![\"11.5.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/First_plaster_long_leg_cast...._-_9383569051-2.jpg\")
Figure 11.5.1 Water and casts don't mix.<\/em>[\/caption]\r\n\r\nBreak A Leg<\/h1>\r\n<\/div>\r\nDid you ever break a leg or other bone, like the man looking longingly at the water in this swimming pool (Figure 11.5.1)? Having a broken bone can really restrict your activity. Bones are very hard, but they will break (or fracture) if enough force is applied to them. Fortunately, bones are highly active organs that can repair themselves if they break. Bones can also remodel themselves and grow. You\u2019ll learn how bones can do all of these things in this section.\r\n\r\nBone Growth<\/h1>\r\n<\/div>\r\nEarly in the\u00a0development\u00a0of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification.\u00a0[pb_glossary id=\"4077\"]Ossification[\/pb_glossary]<\/strong> is a process in which bone tissue is created from cartilage. The steps in which bones of the skeleton form from cartilage are illustrated in 11.5.2. The steps are as follows:\r\n\r\n \t- Cartilage \u201cmodel\u201d of bone forms. This model continues to grow as ossification takes place.<\/li>\r\n \t
- Ossification begins at a primary ossification center in the middle of bone.<\/li>\r\n \t
- Ossification then starts to occur at secondary ossification centers at the ends of bone.<\/li>\r\n \t
- The medullary cavity forms. This cavity will contain red bone marrow.<\/li>\r\n \t
- Areas of ossification meet at epiphyseal plates, and articular cartilage forms. Bone growth ends.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_4078\" align=\"aligncenter\" width=\"520\"]
![\"11.5.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Bone_growth-2.png\")
Figure 11.5.2 The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.\r\n\r\n<\/div>\r\nPrimary and Secondary Ossification Centers<\/h2>\r\nWhen bone forms from cartilage, ossification begins with a point in the cartilage called the\u00a0[pb_glossary id=\"4079\"]primary ossification center[\/pb_glossary]<\/strong>. This generally appears during fetal\u00a0development, although a few short\u00a0bones\u00a0begin their primary ossification after birth. Ossification happens\u00a0toward both ends of the bone from the primary ossification center, and \u2014 in the case of long bones \u2014 it eventually forms the shaft of the bone.\r\n\r\n[pb_glossary id=\"4080\"]Secondary ossification centers<\/strong>[\/pb_glossary] form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.\r\nSkeletal Maturity<\/h2>\r\nThroughout childhood, the [pb_glossary id=\"3951\"]cartilage[\/pb_glossary] remaining in the skeleton keeps growing, and allows for bones to grow in size. Once all of the cartilage has been replaced by bone, and fusion has taken place at the [pb_glossary id=\"4108\"]epiphyseal plate[\/pb_glossary]s, bones can no longer keep growing in length. At this point, skeletal maturity has been reached. It generally takes place by age 18 to 25.\r\n\r\nThe use of [pb_glossary id=\"3438\"]anabolic steroids[\/pb_glossary]\u00a0by teens can\u00a0speed\u00a0up the process of skeletal maturity, resulting in a shorter period of cartilage growth before fusion takes place. This means that teens who use steroids are likely to end up shorter as adults than they would otherwise have been.\r\n\r\nBone Remodeling<\/h1>\r\n<\/div>\r\nEven after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process\u00a0called\u00a0[pb_glossary id=\"4109\"]bone remodeling[\/pb_glossary]<\/strong>. In this lifelong process, mature bone tissue is continually turned over, with about ten per cent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of [pb_glossary id=\"4042\"]osteoclasts[\/pb_glossary]<\/strong>\u00a0\u2014\u00a0<\/strong>which are bone\u00a0cells\u00a0that resorb bone and dissolve its\u00a0minerals\u00a0\u2014 and\u00a0[pb_glossary id=\"4040\"]osteoblasts[\/pb_glossary]<\/strong>, which are bone\u00a0cells\u00a0that make new bone matrix.\r\n\r\nBone remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary], because it either releases mineral from bones into the blood or absorbs mineral from the blood into bones. Figure 11.5.3 shows how osteoclasts in bones are involved in calcium regulation.\r\n\r\n[caption id=\"attachment_4110\" align=\"aligncenter\" width=\"825\"]![\"11.5.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Calcium_Homeostasis-2.jpg\")
