In people with osteogenesis imperfecta, the lack of collagen or presence of defective collagen prevents the extracellular matrix from forming correctly. The mineral structure of the bone is weakened, and as a result, the bones are unusually brittle. The bones of people with osteogenesis imperfecta break extremely easily, sometimes with only small amounts of pressure. They also typically take longer to heal and may be more prone to infection.
Bones become stronger and thicker when they are exposed to regular stress. Conversely, bones that do not experience regular stress may lose extracellular matrix and become less dense. In a person with osteogenesis imperfecta, bones that break often and therefore spend a lot of time immobilized in casts or restraints are exposed to much less stress.
As a result, they may lose even more extracellular matrix and become even more brittle. You have already learned about the hydroxyapatite and collagen combination, which is responsible for giving bones their rigidity.
The skeletal system is an excellent example of a system that is primarily composed of extracellular matrix. Generally, cells of the body function within a matrix to hold them together. In the case of bone, the extracellular matrix ECM is composed of crystals of hydroxyapatite and long collagen proteins.
The hydroxyapatite and collagen work together to give bone its unique properties of strength, compressive resistance, and flexibility.
The previous section described the importance of the bone matrix components calcium and hydroxyapatite in maintaining bone rigidity. However, bone remodeling is equally important to maintaining bone health.
Throughout your lifetime, your bones are constantly remodeled and repaired. There are three main cell types in bone, which maintain bone balance and strength:. Think of the bones of your body as a rapidly expanding city. When the old buildings get outdated, they are either remodeled to make enhancements or knocked down to make room for the new ones. As soon as a building is knocked down, a new one is erected in its place. The construction crew recycles the old building materials.
Many city planners constantly monitor the building progress and talk with one another about which sections of the city will get the available resources. Osteoclasts secrete an acid that dissolves the inorganic components of bone: calcium and phosphate. Osteoclasts also contain enzymes that digest the organic components of the bone matrix, such as the proteins and proteoglycans.
Bone resorption osteolysis is complete when small cavities remain on the bone surface. Osteoclast activity is important for creating the medullary cavities of diaphyses, which house bone marrow.
Remember that the diaphysis is the main shaft of a long bone. Osteoclasts are important for removing calcium from bones if the blood calcium levels fall, such as when a diet is deficient in calcium. Osteoblasts build bone and fill in the cavities created by osteoclasts. Osteoblast cells secrete osteoid the organic portion of the bone matrix around them, and then mineralize the matrix to generate rigid bone tissue.
Bone mineralization occurs through osteoblast secretion of an enzyme called alkaline phosphatase. Osteoclasts: These are large cells with more than one nucleus. Their job is to break down bone. They release enzymes and acids to dissolve minerals in bone and digest them. This process is called resorption. Osteoclasts help remodel injured bones and create pathways for nerves and blood vessels to travel through. The marrow is responsible for making around 2 million red blood cells every second.
It also produces lymphocytes or the white blood cells involved in the immune response. Bones are essentially living cells embedded in a mineral-based organic matrix. This extracellular matrix is made of:. Inorganic components , including hydroxyapatite and other salts, such as calcium and phosphate. Collagen gives bone its tensile strength, namely the resistance to being pulled apart.
Hydroxyapatite gives the bones compressive strength or resistance to being compressed. Bones provide a frame to support the body. Muscles, tendons, and ligaments attach to bones.
Without anchoring to bones, muscles could not move the body. For instance, the skull protects the brain, and the ribs protect the heart and lungs. Cancellous bone produces red blood cells, platelets, and white blood cells.
Also, defective and old red blood cells are destroyed in bone marrow. Osteoblasts produce a protein mixture called osteoid, which is mineralized and becomes bone. They also manufacture hormones, including prostaglandins. Osteocytes: These are inactive osteoblasts that have become trapped in the bone that they have created.
They maintain connections to other osteocytes and osteoblasts. They are important for communication within bone tissue. Osteoclasts: These are large cells with more than one nucleus. Their job is to break down bone. They release enzymes and acids to dissolve minerals in bone and digest them.
This process is called resorption. Osteoclasts help remodel injured bones and create pathways for nerves and blood vessels to travel through. The marrow is responsible for making around 2 million red blood cells every second.
It also produces lymphocytes or the white blood cells involved in the immune response. Bones are essentially living cells embedded in a mineral-based organic matrix. This extracellular matrix is made of:. Inorganic components , including hydroxyapatite and other salts, such as calcium and phosphate.
Collagen gives bone its tensile strength, namely the resistance to being pulled apart. Hydroxyapatite gives the bones compressive strength or resistance to being compressed. Bones provide a frame to support the body.
Muscles, tendons, and ligaments attach to bones. Without anchoring to bones, muscles could not move the body. For instance, the skull protects the brain, and the ribs protect the heart and lungs. Cancellous bone produces red blood cells, platelets, and white blood cells. Also, defective and old red blood cells are destroyed in bone marrow. Storing minerals: Bones act as a reserve for minerals, particularly calcium and phosphorous. Detoxification: Bones can absorb heavy metals and other toxic elements from the blood.
The actual width is about 1 cm. Move mouse over image for labels Further magnification demonstrates the organization of the cortical bone into Haversian systems, consisting of concentric layers of bone and a central canal which supplies blood. The small black dots are spaces that contain osteocytes. The boundaries between Haversian systems are the cement lines. Move mouse over image for labels Mineral reservoir In addition to its mechanical functions, the bone is a reservoir for minerals a "metabolic" function.
Changes with aging This graph shows values for bone mineral density at the hip in Caucasian men and women and African-American men and women.
With aging, bone density decreases in all groups. This inevitable bone loss is frequently the cause of osteoporosis. Data are from a study by Looker, Osteoporosis International This image shows trabecular bone structure in the lower spine of a young adult compared to an osteoporotic elderly adult.
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