Bone Development

Bone Development, Modeling, and Repair 1. Define and differentiate between endochondral and intramembranous ossification. Endochondrial ossification occurs within a hyaline cartilage model of the bone. The hyaline cartilage model in the shape of the bone is formed prior to any bone tissue being laid down. The cartilage is removed and then replaced with bone tissue. The process occurs in two different places: during the initial development of the embryo/fetus and in the epiphyseal plate of a young child or adolescent. In the fetus, the process starts with a hyaline cartilage model of the bone. The first change occurs midshaft, where hypertrophy of chondrocytes happens. The chondrocytes enlarge and secrete alkaline phosphatase. This causes the cartilage matrix to calcify. The cartilage cells begin to die because the flow of nutrients is cut off by the hydroxyapetite crystals. At the same time as the hyperatrophy of the chondrocytes, the perichondrium becomes active. The mesenchymal cells in the perichondrium differentiate into osteoblasts. The osteoid (unmineralized bone matrix) is laid down by the osteoblasts. Then mineralization of the osteoid can occur. This activity forms a periosteal collar (subperiosteal bone collar) around the entire midshaft of the cartilage model. The bony collar is formed outside of the cartilage. The bony collar is formed by intramembranous ossification within the periosteum. Next, comes the invasion of blood vessels in the form of periosteal buds, which enter into the cartilage matrix via holes in the forming bone collar. The periosteal buds erode away the uncalcified cartilage matrix. These blood vessels bring in osteoprogenitor cells into the forming cavities in the cartilage matrix. The osteoprogenitor cells differentiate into osteoblasts. In order for this to happen, the osteoprogenitor cells migrate from the blood vessel to the surface of the calcified cartilage matrix. The newly differentiated osteoblasts lay down osteoid on the surface of the remaining calcified cartilage matrix. The osteoblasts then lay down new bone osteoid, layer by layer. Calcification of the bone osteoid builds up. This is always on the free surface of the cavity so it is appositional growth of bone. Bone trabeculae are formed with a center of calcified cartilage matrix surrounded by bone tissue. The bone that is formed is spongy bone consisting of trabecula. This region is called primary spongiosa because it is the first area that trabecular bone is formed. The bone first laid down here is woven bone (immature) due to its random organization of collagen fibers. If compaction occurs (the spongy bone is converted into dense bone) the osteons formed will be primitive osteons. The next step is secondary bone formation. This is done by internal remodeling which removes the woven bone and replaces it with lamellar bone (mature). Endochondrial ossification is a continuous process that starts in the center of the bone and moves toward each end. The process starts at the primary ossification center where ossification first appears in cartilage model. At the long bones of the body, this occurs in the center of diaphysis. A secondary ossification center is where ossification begins anew in the same cartilage model of bone, but, in a different place after the primary ossification

center has started. In long bones, this is in the epiphysis. Irregular bones have multiple ossification centers. Ossification at the epiphyseal plate occurs at the plate between the diaphysis and epiphysis of a long bone. The plate is made up of hyaline cartilage between the bone tissue of the two centers (a synchondroses cartilage joint). The plate is organized into fibe different zones. The zone of reserve cartilage (resting zone) is located nearest to the epiphysis. It consists of normal hyaline cartilage and functions to attach cartilage to the bony epiphysis. The zone of proliferation is a region of rapid proliferation of chondroblasts. It is recognized as columns of flattened cells that are really isogenous groups of chondroblasts originating from one stem cell. The ECM is slowly laid down to force these cells apart. The zone of proliferation is the interstitial growth of the cartilage. This is key to the function of the plate. The zone of maturation and hypertrophy is where the chondrocytes round up and enlarge. This is due to the increasing cell size as the matrix between cells in the column decreases. The cells here begin to secrete alkaline phosphatase. The zone of calcification is a place where the cartilage matrix becomes mineralized. The amount of mineral being deposited in the matrix cuts off the nutrients and the cells die. It takes special calcium stains to visualize this zone so we can identify it by location. It is the last two rows of enlarged cells adjacent to the resorption front. The zone of ossification is where the blood vessels invade into the cartilage matrix and erode it away. The resorption front is the same thing as the invasion zone. The dead chondrocytes and uncalcified cartilage matrix is removed. Osteoprogenitor cells are brought in with the invading blood vessels. They migrate across the space to the surface of the remaining cartilage matrix. Osteoblasts then differentiate on the surface of calcified cartilage matrix, laying down bone osteoid on the cartilage surface. The osteoid is mineralized and the new deposited bone is woven bone. The trabeculae tha tare formed are mostly longitudinal in orientation. The bone typed formed here is spongy bone and it is called the primary spongiosa or zone of primary ossification. The zone of secondary ossification is further away from the epiphyseal plate and not part of it. This is where internal remodeling takes place and the woven bone is replaced by lamellar bone. Intramembranous ossification starts in the vascular mesenchyme, where the pluripotential mesenchymal cells differentiate to form osteoprogenitor cells. The mesenchymal cells group together in the vascular mesenchyme. These cells then round up and make cellular contact with the adjacent cells. Big thing to remember here are the many small blood vessels running through the mesenchyme. When the O2 content is low, then cartilage is formed, O2 high, vascularized bone. The cells then secrete unmineralized organic bone matrix called osteoid. This pushes the cells further apart. The long cell processes still maintain contact with each other. Mineralization now occurs and forms small bone islands. As the mineralization progresses, the osteoblasts become embedded in the mineralized matrix to form osteocytes. The next generation of osteoblasts, comes from osteoprogenitor cells, differentiates and lines the surface of the bone island to lay down the next layer of osteoid. The osteoblasts

