I've been studying my muscle anatomy and physiology for the past few hours, and I just figured out why each muscle cell (or fiber; terms are interchangeable) can have over a hundred distinct nuclei, but no reproductive capability: It turns out that during embryonic development, hundreds of mesodermal myoblasts fuse together into a cohesive unit not too dissimilar from the way that osteons a are cohesive units of osteoblasts and osteocytes (among other things). Not long after they fuse, they become "myocytes" (made that term up) and lose their reproductive capability, and retain only enough regenerative ability to fix themselves if broken. Of course, the generous blood supply is wonderous at helping the healing process along. I just thought I'd share that
who? No but seriously, could someone put that in a simpler form? Perhaps turn it into some practical advice for me maybe? Haha, way over my head.
well, to drop it out of the academic stratosphere into something more manageable, when a person's physically developing within the womb, starting from the very first cell devisions, you have three "derms" which through processes yet unknown to me provide all the tissue in our bodies. The endoderm produces epithelial tissue, which lines and covers glands, ducts and organs; the ectoderm makes nervous tissue and the remainder of the epithilia, and the mesoderm is responsible for skeletal, cardiac, and most of the smooth muscle in the body. It also gives rise to the blood, lympth and red bone marrow, the skin and other things. Anyway Purely from deductive reasoning, since I haven't gotten this far in class, though I'll ask about it, mesoderm contains cells called myoblasts. If they're anything like osteoblasts, which secrete a calcium phosphate matrix (stuff that makes up bones) among other things, then myoblasts secrete some kind of fluid or gel matrix that helps to eventually gives rise to muscle fibers. Now, each myoblast doesn't make one muscle fiber, but rather, potentially hundreds of myoblasts can fuse into one giant cell, which, in becoming a muscle fiber, loses all reproductive capability (ie: for purposes of replication, all those hundreds of nuclei go dormant). While the muscle cell can't reproduce, it still has the ability to repair itself largely due to the huge amount of blood that it has access to. There are probably thousands of capillaries (tiny blood vessels) that permeate each muscle organ, not to mention the ones that penetrate the bones. All that blood is really good for the healing process, and is largely the reason why muscle inflammation is a pain in the neck: all that blood can concentrate a lot of heat in one area, not to mention that one of muscle's primary functions is to generate heat. It's not really advice so much as "hey, I didn't know that. Cool!"
So you're saying hyperplasia (the splitting of muscle fibers to create more muscle fibers) isn't possible? So whatever number of muscle fibers we develop early on in life is the maximum number we'll have no matter what, and the only way to increase the size of the muscle would be to increase sarcoplasm (bodybuilder muscle) or increase the cross section of each muscle fiber (powerlifter muscle). But it's just not possible for fibers to split and create more fibers? Very neat stuff d33pthought!
Yep, no new muscle fibers after birth. While there are residual myoblasts in mature muscle fibers, they function to repair existing fibers instead of creat new ones. When muscles get bigger, it's really because the muscle fibers themselves increase in diameter, from the production of more mitochondria, sarcoplasm and sarcoplasmic reticulum (which stores calcium ions, a necessary ingredient for every muscle contraction). Interestingly, while muscle fibers themselves can't replicate, the contractile elements within them, muscle filaments, apparently can (from what I'm reading). Muscle filaments come in two types: thick and thin. They're the major parts of motor units, or sarcomeres, and when a muscle contracts, the thick filaments attatch to and ratchet themselves along the many binding sites on the thin filaments, which pulls the thin ones towards the center of the sarcomere. This sort of sliding-fiber mechanism is a lot like tugging on a string attatched to a rubber band: while the myofibrils themselves don't shorten, the titin proteins and z-discs at the borders of each sarcomere get pulled towards the center. EDIT: Actually, I misread the text concerning motor units. A motor unit is really a motor neuron and all the muscle fibers that it stimulates. Sorry.
A lot of recent research is showing that hyperplasia is actually possible and does occur. Obviously this isn't to the level that hypertrophy occurs but it does happen. There was a discussion on it a while back on another thread, but I can't find it Edit - Found it http://www.martialartsplanet.com/forums/showthread.php?t=28521&page=5&pp=15&highlight=hyperplasia
maybe my textbook's just behind on the times a little. Even the presence of myoblasts in mature muscle cell lends possibility to the existence of hyperplasia, but, like is was said in that other thread, it's neither wide-spread nor does it yield significant difference in skeletal muscle. Who knows: maybe it's more likely in smooth muscle? I won't find that out in class till next semester, though.
If you look at the link I posted on that thread, it should answer your questions. The research is fairly recent and still ongoing and debatable, so it doesn't really appear in many text books.
The way I understand it is that they know hyperplasia occurs in animals (maybe some) but they can't prove that it occurs in humans.