Figure 11.5.3 Keeping the calcium level in homeostasis involves the work of osteoclasts, the bone cells that resorb bone and release calcium into the blood.<\/em>[\/caption]\r\n\r\nThe action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of [pb_glossary id=\"5757\"]enzymes[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], and other substances that either promote or inhibit the activity of the cells. In this way, these substances control the rate at which bone is made, destroyed, and changed in shape. For example, the rate at which osteoclasts resorb bone and release calcium into the blood is promoted by [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] (PTH) and inhibited by calcitonin, which is produced by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary] (see the diagram in Figure 11.5.3). The rate at which osteoblasts create new bone is stimulated by growth hormone, which is produced by the anterior lobe of the pituitary gland. Thyroid hormone and sex hormones (estrogens and androgens) also stimulate osteoblasts to create new bone.\r\n\r\nBone Repair<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"4111\"]Bone repair[\/pb_glossary]<\/strong> (or healing) is the process in which a bone repairs itself following a bone fracture. You can see an X-ray of a bone fracture in Figure 11.5.4. In this fracture, the humerus in the upper arm has been completely broken through its shaft. Before this fracture heals, a physician must push the displaced bone parts back into their correct positions. Then, the bone must be stabilized \u2014 with a cast and\/or pins surgically inserted into the bone, for example (as shown in Figure 11.5.5) \u2014 until the bone\u2019s natural healing process is complete. This process may take several weeks.\r\n
Bone Growth<\/h1>\r\n<\/div>\r\nEarly in the\u00a0development\u00a0of a human fetus, the skeleton is made almost entirely of cartilage. The relatively soft cartilage gradually turns into hard bone through ossification.\u00a0[pb_glossary id=\"4077\"]Ossification[\/pb_glossary]<\/strong> is a process in which bone tissue is created from cartilage. The steps in which bones of the skeleton form from cartilage are illustrated in 11.5.2. The steps are as follows:\r\n\r\n \t- Cartilage \u201cmodel\u201d of bone forms. This model continues to grow as ossification takes place.<\/li>\r\n \t
- Ossification begins at a primary ossification center in the middle of bone.<\/li>\r\n \t
- Ossification then starts to occur at secondary ossification centers at the ends of bone.<\/li>\r\n \t
- The medullary cavity forms. This cavity will contain red bone marrow.<\/li>\r\n \t
- Areas of ossification meet at epiphyseal plates, and articular cartilage forms. Bone growth ends.<\/li>\r\n<\/ol>\r\n[caption id=\"attachment_4078\" align=\"aligncenter\" width=\"520\"] Figure 11.5.2 The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.\r\n\r\n<\/div>\r\n
Primary and Secondary Ossification Centers<\/h2>\r\nWhen bone forms from cartilage, ossification begins with a point in the cartilage called the\u00a0[pb_glossary id=\"4079\"]primary ossification center[\/pb_glossary]<\/strong>. This generally appears during fetal\u00a0development, although a few short\u00a0bones\u00a0begin their primary ossification after birth. Ossification happens\u00a0toward both ends of the bone from the primary ossification center, and \u2014 in the case of long bones \u2014 it eventually forms the shaft of the bone.\r\n\r\n[pb_glossary id=\"4080\"]Secondary ossification centers<\/strong>[\/pb_glossary] form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.