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Development of the Muscular System
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By eHow Contributing Writer
Embryonic Cells
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In the beginning, when human life is in the embryonic stage, no muscular system exists per se--only embryonic cells. Some of these embryonic cells, called somites, differentiate, forming the building blocks of skeletal muscle fibers: myoblasts. More and more myoblasts are produced as these naive embryonic cells continue to differentiate.
Sticking Together
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Once a myoblast has been produced, it begins to seek out other myoblasts. When one myoblast comes into contact with another myoblast, the two stick together, so to speak. Still other myoblasts then attach to these two, until many are joined together. Finally, the cell membranes of each of these individual myoblasts actually open up to form one large cell membrane--and one large cell made up of what were once many myoblasts. This creates what are called myotubes. As myotubes fuse or are joined with still more myoblasts, actual skeletal muscle fiber is created.
Aggregating
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These newly-formed muscle fibers now aggregate, first forming the muscles of the neck, chest and back. As groups of fibers fuse together, move away from their place of origin or subdivide into still other groups, the embryonic skeleton is slowly covered in a full muscular system, and at about 10 weeks, the embryo begins to move spontaneously. As the embryo grows to fetus and the fetus grows in size, the muscular system, already developed and in place, grows with it, just as it will once the baby is born and the child grows to adulthood.
Further Growth
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As an adult, the muscular system can be further developed by working out. This is particularly true with fast-twitch muscles (or Type II muscles). Fast-twitch muscles are muscles that become fatigued quickly with repeated contraction (as in, for example, a weightlifting situation), as opposed to slow-twitch muscles, which can be utilized over a long period of time before becoming fatigued (like the muscles used to go on a long-distance run). As an adult lifts weights, for example, fast-twitch muscle fibers literally tear. If the weight-lifter has been eating a high-protein diet, that protein helps the torn muscle fibers to grow back--and larger. This is repeated over an extended period of time until the fast-twitch muscles being worked out are actually visibly bigger than they were before. In this way, the muscular system continues to be developed even in the midst of adulthood.
eHow Article: Development of the Muscular System
