Re: Do the different types of muscle cells, have different ways of repair.

Sorry it's taken so long, but I was unable to get answers from several other scientists to whom this question had been sent. There are essentially three types of muscle cells in the body: skeletal muscle, smooth muscle, and cardiac muscle. Each of these muscle types performs different sets of functions and is formed differently during development, so each has a different way of repairing itself. Since skeletal muscle is the most complicated, I'll leave it for last.

First then, let's look at smooth muscle. Smooth muscle is important for the contraction of many tissue types, the most familiar being the gut and the blood vessels. In the intestines, the action of the smooth muscle cells (SMC's) propels food through to the colon. In the blood vessels, smooth muscle cells open and close veins and arteries to control the flow of blood to different parts of the body. Beside being contractile, SMC's are responsible for maintaining the connective tissues they occupy, and perform several functions similar to non-contractile connective tissue cells (like fibroblasts). This independence is also seen in their growth and repair: smooth muscle cells can replicate and divide to expand their numbers or fill in for losses due to injury, while maintaining their contractile activities, i.e. fully differentiated smooth muscle cells maintain the capacity to proliferate.

Cardiac muscle is intermediate between smooth and skeletal. The muscular walls of the heart are composed of a vast network of cardiomyocytes ("heart muscle cells"), which serve not only as the contractile force that pumps the heart, but also as the conductive system that carries the electrochemical impulses that set the pace of the contractions. Each cardiomyocyte constructs elaborate connection with its neighbors called intercalary disks. These intercalary disks are composed primarily of gap junctions, which allow free flow of ions between the cytoplasms of the cells, such that an ionic change in one part of the heart (e.g. a neural pulse from a node) travels through the entire myocardium (muscular layer of the heart) as if it were a single cytoplasm. But even with these specialized functions, cardiomyocytes, like SMC's, maintain the capacity to proliferate. That is, in cases of injury to the myocardium, the cardiomycytes can grow and divide to compensate for lost cells. In fact, some of the signals that tell these cells that injury has occured can be switched on by other factors, causing the cardiomyocytes to proliferate too much, and resulting in cardiac hypertrophy. I have worked extensively with embryonic cardiomyocytes, and one of the cooler things about them is that they throb on their own when grown in culture, demonstrating that they can be functional and proliferative at the same time. When they become dense enough to contact each other, they all beat in unison.

Skeletal muscle, on the other hand, is not poliferative. The main reason for this is that skeletal muscle takes the shared cytoplasms a step further than heart muscle: each skeletal muscle fiber is actually a syncytium formed from the fusion of hundreds of myoblasts into a single cell with hundreds of nuclei. Because muscle fibers can only be formed from the fusion of myoblasts, they cannot themselves produce more fibers (or even, more nuclei within a fiber). But, skeletal muscle can repair itself. It accomplishes this by keeping a very few unfused myoblasts in their undifferentiated, proliferative state. These reserve myoblasts are called satellite cells, and reside in special pockets interspersed along the sides of the muscles fibers. In cases of muscle injury, the inflammation around the wound activates the satellite cells, which grow and divide to fill in the lost muscle, and then fuse to form new fibers. I've also grown satellite cells in culture, and they grow and proliferate like fibroblasts, until their growth factors are taken away, at which point they fuse into fibers, some of which will even twitch.