Over the next few months, the soft spot gradually hardens. What explains this? Jana is 17 years old and cm tall. She plays basketball and hopes to grow at least 4 cm more before she turns 18 and goes to college. Jana recently injured her leg, and her doctor took an X-ray of it.
Based on the X-ray, the doctor determined that Jana had reached skeletal maturity. How much taller is Jana likely to grow? Explain your answer. Bone development continues throughout adulthood. Even after adult stature is attained, bone development continues for repair of fractures and for remodeling to meet changing lifestyles.
Osteoblasts, osteocytes and osteoclasts are the three cell types involved in the development, growth and remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are mature bone cells and osteoclasts break down and reabsorb bone. Intramembranous ossification involves the replacement of sheet-like connective tissue membranes with bony tissue. Bones formed in this manner are called intramembranous bones. They include certain flat bones of the skull and some of the irregular bones.
The future bones are first formed as connective tissue membranes. Osteoblasts migrate to the membranes and deposit bony matrix around themselves. When the osteoblasts are surrounded by matrix they are called osteocytes. Bone growth continues until approximately age Bones can grow in thickness throughout life, but after age 25, ossification functions primarily in bone remodeling and repair.
Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles.
Ossification begins as mesenchymal cells form a template of the future bone. They then differentiate into osteoblasts at the ossification center. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized portion of the bone or osteoid continues to form around blood vessels, forming spongy bone. Connective tissue in the matrix differentiates into red bone marrow in the fetus.
The spongy bone is remodeled into a thin layer of compact bone on the surface of the spongy bone. Endochondral ossification is the process of bone development from hyaline cartilage. All of the bones of the body, except for the flat bones of the skull, mandible, and clavicles, are formed through endochondral ossification. In long bones, chondrocytes form a template of the hyaline cartilage diaphysis.
Responding to complex developmental signals, the matrix begins to calcify. This calcification prevents diffusion of nutrients into the matrix, resulting in chondrocytes dying and the opening up of cavities in the diaphysis cartilage. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix into spongy bone.
Osteoclasts then break down some of the spongy bone to create a marrow, or medullary, cavity in the center of the diaphysis. Dense, irregular connective tissue forms a sheath periosteum around the bones.
The periosteum assists in attaching the bone to surrounding tissues, tendons, and ligaments. The bone continues to grow and elongate as the cartilage cells at the epiphyses divide. It is considered a part of the growth plate: the part of the bone that grows during childhood, which, as it grows, ossifies near the diaphysis and the epiphyses. After the zone of calcified matrix, there is the zone of ossification, which is actually part of the metaphysis.
Arteries from the metaphysis branch through the newly-formed trabeculae in this zone. The newly-deposited bone tissue at the top of the zone of ossification is called the primary spongiosa. The older bone at the bottom of the zone of ossification is called the secondary spongiosa.
Bones continue to grow in length until early adulthood with the rate of growth controlled by hormones. When the chondrocytes in the epiphyseal plate cease their proliferation and bone replaces the cartilage, longitudinal growth stops. All that remains of the epiphyseal plate is the epiphyseal line.
While bones are increasing in length, they are also increasing in diameter; growth in diameter can continue even after longitudinal growth ceases. This is called appositional growth. Osteoclasts, cells that work to break down bone, resorb old bone that lines the medullary cavity. At the same time, osteoblasts via intramembranous ossification, produce new bone tissue beneath the periosteum.
The erosion of old bone along the medullary cavity and the deposition of new bone beneath the periosteum not only increase the diameter of the diaphysis, but also increase the diameter of the medullary cavity.
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