Parts of the skeletal system begin to form during the first few weeks of life, and bony structures continue to develop and grow into adulthood.
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Bones form by the replacement of existing connective tissues, in one of two ways. Some appear between sheetlike layers of connective tissue and are called intramembranous bones. Others begin as masses of cartilage, which are later replaced by bone tissue. They are called endochondral bones. |
Examples of intramembranous bones are the broad, flat bones of the skull. During their development, sheetlike masses of connective tissues appear at the sites of the future bones. Then some of the primitive connective tissue cells enlarge and differentiate into bone-forming cells, called osteoblasts. The osteoblasts become active within the membranes and deposit bony intercellular materials around themselves. As a result, spongy bone tissue is produced in all directions within the membranes. Eventually cells of the membranous tissues that persist outside the developing bone give rise to the periosteum. At the same time osteoblasts on the inside of the periosteum form a layer of compact bone over the surface of the newly built spongy bone. When osteoblasts are completely surrounded by matrix, they are called osteocytes.
Endochondral Bones
Most of the bones of the skeleton are endochondral bones. They develop from masses of hyaline cartilage with shapes similar to future bony structures. These cartilaginous models grow rapidly for a while, and then begin to undergo extensive changes. In a long bone, for example, the changes begin in the center of the diaphysis where the cartilage slowly breaks down and disappears. At about the same time, a periosteum forms from connective tissues which encircles the developing diaphysis. Blood vessels and osteoblasts from the periosteum invade the disintegrating cartilage, and spongy bone is formed in its place. This region of bone formation is called the primary ossification center, and bone tissue develops from it toward the ends of the cartilaginous structure.
Meanwhile, osteoblasts from the periosteum deposit a thin layer of compact bone around the primary ossification center. The epiphyses of the developing bone remain cartilaginous and continue to grow. Later secondary ossification centers appear in the epiphyses, and spongy bone forms in all directions from them. As bone is deposited in the diaphysis and epiphysis, a band of cartilage, called the epiphyseal disk (physis), is left between these two ossification centers.
The cartilaginous cells of an epiphyseal disk include rows of young cells that are undergoing mitosis and producing new cells. As these cells enlarge and intercellular material is formed around them, the cartilaginous disk thickens. Consequently, the length of the bone increases. At the same time, calcium salts accumulate in the matrix adjacent to the oldest cartilaginous cells, and as the matrix becomes calcified, the cells begin to die.
Later, this calcified intercellular substance is broken down by lager multinucleated cells called osteoclasts. These large cells originate in bone marrow by the fusion of certain single-nucleated white blood cells (monocytes).
Osteoclasts secrete an acid that dissolves the inorganic component of the calcified substance, and at the same time, their lysosomal enzymes digest the organic components of the substance. After the matrix is removed, bone-building osteoblasts invade the region and deposit bone tissue in place of the calcified cartilage.
A long bone will continue to grow in length while at the cartilaginous cells of the epiphyseal disks are active. However, once the ossification centers of the diaphysis and epiphyses come together and the epiphyseal disks become ossified, growth in length is no longer possible.
A developing long bone grows in thickness as compact bone tissue is deposited on the outside, just beneath the periosteum. As this is occurring, other bone tissue is being eroded away on the inside by the activity of the osteoclasts. The space that is created becomes the medullary cavity of the diaphysis, which later fills with marrow.
Bone in the central regions of the epiphyses and diaphysis remains spongy, and hyaline cartilage on the ends of the epiphyses persists throughout life as articular cartilage.
Homeostasis Of Bone Tissue
After the intramembranous and endochondral bones have been formed, they are continually remodeled by the activities of osteoclasts and osteoblasts. Thus, throughout life, osteoclasts are being stimulated to resorb bone tissue at specific sites, and osteoblasts are being activated to replace the bone. These opposing process of resorption and replacement, however, are well regulated. Consequently, the total mass of bone tissue within an adult skeleton normally remains nearly constant, even though 3 percent to 5 percent of bone calcium is exchanged each year.