There seems to be some controversy as to whether or not
human muscle cells and fat cells(?) acheive hyperplasia
(vs hypertrophy) or whether the number of cells is
genetically fixed. My personal opinion is that the idea of
either cell type being fixed in number is silly. You may
genetically start out with a certain number of muscle and
fat cells, but I certainly think it is possible to increase
the number of cells through hyperplasia. I theorize that
is what occurs in the so-called “muscle memory” effect BBs
talk about: training eventually causes hyperplasia
(initially only causes hypertrophy), then detraining
causes the cells to shrink, but retraining causes the
greater number of cells to quickly swell back up. Also,
have you ever noticed that people who have previously been
obese and “lose” the fat never get really cut, and they
easily regain fat unless they always eat a really strict
diet? I hypothosize that they have been very successful
at fat cell hyperplasia and when they diet, they are only
shrinking the cells, not “destroying” them. This is a sort
of “fat cell memory” effect. Now my question is this: once
fat cell (or muscle cell) hyperplasia occurs, does complete
cell “destruction” ever occur, or will you always have
those cells? EG would a previously obese person have to
have the fat cells physically removed (lipo) to actually
get rid of them completely rather than just shrinking them?
And if it is possible, how does this occur, and how can it
be achieved? Thanks y’all. 
You may intuitively feel that a genetic set point for muscle and fat cell number is silly, but it is pretty well established by research. The muscle cell hyperplasia/necrosis is really a different issue than the fat cell hyperplasia/necrosis.
With both cell types, your absolute number is pretty well established somewhere between birth and adolescence. In animals it can be pretty firmly nailed down, but it’s not possible in humans (research ethics and all). Anyway, you are probably right about the fat people who never really get cut, but these are fat individuals who hyperplased early in childhood most likely. So, when they grow up, they have a lot of fat cells which can only be shrunk, but not eliminated. Again, I don’t think this can be definitively established in humans though.
As far as muscle, there is some experimental evidence in animals that hyperplasia does occur in adult muscle. It’s next to impossible to prove in humans, due to the aforementioned limitations. I think, taking into account a lot of indirect evidence that hyperplasia does occur. There are many growth factors which cause satelite cells to multiply and when we look at regenerating muscle in the lab, it appears as if new novel fibers are being generated. As far as eliminating fibers, this can occur. Under extreme conditions (ie. space flight, denervation) a process called apoptosis is initiated whereby cells that are not needed are eliminated quietly, so as not to cause an immune response. In space your body perceives it doesn’t need the muscle anymore, so this occurs. Also, when innervation is lost to the muscle, it loses its ability to contract so the stimulus which maintains the muscle is lost. Eventually this muscle cell will be eliminated unless some type of therapeutic intervention can be innitiated.
I forgot to address destruction of fat cells. I am not aware of any research looking at apoptosis in fat cells. To go back to muscle for a second, there are investigators that say it doesn’t happen in muscle, but I’m not one of them. Anyway, I’m sure someone has tried to look at it, the problem is though fat cells are really hard to isolate and manipulate experimentally. The apoptosis (destruction) may happen in them, but I would expect that there are very strong evolutionary mechanisms in place to prevent this. Because mammals are designed to store fat, just because you are not using the cells, your body may think that it may need them to survive down the road. Just a hunch, no experimental evidence to back it up, sorry. Certainly the most effective way to get rid of the cells is to suck them out.
Thanks for the great answer, Steve. Anyone else?
I’d like to ask a related question; once liposuction has occurred (for example, on the abdominal area), is it safe to assume those fat cells will absolutely never regenerate? I know this seems obvious, but Lipo is relatively new in the evolutionary scheme of things, so maybe the body could get stimulated to generate new cells in the presence of excessive calories. If not, will other fat cells in the body “take up the slack” and store the fat that would have been stored in the removed cells? I’ve never seen these adequately answered. Thanx
After Steve’s answer what more can I add? Well, maybe one thing. New research in androgen users indicates that hyperplasia MAY occur with long term use of androgens and weight training. Otherwise, as Steve said, it’s probably not going to happen.
I need to apologize. In my earlier posts, I think my point got lost in all of the info. Although some data supports the contention that hyperplasia (of adult muscle) does not occur, I personally believe that it does. There are two caveats though. First, it probably only happens under extreme conditions. Extreme conditions might include dedicated, prolonged strength training. Other extreme conditions would be muscle injury and regeneration. As John alluded, pharmacological growth factors could potentially impact this also. Second, it probably happens to a very small extent. For example if hyperplasia occurred in 1 % of the musculature, this would not become noticeable until it were compounded over several years. This is why it would be so hard to establish experimentally.