The skeletal and muscular systems are taught as individual subjects in traditional anatomy courses. However, they are intimately linked in both function and development. Early in development, the skeletal system is formed by cartilage. This embryonic structure is often labeled as a template from which bone cells deposit and maintain the health of growing bones. In turn, muscle cells have a completely different mode of development. Eventually, it is the action and growth of both systems that contribute to the formation of each other.
At first glance, bone may appear to be anything but a tissue. It is, however, a connective tissue complete with cells, fibers, and ground substance (minerals that make crystalline structures). Early in the growth process, osteogenic (osteoprogenitor) cells form from embryonic mesenchyme tissue. They are found in the endosteum and lie within the Haversian (central) canals. Many osteogenic cells are also located on the inner surface of the periosteum, which is logical since bone growth continues to occur from the inside to the outside of the bone. These cells are the only bone cells capable of mitosis. As a result, they are the only cells to produce new bone cell types.
One of the cells to develop from the osteogenic cells is the osteoblast. These cells are the actual bone-forming cells. The osteogenic cells lie on the surface of the cartilaginous template forming a small depression or lacuna. When they divide, a permanent resting place for future bone cells, the lacuna, becomes the site of bone formation. The osteoblasts synthesize collagen and glycosaminoglycans (GAGs), which are part of the bone matrix (supporting tissue). Osetoblasts cannot undergo mitosis, but the osteogenic cells rapidly multiply under stress or fractures and produce many more osteoblasts. This process of bone formation under stress will become very important when the role of muscle formation is discussed.
As the osteoblasts continue to deposit bone matrix by depositing the two molecules just discussed in addition to proteoglycans and plycoproteins. The mineral portion of the bone, hydroxyapatite is also deposited and is about 85% of the individual bone's mineral composition. Other depositional minerals are magnesium, sodium, potassium, fluoride, sulfate, carbonate, and hydroxyl ions.
Osteoblasts eventually become trapped in the lacunae as the bone grows around them. At this point the cells are renamed osteocytes. The osteocytes no longer produce bone matrix. Instead, they remain as the active and living part of bone. They monitor calcium and phosphate levels and work to maintain the correct balance between the two. Nutrients for proper bone growth and maintenance are passed from one osteocyte to another through small connections (canaliculi) between one osteocyte and another.
Physical stress is one of the most important factors in bone growth. Most of the physical stress put on bones during early development comes from the simultaneous growth of muscles. Skeletal muscles of the trunk arise from cells located along the neural crest. The somites first appear as wedge-shaped segments along either side of the notochord. They arise from the mesoderm and at first only one-three pairs are visible. At various stages during growth, more somite pairs appear and may reach up to 40 pairs. The first four are occipital and eventually migrate to form muscles of the head and neck region. The next eight somites form the cervical section. There are twelve thoracic and five lumbar somites. The first few sacral and the remaining caudal somites will develop into the muscles of the lower region and limbs.
The cells of the somites follow two different paths. The first group (the myotome) becomes the muscles of the axial skeleton. The second give rise to the muscles of the limbs and body wall. Cells for these latter structures migrate from their source and become the precursor cells for later development.
As the somites grow, they begin their origination on certain bones. Their insertion is on other target bones. With continued growth, the muscles begin to contract and gain strength. This puts stress on the individual bones and stimulates the deposition of osteoblasts and formation of newer bone. In the case of the spinal cord, each embryonic somite pulls on a region of the notochord. This biomechanical stress causes the bone to fortify. Differentiation of the notochord occurs and the individual sections become the vertebrae. This is the inseparable relationship between bone and muscle growth. As muscle grows so does bone. As bones become larger and stronger they provide a greater surface area for additional muscle fibers of the somites to grow upon. This then causes the muscles to enlarge and become stronger. While the two systems are quite different in growth mechanics and tissue structure, they cannot function and grow without each other.
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