19.2 Bone - Biology Concepts - 1st Canadian Edition (2023)

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NahtknochenThey are small, flat, irregularly shaped bones. They are located between the flat bones of the skull. They vary in number, shape, size and position.

While compact bone tissue forms the outer layer of all bones, cancellous boneor cancellous bone forms the inner layer of all bones. Spongy bone tissue does not contain osteons, which form compact bone tissue. Instead it consists of bunch, which are lamellae that are arranged like rods or plates. There is red bone marrow between the trabuculae. Blood vessels within this tissue provide nutrients and remove waste products to osteocytes. The red bone marrow of the femur and the interior of other large bones, such as the ileum, make up the blood cells.

Cancellous bone reduces bone density and allows the ends of long bones to compress as a result of the stress placed on the bone. Cancellous bone occurs in areas of the bone that are not heavily used or where loads come from multiple directions. The epiphyses of bones such as B. the femoral neck are exposed to loads from different directions. Imagine placing a heavy framed painting on the floor. You can hold one side of the picture with a toothpick if the toothpick is perpendicular to the floor and the picture. Now drill a hole and stick the toothpick into the wall to hang the picture. In this case, the function of the toothpick is to transfer downward pressure from the painting to the wall. The force on the picture is directly on the floor, but the force on the rod is both pulling down the picture wire and pushing up the bottom of the hole in the wall. The toothpick will break straight into the wall.

The femoral neck is horizontal like the toothpick on the wall. The weight of the body pushes it down near the joint, but the vertical shaft of the femur pushes it up at the other end. The femoral neck must be strong enough to transmit the downward force of body weight horizontally to the vertical axis of the femur (Fig. 19.20).

Bones are made up of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. Osteoblastenare bone cells that are responsible for the formation of bones. Osteoblasts synthesize and secrete the organic and inorganic portions of the extracellular matrix of bone tissue and collagen fibers. Osteoblasts become entrapped in these secretions and differentiate into less active osteocytes. Osteoclastsare large bone cells with up to 50 cell nuclei. They remove the bone structure by releasing lysosomal enzymes and acids that dissolve the bone matrix. Released from the bones into the blood, these minerals help regulate calcium levels in body fluids. Bone can also be resorbed for remodeling when the applied loads are changed. Osteozytenthey are mature bone cells and the chief cells of bone connective tissue; these cells cannot divide. Osteocytes maintain normal bone structure by recycling mineral salts from the bone matrix. osteoprogenitor cellsare squamous stem cells that divide to produce daughter cells that differentiate into osteoblasts. Osteoprogenitor cells are important in fracture healing.

Ossification, or osteogenesis, is the process of bone formation by osteoblasts. Ossification differs from the calcification process; While calcification occurs during bone ossification, it can also occur in other tissues. Ossification begins about six weeks after fertilization in an embryo. Before this period, the embryonic skeleton consists entirely of fibrous membranes and hyaline cartilage. The development of bone from fibrous membranes is called intramembranous ossification; The development of hyaline cartilage is called endochondral ossification. Bone growth continues until about age 25. Bones can increase in thickness throughout life, but after age 25 ossification is primarily for bone remodeling and repair.

intramembranous Ossificationis the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, jaw and collarbones. Ossification begins when mesenchymal cells form a blueprint for future bone. They then differentiate into osteoblasts in the ossification center. Osteoblasts secrete the extracellular matrix and deposit calcium, which hardens the matrix. The non-mineralized portion of bone or osteoid continues to form around the blood vessels, forming cancellous bone. Connective tissue in the matrix differentiates into red bone marrow in the fetus. The cancellous bone is remodeled into a thin layer of compact bone on the surface of the cancellous bone.

is the process of bone development from hyaline cartilage. All bones in the body, except for the flat bones of the skull, jaw, and collarbones, are formed by endochondral ossification.

In long bones, chondrocytes form a cast of the diaphysis of hyaline cartilage. In response to complex developmental signals, the matrix begins to calcify. This calcification prevents the diffusion of nutrients into the matrix, leading to the death of chondrocytes and the opening of cavities in the cartilage of the diaphysis. Blood vessels invade the cavities, and osteoblasts and osteoclasts modify the calcified cartilage matrix in spongy bone. Osteoclasts then break off some of the cancellous bone to create a medullary or medullary cavity in the middle of the diaphysis. Dense, irregular connective tissue forms a sheath (periosteum) around the bones. The periosteum helps attach bones to surrounding tissues, tendons, and ligaments. The bone continues to grow and lengthen as the cartilage cells in the epiphyses divide.

In the final stage of prenatal bone development, the centers of the epiphyses begin to calcify. Secondary ossification centers form in the epiphyses when blood vessels and osteoblasts invade these areas and transform hyaline cartilage into cancellous bone. Hyaline cartilage persists until puberty Epiphysenfuge(growth plate), this is the area between the diaphysis and epiphysis that is responsible for the length growth of the long bones (Fig. 19.21).

Long bones continue to lengthen, possibly into adolescence, with the addition of bone tissue at the epiphyseal plate. They also increase in width by appositional growth.

Bone turnover continues after birth into adulthood. bone reconstructionIt is the replacement of old bone tissue with new bone tissue. It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. Normal bone growth requires vitamins D, C and A and minerals such as calcium, phosphorus and magnesium. Hormones such as parathyroid hormone, growth hormone and calcitonin are also required for proper bone growth and maintenance.

Bone turnover rates are quite high, with five to seven percent of bone mass being recycled every week. There are differences in the rate of turnover in different areas of the skeleton and in different areas of a bone. For example, the bone at the femoral head can be completely replaced every six months, while the bone along the shaft changes much more slowly.

Bone remodeling allows bones to adapt to stress and to grow thicker and stronger when subjected to stress. Bones that are not subjected to normal loading, for example when a limb is in plaster, begin to lose mass. A broken or fractured bone is repaired in four stages: