Studying cell physiology is all about how and why cells act the way they do. How do cells change their behavior based on the environment, like dividing in response to a signal from your body saying you need more new cells and how do cells interpret and understand those environmental signals?
Just as important as why cells act the way they do is why they go where they go, and that's where cell motility comes in. Cell motility is the movement of the cell from one place to another via the consumption of energy.
It's sometimes called cell mobility, but cell motility is the more correct term, and the one you should get used to using.
So Why Are Motile Cells Important?
Your body relies on your cells and tissues to function properly in order to stay healthy, but it also relies on those cells and tissues to be in the right place at the right time.
Think about it: You couldn't rely on your skin cells to help keep pathogens out of your system, for instance, if they weren't organized properly on the outside of your body. And your kidney cells? Good luck getting them to function well if they aren't properly organized within your kidneys, where they can filter your blood.
Cell motility helps ensure that your cells get to where they're supposed to be. That's especially important in developing tissues. Often, the progenitor, "stem-like" cells aren't found alongside fully mature cells. Those cells develop into mature tissue, then migrate to wherever they're supposed to go.
What Is Involved in Cell Motility?
Think back to your skin cells, for example. The outer layers of skin cells play some of the most important functions in your body. They form a waterproof layer that keeps outside moisture out and your bodily fluids in, they help block pathogens from getting into your body and they help regulate your body temperature.
But what about the progenitor cells that develop into mature skin cells? They're found in the deeper layers of your skin, and then move to the surface as they mature.
Without cell mobility, your skin wouldn't be able to regenerate itself properly, which would have far-reaching effects for your health. And the same concept applies to other tissues: mature cells that can't migrate to the right place in your body simply don't help keep you healthy.
Cell mobility is also important for single-celled organisms. Okay, so you understand why cell mobility is important in animals, plants and other multicellular organisms. But what about single-celled organisms, like bacteria?
Migration is also crucial for single cells. Motility allows bacteria, for instance, to move toward sources of nutrients and away from harmful compounds that could otherwise kill them. Motility helps bacteria survive longer and continue to divide, so they can pass on their genes to the next generation.
How Do Cells Move?
When you're talking cell mobility, two organelles do the majority of the work: cilia and flagella.
Cilia are small, hair-like structures that project out of the cell. They're driven by motor proteins, and they're able to move back and forth in a rowing-like motion, helping to propel the cell forward. Cilia can also move the environment around the cell. For example, the cilia on the cells that line your airways continually "row" unwanted particles up and out of your lungs.
Certain cells, like sperm cells and bacteria, get most of their mobility via flagella. Flagella are whip-like structures that move like a propeller, driving the cell forward. They allow cells to "swim" away from or toward stimuli.
The Cytoskeleton and Cell Movement
While both cilia and flagella can directly propel the cell, the cytoskeleton, the group of structural proteins important for maintaining the shape of the cell, also play a key role in cell motility.
Specifically, your cells use a protein called actin, a part of the cytoskeleton, to help drive motility. Actin fibers are highly dynamic, and they can get shorter or longer according to the cell's needs. Elongating actin fibers in one direction while retracting them in the other pushes the cell forward, allowing the cell to move.
What Guides Cell Locomotion?
So now you know how cells move, but how do they know where to go? One answer is chemotaxis, or movement in response to a chemical stimulus.
Cells naturally contain special proteins, called receptors, which are located on the cells' surface. Those receptors can sense conditions in the cells' environment and relay signals to the rest of the cells to move this way or that.
Positive chemotaxis promotes movement toward a stimulus. It's what drives the sperm cell to swim towards the ovum, in the hope of fertilization. Your body also uses positive chemotaxis to set "destinations" for newly-developed cells so that when a newborn cell gets to a certain place in your body, it'll stop moving and stay there.
Negative chemotaxis means movement away from a stimulus. For example, bacteria might attempt to move away from harmful compounds, and instead swim toward a friendlier environment where they can grow and divide more rapidly.
Cell motility can also be hard-wired into your cells, so cells know where to move based on their genetics.
Types of Cell Motility
Now that you know the basics of why and how cells move, let's look at some real world examples.
Take the white blood cells that make up part of your immune system. The cells work by circulating throughout your body, looking for foreign particles that might be harmful. When your immune system finds something harmful, it releases chemicals, called cytokines, at the site of infection.
Those cytokines trigger positive chemotaxis. They draw more immune cells to the area, so your body can mount a proper immune response.
More Cell Motility Examples
Another important instance of cell motility is wound healing. Torn and damaged tissue needs to be repaired, so damage to your tissues tells your body to start making new cells to replace the damaged ones. Simply creating new cells isn't enough, though, those cells also need to move across the torn tissue, gradually filling the wound in.
An example of cell movement gone wrong is cancer. Normally, your cells only migrate to defined areas of your body. You want them to migrate to wherever they're needed, and stay out of areas of the body where they aren't needed.
Cancer cells, though, break the rules. They can tunnel through the "borders" between tissues (called the extracellular matrix) and invade neighboring tissues. That's how breast cancer, for instance, can end up in the bones or brain or places where you definitely wouldn't find breast tissue under normal circumstances.
Cell Motility: What You Need to Know
Here's a general review of the key points to remember:
- Cell motility is the movement of the cell from one place to another. It's a process that uses energy.
- Movement is guided by the cell's cytoskeleton and can involve specialized organelles like cilia and flagella.
- Cells can know where and how to move based on genetics. They can also respond to chemical signals from the environment, which is called chemotaxis.
- Positive chemotaxis is movement toward a stimuli, while negative chemotaxis is movement away from it.
- Cell motility is important for the overall functioning of an organism. In the human body, it plays an important role in immunity and healing.
- When cell motility goes wrong, it can contribute to diseases, including cancer.
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