\r\nSkeletal Maturity<\/h2>\r\nThroughout childhood, the [pb_glossary id=\"3951\"]cartilage[\/pb_glossary] remaining in the skeleton keeps growing, and allows for bones to grow in size. Once all of the cartilage has been replaced by bone, and fusion has taken place at the [pb_glossary id=\"4108\"]epiphyseal plate[\/pb_glossary]s, bones can no longer keep growing in length. At this point, skeletal maturity has been reached. It generally takes place by age 18 to 25.\r\n\r\nThe use of [pb_glossary id=\"3438\"]anabolic steroids[\/pb_glossary]\u00a0by teens can\u00a0speed\u00a0up the process of skeletal maturity, resulting in a shorter period of cartilage growth before fusion takes place. This means that teens who use steroids are likely to end up shorter as adults than they would otherwise have been.\r\n\r\nBone Remodeling<\/h1>\r\n<\/div>\r\nEven after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process\u00a0called\u00a0[pb_glossary id=\"4109\"]bone remodeling[\/pb_glossary]<\/strong>. In this lifelong process, mature bone tissue is continually turned over, with about ten per cent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of [pb_glossary id=\"4042\"]osteoclasts[\/pb_glossary]<\/strong>\u00a0\u2014\u00a0<\/strong>which are bone\u00a0cells\u00a0that resorb bone and dissolve its\u00a0minerals\u00a0\u2014 and\u00a0[pb_glossary id=\"4040\"]osteoblasts[\/pb_glossary]<\/strong>, which are bone\u00a0cells\u00a0that make new bone matrix.\r\n\r\nBone remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary], because it either releases mineral from bones into the blood or absorbs mineral from the blood into bones. Figure 11.5.3 shows how osteoclasts in bones are involved in calcium regulation.\r\n\r\n[caption id=\"attachment_4110\" align=\"aligncenter\" width=\"825\"] Figure 11.5.3 Keeping the calcium level in homeostasis involves the work of osteoclasts, the bone cells that resorb bone and release calcium into the blood.<\/em>[\/caption]\r\n\r\nThe action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of [pb_glossary id=\"5757\"]enzymes[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], and other substances that either promote or inhibit the activity of the cells. In this way, these substances control the rate at which bone is made, destroyed, and changed in shape. For example, the rate at which osteoclasts resorb bone and release calcium into the blood is promoted by [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] (PTH) and inhibited by calcitonin, which is produced by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary] (see the diagram in Figure 11.5.3). The rate at which osteoblasts create new bone is stimulated by growth hormone, which is produced by the anterior lobe of the pituitary gland. Thyroid hormone and sex hormones (estrogens and androgens) also stimulate osteoblasts to create new bone.\r\n\r\nBone Repair<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"4111\"]Bone repair[\/pb_glossary]<\/strong> (or healing) is the process in which a bone repairs itself following a bone fracture. You can see an X-ray of a bone fracture in Figure 11.5.4. In this fracture, the humerus in the upper arm has been completely broken through its shaft. Before this fracture heals, a physician must push the displaced bone parts back into their correct positions. Then, the bone must be stabilized \u2014 with a cast and\/or pins surgically inserted into the bone, for example (as shown in Figure 11.5.5) \u2014 until the bone\u2019s natural healing process is complete. This process may take several weeks.\r\n
Figure 11.5.2 The ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.<\/em>[\/caption]\r\n\r\n\r\n\r\nThe ossification of cartilage in the human skeleton is a process that lasts throughout childhood in some bones.\r\n\r\n<\/div>\r\nPrimary and Secondary Ossification Centers<\/h2>\r\nWhen bone forms from cartilage, ossification begins with a point in the cartilage called the\u00a0[pb_glossary id=\"4079\"]primary ossification center[\/pb_glossary]<\/strong>. This generally appears during fetal\u00a0development, although a few short\u00a0bones\u00a0begin their primary ossification after birth. Ossification happens\u00a0toward both ends of the bone from the primary ossification center, and \u2014 in the case of long bones \u2014 it eventually forms the shaft of the bone.\r\n\r\n[pb_glossary id=\"4080\"]Secondary ossification centers<\/strong>[\/pb_glossary] form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.\r\n
