Yes, you constantly lose skin cells, even in the shower. They get replaced regularly with new ones. it may sound gross, but if we used a microscope to look at your pillowcase, we would see thousands, maybe even millions of dead cells after just one nights' sleep! You constantly make new skin cells to replace these dead ones. Can you think of a good reason why our bodies would keep making new skin (instead of just using the same skin our entire lives)?

And yes, cells can indeed "sense" their environment. It's amazing!!!
For example, How do you think that your brain tells your pancreas to produce more insulin so that your sugar levels will be "just right" for your brain to function? If you were designing an electrical circuit so that a "switch" thrown in the next room caused a light to blink in an adjacent room, how would you do that? Animals (and plants) with many cells have the same engineering problem that must be overcome, but they don't have metal wires or RadioShack to supply them with parts. So, cells use macromolecules instead--proteins, sugars, lipids-- all with a kind of "coded" information contained in their structures that serve as signals. When a cell "binds to" a signal molecule, this triggers a biochemical circuit inside the cell that causes that cell to respond to that signal--which in fact is a response to "the environment." The study of these "cellular circuits"
--how the are built and maintained in a cell and how they are regulated--is called SIGNAL TRANSDUCTION BIOLOGY.

You might have heard of a technology called "biosensors." Let's say that a certain kind of cell has the ability to sense a toxin (let's
say nerve gas) at very low levels. If you could make a "chip" that contained these cells you could use that chip as a sort of sensor or detector for the nerve gas. You just have to know what the cell "does" in response to sensing the nerve gas so that you can "read
it." (Maybe you can engineer the cell to change color or send an electrical impulse to a computer). Guess what? These nerve gas biosensors exist and are being used in subways in several major
cities throughout the world as a way to protect against terrorist acts! Can you think of some other useful biosensors?

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For all of the questions: students interested in "cells and how they
are studied" as well as "what is a cell biologist?" can contact the
American Society for Cell Biology (ASCB) for free brochures and
on-line information at:

http://www.ascb.org
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Yes - dead cells at the top of the skin fall off when we wash.Yes - cells sense stuff. All cells sense some things, like how much food is coming to them in the blood and whether there are other cells touching them. Some cells sense things like pain and heat and seeing and hearing, too.

The cells that make up your skin are just about the fastest growing cells in your whole body! You are constantly making new skin. As the old cells on the surface die and flake off your body they just fall off where ever. Some of these cells get washed away in the shower, some get rubbed off when you towel off and put on your clothes, and tons get rubbed off as you sleep. Your mattress is actually loaded with dead skin cells. If you examined most household dust under a microscope you would find that ~80% of it is made up of dead skin cells!
Your second question concerns the very topic of my PhD project! Yes, cells do sense things. In fact, many cells can sense as many things as a human body can sense: light, smells, tastes, noises, and touch. Isn't that amazing? But, when you think about it, it is these single cells within our body that allow us to sense these things. For example, we have special cells on our tongue that are very sensitive to chemicals. Whenever a certain type of chemical lands on that cell, it triggers the cell to send messages to the brain that that particular chemical is present. This is how you taste things. The way we see is via single cells in our eyeballs called photoreceptors.
These single cells send signals to our brain when light strikes the cell. So we humans experience our world only through the efforts of millions and billions of little cells doing their jobs sensing their various items they are tuned to sense.