Mechanisms of
Sperm Motility:
Dr. Charles Lindemann
Dr. Lindemann's Cilia and Flagella Page
WHAT ARE CILIA AND FLAGELLA?
Cilia and flagella are whip-like appendages of many living cells that are used to move fluid or to propel the cells. Cilia beat with an oar-like motion and flagella have a snake-like motion as illustrated in Figure 1. The cilia in your lungs keep dirt and dust from clogging your breathing tubes (the bronchi) by moving a layer of sticky mucous along to clean out the airways. Sperm cells use a flagellum as a propeller to move the cell through the fluid of the oviduct to reach the egg. Thousands of animals and plants use cilia and flagella for swimming (example: paramecium), or feeding (example: clams and mussels) or mating (example: green algae). It is a curious fact that all of these cilia and flagella have a very similar internal arrangement of tubes (the outer doublets) and protein connectors (the nexin links and dynein arms) that suggest that there is something very special about this particular way of building a cell propeller. Figure 2 is a diagram of these internal parts of a flagellum. Nature tends to keep designs that work well. If we can possibly understand why this particular design works so well, we might be able to design miniature devices that use the same principles of operation!
Figure 1
 
Figure 2
 
An electron-microscopy image of a numbered bull axoneme and mouse axoneme, respectively.
 
THE GEOMETRIC CLUTCH MODEL

The Geometric Clutch model of ciliary and flagellar beating is a hypothesis that attempts to explain the way that cilia and flagella work. A computer model based on this hypothesis can imitate a cilium or a flagellum. The basic underlying idea of the Geometric Clutch hypothesis is rather simple to understand. When the molecular motors (dynein arms in the picture) that power the beat of the cilium or flagellum are activated, they pull and push on the outer doublets and induce a strain on the structure that causes the cilium to bend. This part of the story of how cilia beat is agreed upon by all of the scientists that study cilia and flagella. (The Geometric Clutch idea is based on the ida that when the motors push and pull on the outer doublets the strain on each doublet creates a sideways force that is transverse to the doublet. This transverse force (or t-force) pushes some of the doublets closer together and others are pushed apart. The motors on the doublets that are pushed closer together go into action and generate force; the motors on doublets that are pulled apart are forced to stop pulling. In the Geometric Clutch model this is the working principle. The t-force controls the motors and acts like a "clutch", much as the clutch that engages or disengages the motor of your car. When this working principle is built into a computer simulation of a cilium or flagellum, the simulated flagellum can produce repetitive beats that look very much like those of a real cilium or flagellum. A working copy of the Geometric Clutch computer simulation can be downloaded from the "clutch model" page of this web-site (HERE). If you follow the instructions that are built in to the demonstration version you can make the model simulate a beating 10-micron long cilium (provided you are working from an IBM compatible PC).
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