A cell's long, hollow, cylindrically shaped microtubules help maintain the shape of the cell by forming a cytoskeleton. Microtubules consist of helically stacked molecules of the protein tubulin. Floating freely in the cytoplasm, undifferentiated microtubules are regularly assembled and disassembled over the life of the cell. In contrast, differentiated microtubules, or those that have been incorporated into the structure of various organelles such as centrioles, cilia, and flagella, are more stable.
Centrioles are located just outside the nucleus in an area of the cytoplasm known as the centrosome. Appearing as small dark bodies, the two centrioles contained by each cell themselves are composed of two hollow cylindrical structures lying at right angles to each other. Each cylinder is composed of nine groups of microtubules with three tubules per group. Cilia and flagella are moveable hair like structures that project from the surface of a cell. Whereas cilia are short and usually found in profusion on a cell, flagella are relatively longer and usually just a few are found on a cell. Both use a whip-like motion to help move the cell or move liquids or small particles past the cell. Both consist of a membrane-bound appendage containing a cytoplasmic matrix. The matrix surrounds eleven groups of microtubules. Of the eleven groups, nine groups are arranged around the perimeter of the appendage while two are located in the center. The part of the appendage that extends into the cell is called the basal body. Basal bodies anchor the appendage to the cell and have the same structure as a centriole.