Primary and Secondary Ossification Centers<\/h2>\r\nWhen bone forms from cartilage, ossification begins with a point in the cartilage called the\u00a0[pb_glossary id=\"4079\"]primary ossification center[\/pb_glossary]<\/strong>. This generally appears during fetal\u00a0development, although a few short\u00a0bones\u00a0begin their primary ossification after birth. Ossification happens\u00a0toward both ends of the bone from the primary ossification center, and \u2014 in the case of long bones \u2014 it eventually forms the shaft of the bone.\r\n\r\n[pb_glossary id=\"4080\"]Secondary ossification centers<\/strong>[\/pb_glossary] form after birth. Ossification from secondary centers eventually forms the ends of the bones. The shaft and ends of the bone are separated by a growing zone of cartilage until the individual reaches skeletal maturity.\r\nSkeletal Maturity<\/h2>\r\nThroughout childhood, the [pb_glossary id=\"3951\"]cartilage[\/pb_glossary] remaining in the skeleton keeps growing, and allows for bones to grow in size. Once all of the cartilage has been replaced by bone, and fusion has taken place at the [pb_glossary id=\"4108\"]epiphyseal plate[\/pb_glossary]s, bones can no longer keep growing in length. At this point, skeletal maturity has been reached. It generally takes place by age 18 to 25.\r\n\r\nThe use of [pb_glossary id=\"3438\"]anabolic steroids[\/pb_glossary]\u00a0by teens can\u00a0speed\u00a0up the process of skeletal maturity, resulting in a shorter period of cartilage growth before fusion takes place. This means that teens who use steroids are likely to end up shorter as adults than they would otherwise have been.\r\n\r\nBone Remodeling<\/h1>\r\n<\/div>\r\nEven after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process\u00a0called\u00a0[pb_glossary id=\"4109\"]bone remodeling[\/pb_glossary]<\/strong>. In this lifelong process, mature bone tissue is continually turned over, with about ten per cent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of [pb_glossary id=\"4042\"]osteoclasts[\/pb_glossary]<\/strong>\u00a0\u2014\u00a0<\/strong>which are bone\u00a0cells\u00a0that resorb bone and dissolve its\u00a0minerals\u00a0\u2014 and\u00a0[pb_glossary id=\"4040\"]osteoblasts[\/pb_glossary]<\/strong>, which are bone\u00a0cells\u00a0that make new bone matrix.\r\n\r\nBone remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary], because it either releases mineral from bones into the blood or absorbs mineral from the blood into bones. Figure 11.5.3 shows how osteoclasts in bones are involved in calcium regulation.\r\n\r\n[caption id=\"attachment_4110\" align=\"aligncenter\" width=\"825\"] Figure 11.5.3 Keeping the calcium level in homeostasis involves the work of osteoclasts, the bone cells that resorb bone and release calcium into the blood.<\/em>[\/caption]\r\n\r\nThe action of osteoblasts and osteoclasts in bone remodeling and calcium homeostasis is controlled by a number of [pb_glossary id=\"5757\"]enzymes[\/pb_glossary], [pb_glossary id=\"5661\"]hormones[\/pb_glossary], and other substances that either promote or inhibit the activity of the cells. In this way, these substances control the rate at which bone is made, destroyed, and changed in shape. For example, the rate at which osteoclasts resorb bone and release calcium into the blood is promoted by [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary] (PTH) and inhibited by calcitonin, which is produced by the [pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary] (see the diagram in Figure 11.5.3). The rate at which osteoblasts create new bone is stimulated by growth hormone, which is produced by the anterior lobe of the pituitary gland. Thyroid hormone and sex hormones (estrogens and androgens) also stimulate osteoblasts to create new bone.