The skeleton is a dynamic structure, made up of bones. Each bone is an organ since it is formed by various tissues: osseous, cartilaginous, dense connective, epithelial, others that generate blood, adipose and nervous. The hardness of the bone depends on the crystallized organic mineral salts it contains, and its flexibility depends on the collagen fibers. Bones are not completely solid, as they have many spaces. According to the size and distribution of these spaces, the regions of a bone are classified as compact and spongy. In general, compact bone constitutes 80% of the skeleton, and cancellous bone the remaining 20%.
The embryo contains no bones but hyaline cartilage structures. Gradually, ossification and osteogenesis take place, starting from ossification centers made up of clusters of special bone-forming cells called osteoblasts. The Golgi apparatus of the osteoblasts specializes in the synthesis and secretion of mucopolysaccharides, and their endoplasmic reticulum elaborates and secretes a protein called collagen. The mucopolysaccharides accumulate around each osteoblast and the bundles of collagen fibers are embedded in this substance. All this together constitutes the bone matrix; the collagen fibers give it strength. As the bone matrix forms, inorganic compounds such as calcium salts begin to be deposited in them, giving the bone its characteristic hardness.
As long as the longitudinal growth of the bone has not been completed, a layer of cartilage called epiphyseal cartilage remains between each epiphysis and the diaphysis. The proliferation of the cells of the epiphyseal cartilage causes the longitudinal growth of the bone; when the bones have reached their maximum length, this cartilage disappears. Bones increase in diameter by the combined action of two classes of cells: osteoclasts and osteoblasts. The osteoclasts increase the diameter of the medullary cavity by digesting the bone in the walls; the osteoblasts of the periosteum produce new bone on the outside.
By this double phenomenon, a bone with a larger diameter and a more extensive medullary cavity is produced. The formation of bone tissue continues after the bones have finished growing. Bone formation (osteogenesis) and bone destruction (resorption) occur simultaneously throughout life. During childhood and adolescence, osteogenesis has a higher rate than resorption, and bones become larger. From 35 to 40 years of age, bone loss exceeds bone gain.
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