Cilia and Flagella

Cilia and flagella are projections from the cell. They are made up of microtubules , as shown in this cartoon and are covered by an extension of the plasma membrane. They are motile and designed either to move the cell itself or to move substances over or around the cell. The primary purpose of cilia in mammalian cells is to move fluid, mucous, or cells over their surface. Cilia and flagella have the same internal structure. The major difference is in their length.  See page 92,  Gartner and Hiatt

This figure shows a cross section of a cilium next to a longitudinal section. Below, we will see how the microtubules are organized in the core (shown in the cartoon in this figure).  Also shown is the centriole or basal body that organizes the formation and direction of the cilia.

How do Cilia and Flagella move?

Cilia and flagella move because of the interactions of a set of microtubules inside. Collectively, these are called an "axoneme", This figure shows a microtubule (top panel) in surface view and in cross section (lower left hand panel). Two of these microtubules join to form one doublet in the cilia or flagella This is shown in the middle panel. Note that one of the tubules is incomplete. Furthermore, there are important microtubule associated proteins (MAPs) projecting from one of the microtubule subunits.

This figure shows an electron micrograph of a cross section of a cilium. Note that you can see the dynein arms and the nexin links. The dynein arms have ATPase activity. In the presence of ATP, they can move from one tubulin to another. They enable the tubules to slide along one another so the cilium can bend. 

The dynein bridges are regulated so that sliding leads to synchronized bending. Because of the nexin and radial spokes, the doublets are held in place so sliding is limited lengthwise. If nexin and the radial spokes are subjected to enzyme digestion, and exposed to ATP, the doublets will continue to slide and telescope up to 9X their length.

Centrioles and Basal Bodies

Cilia and flagella are organized from centrioles that move to the cell periphery. These are called "basal bodies" and are shown in this electron micrograph (bb). Note the numerous cilia projecting from the cell membrane (cm). Basal bodies control the direction of movement of the cilia. This can be shown experimentally. Read pp 48-49 in Gartner and Hiatt

Centrioles control the direction of cilia or flagella movement.

Paramecium have parallel rows of cilia all aligned so that they will beat in the same direction. However, in the 1960's rows of cilia/basal bodies were grafted into Paramecium and they were able to show a change in direction of the beat. The cells passed on the change to future generations even though this was not a genetic alteration. 

Centriole structure

Like Cilia and Flagella, Centrioles are also made of microtubules. The difference is that they contain 9 sets of triplets and no doublet in the center. How the triplets in the basal body turn into the cilium doublet remains a mystery. Centrioles come in pairs, each organized at right angles to the other. This figure shows an electron micrograph of a pair of centrioles and the cartoon compares the cross section of a cilium (above)  with that of a centriole. Centrioles organize the spindle apparatus on which the chromosomes move during mitosis. 

Centriole Replication. Centrioles replicate autonomously like mitochondria and peroxisomes. They begin from centers which contain proteins needed for their formation (tubulin, etc.), Then the procentrioles form. Each grows out a single microtubule from which the triplet can form. Once a centriole is made, daughter centrioles can grow out from the tubules at right angles. These then add to the daughter cell (in a dividing cell), or they move to the periphery and form the basal body for the cilium.
To understand the cytoskeletal system:
Learn about microtubules, intermediate filaments, and actin filaments.

Last updated: 08/14/01
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URL Address: Cilia, flagella, and centrioles
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text copyright 1996 Gwen V. Childs, Ph.D.