Long, hollow, cylindrical structures, the microtubules, as seen under digital compound microscopes, seem to play a role in intracellular movement and support, like the microfil¬aments. Though not recognized as tubules at the time, they were probably first seen in the spindle of dividing cells. (The spindle, a basketlike arrangement of tubules formed during the process of cell division, is instrumental in moving chromosomes into new nuclei. Later microtubules were detected elsewhere in the cell, and evidence accumulated that they may help guide the move¬ment of a variety of materials within the cell. For example, they may fix the pathways followed by secretory vesicles as they move from their sites of formation to the points on the plasma membrane where they will release their contents by exocytosis.

There is evidence that microtubules also provide a supportive cy¬toskeleton that helps maintain the cell’s shape. If the cell is treated with colchicine, a substance that causes breakdown of micro¬tubules and prevents assembly of new ones, the distinctive shape of the cell is quickly lost.

The microtubules of the spindle appear at one stage in the process of cell division and disappear at another stage. Microtubules in other parts of the cell are sometimes similarly transitory. In short, except when incorporated in other organelles, such as centrioles, cilia, and flagella, microtubules are not stable features of the cell, but may be assembled and disassembled at various times. They are made of a globular protein called tubulin, visible under digital compound microscopes. Assembly of a microtubule involves the stacking, in helical fashion, of tubulin molecules to form the wall of the tube.

CENTRIOLES

Centrioles, as seen under digital compound microscopes, are small dark bodies located just outside the nucleus of most animal cells in a region of specialized cytoplasm. As electron microscope studies have revealed, they are cylindrical structures, and two of them normally lie close together, oriented at right angles to each other. In cross section, each is seen to contain nine groups of microtubules, with three tubules in each group.

Centrioles are associated with the formation of the spindle in divid¬ing animal cells. They are absent from most of the cells of higher plants, although they do occur in some algae and fungi and in a few reproductive cells of higher plants.

CILIA AND FLAGELLA

Some cells of both plants and animals have one or more movable hair like structures projecting from their free surfaces. If there are only a few of these appendages and they are relatively long in propor¬tion to the size of the cell, they are called flagella. If there are many and they are short, they are called cilia. Actually, the basic structure of flagella and cilia is similar, as seen under a microscope, and the terms are often used interchangeably. Both usually function either in moving the cell or in moving liquids or small particles across the surface of the cell. They occur commonly on unicellular and small multicellular organisms and on the male reproductive cells of most animals and many plants, in both of which they may be the principal means of locomotion. They are also common on the cells lining many internal passageways and ducts in animals, where their beating aids in moving materials through the passageways.

Electron-microscope studies of the flagella and cilia of eucaryotic cells have revealed a remarkable uniformity in their internal struc¬ture, regardless of the organism to which they belong, whether plant or animal, simple or complex. The slender cylindrical stalk contains a cytoplasmic matrix, with eleven groups of microtubules embedded in it. Nine of these groups are arranged around the periphery of the cylinder, and the other two are in the center. At the base of the stalk, within the main portion of the cell, is a basal body, whose structure is the same as that of a centriole. The basal body is essential to the function of the cilium or flagellum, and it is the part that gives rise to the stalk.