tubules according to specific needs. The action of the plant alkaloid colchicine halts the assembly of microtubules. This compound stops cell mitosis by interfering with for­mation of the mitotic spindle. Colchicine is often used for

cytogenetic (chromosome) studies. It is also used as a drug for treating gout. The drug's ability to reduce the inflam­matory reaction associated with this condition may stem from its ability to interfere with microtubular function of white blood cells and their migration into the area.

Cilia and Flagella. Cilia and flagella are hairlike pro­cesses extending from the cell membrane that are capable of sweeping and flailing movements, which can move surrounding fluids or move the cell through fluid media (Fig. 4-8). Both contain identically organized cores of mi­crotubules that consist of two microtubules surrounded at the periphery by clusters of paired microtubules. Each cil-ium and flagellum is anchored to a basal body that is re­sponsible for the formation of the microtubular core. Cilia are found on the apical (luminal) surfaces of many epi­thelial linings, including the nasal sinuses or passages such as the upper respiratory system. Removal of mucus from the respiratory passages is highly dependent on proper function of the cilia. Flagella form the tail-like structures that provide motility for sperm.

Genetic defects can result in improper microtubule formation and, as a result, the cilia may be nonfunctional. One of these disorders, the immobile cilia syndrome, impairs sperm motility, causing male sterility while also immobi­lizing the cilia of the respiratory tract, thus interfering with clearance of inhaled bacteria, leading to a chronic lung disease called bronchiectasis.

Centrioles and Basal Bodies. Centrioles and basal bodies are structurally identical organelles composed of an array of highly organized microtubules. Internally, centrioles and basal bodies have an amorphous central core surrounded by clusters formed of triplet sets of microtubules. In divid­ing cells, the two cylindrical centrioles form the mitotic spindle that aids in the separation and movement of the chromosomes. Basal bodies are more numerous than cen­trioles and are found near the cell membrane in association with cilia and flagella. They are responsible for the forma­tion of the highly organized core of microtubules found in cilia and flagella. The internal microtubular arrangement of centrioles and basal bodies is different from that found in cilia and flagella.