\r\n\r\nBone Repair<\/h1>\r\n<\/div>\r\n[pb_glossary id=\"4111\"]Bone repair[\/pb_glossary]<\/strong> (or healing) is the process in which a bone repairs itself following a bone fracture. You can see an X-ray of a bone fracture in Figure 11.5.4. In this fracture, the humerus in the upper arm has been completely broken through its shaft. Before this fracture heals, a physician must push the displaced bone parts back into their correct positions. Then, the bone must be stabilized \u2014 with a cast and\/or pins surgically inserted into the bone, for example (as shown in Figure 11.5.5) \u2014 until the bone\u2019s natural healing process is complete. This process may take several weeks.\r\n
\r\nBone Remodeling<\/h1>\r\n<\/div>\r\nEven after skeletal maturity has been attained, bone is constantly being resorbed and replaced with new bone in a process\u00a0called\u00a0[pb_glossary id=\"4109\"]bone remodeling[\/pb_glossary]<\/strong>. In this lifelong process, mature bone tissue is continually turned over, with about ten per cent of the skeletal mass of an adult being remodeled each year. Bone remodeling is carried out through the work of [pb_glossary id=\"4042\"]osteoclasts[\/pb_glossary]<\/strong>\u00a0\u2014\u00a0<\/strong>which are bone\u00a0cells\u00a0that resorb bone and dissolve its\u00a0minerals\u00a0\u2014 and\u00a0[pb_glossary id=\"4040\"]osteoblasts[\/pb_glossary]<\/strong>, which are bone\u00a0cells\u00a0that make new bone matrix.\r\n\r\nBone remodeling serves several functions. It shapes the bones of the skeleton as a child grows, and it repairs tiny flaws in bone that result from everyday movements. Remodeling also makes bones thicker at points where muscles place the most stress on them. In addition, remodeling helps regulate mineral [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary], because it either releases mineral from bones into the blood or absorbs mineral from the blood into bones. Figure 11.5.3 shows how osteoclasts in bones are involved in calcium regulation.\r\n\r\n[caption id=\"attachment_4110\" align=\"aligncenter\" width=\"825\"]
![\"11.5.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Broken-Arm-by-Ashley-Chung-scaled-e1591658172394-2.jpg\")
Figure 11.5.4 A bone fracture does not always involve a complete break in the bone, as in this X-ray. Sometimes, a fracture is just a crack in the bone. In other cases, the bone not only breaks all the way through, but also breaks through the soft tissues around it so it protrudes from the skin. This is called an open fracture.<\/em>[\/caption]<\/td>\r\n![\"11.5.5](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Broken-Arm-with-plate-and-pins-by-Ashley-Chung-scaled-3.jpg\")
Figure 11.5.5 While some bones can heal by wearing a cast, others may require more invasive treatments, such as bone fracture repair. Bone fracture repair is a surgery to fix a broken bone using metal screws, pins, rods, or plates to hold the bone in place. It's also known as open reduction and internal fixation (ORIF) surgery.<\/em>[\/caption]<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nAlthough bone repair is a natural physiological process, it may be promoted or inhibited by several factors.\u00a0Fracture repair is more likely to be successful with adequate nutrient intake. Age, bone type, drug therapy, and pre-existing bone disease are additional factors that may affect healing. Bones that are weakened by disease (such as osteoporosis or bone cancer) are not only likely to heal more slowly, but are also more likely to fracture in the first place.\r\n![First_plaster_long_leg_cast\u2026._-_9383569051<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:First_plaster_long_leg_cast...._-_9383569051.jpg\"){kind=link}
![Bone_growth<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Bone_growth.png\"){kind=link}
![Illu_bone_growth.jpg<\/a> is by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Illu_bone_growth.jpg\"){kind=link}
![Calcium_Homeostasis<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:625_Calcium_Homeostasis.jpg\"){kind=link}