come from the surrounding osteoprogenitor cells in the mesenchyme. These individual bone islands increase in size by appositional growth. The separate islands enlarge until they touch to form trabecula. Appositional growth continues to enlarge the islands and lengthen the trabecula. This process is how trabecular bone (spongy) is first formed. Trabeculae are not straight, but follow the winding route of the blood vessels. Compaction is the formation of compact bone from spongy bone. This is simply a continuation of laying down new layers of osteoid and osteocytes on the surface of the trabecula. The result is the slow build up of the trabecula until the edges of the adjacent trabecula meet to form primitive osteons (made up of woven bone). This osteon may have concentric rings of osteocytes like a mature osteon. However, the new bone tissue is woven bone (immature, primary bone). The collagen fibers (type I) are laid down in interwoven bundles, not in the layers of parallel fibers as in lamellar bone. The skull bones are spongy bone until about 8 years old. They are gradually remodeled into inner & outer tables of compact bone. These layers of dense cortical bone are separated by a middle layer, the diploe, which consists of spongy bone. Ossification starts in the center where the bone is going to form. The place where ossification starts for an individual bone is called the ossification center. The ossification process starts in the center and radiates out towards the periphery. Therefore, the oldest and most developed bone is in the middle of the ossification center. You can see all the sequential steps of ossification by looking at a cross section of that ossification center. The most matrue bone is found in the center and the most immature is in the periphery. Remember that you have more than one ossification center in each bone. 2. Define and differentiate between bone formation and bone growth. Bone formation is making new bone when nothing was originally there. Bone formation equals ossification which equals bone development. Bone growth is always appositional and involves getting larger through calcification and remodeling. 3. Describe what type of growth occurs in bone. All bone growth is appositional. Osteoprogenitor cells differentiate into osteoblasts; the osteoblasts lay down osteoid on the surface. Mineralization of osteoid occurs and osteoblast becomes osteocyte. This is all appositional growth. 4. List and describe the steps of intramembranous bone formation. Intramembranous ossification starts in the vascular mesenchyme, where the pluripotential mesenchymal cells differentiate to form osteoprogenitor cells. The mesenchymal cells group together in the vascular mesenchyme. These cells then round up and make cellular contact with the adjacent cells. Big thing to remember here are the many small blood vessels running through the mesenchyme. When the O2 content is low, then cartilage is formed, O2 high, vascularized bone. The cells then secrete unmineralized organic bone matrix called osteoid. This pushes the cells further apart. The long cell processes still

maintain contact with each other. Mineralization now occurs and forms small bone islands. As the mineralization progresses, the osteoblasts become embedded in the mineralized matrix to form osteocytes. The next generation of osteoblasts, comes from osteoprogenitor cells, differentiates and lines the surface of the bone island to lay down the next layer of osteoid. The osteoblasts come from the surrounding osteoprogenitor cells in the mesenchyme. These individual bone islands increase in size by appositional growth. The separate islands enlarge until they touch to form trabecula. Appositional growth continues to enlarge the islands and lengthen the trabecula. This process is how trabecular bone (spongy) is first formed. Trabeculae are not straight, but follow the winding route of the blood vessels. Compaction is the formation of compact bone from spongy bone. This is simply a continuation of laying down new layers of osteoid and osteocytes on the surface of the trabecula. The result is the slow build up of the trabecula until the edges of the adjacent trabecula meet to form primitive osteons (made up of woven bone). This osteon may have concentric rings of osteocytes like a mature osteon. However, the new bone tissue is woven bone (immature, primary bone). The collagen fibers (type I) are laid down in interwoven bundles, not in the layers of parallel fibers as in lamellar bone. The skull bones are spongy bone until about 8 years old. They are gradually remodeled into inner & outer tables of compact bone. These layers of dense cortical bone are separated by a middle layer, the diploe, which consists of spongy bone. Ossification starts in the center where the bone is going to form. The place where ossification starts for an individual bone is called the ossification center. The ossification process starts in the center and radiates out towards the periphery. Therefore, the oldest and most developed bone is in the middle of the ossification center. You can see all the sequential steps of ossification by looking at a cross section of that ossification center. The most matrue bone is found in the center and the most immature is in the periphery. Remember that you have more than one ossification center in each bone. 5. Describe how trabecular bone is transformed into compact bone. Compaction is the formation of compact bone from spongy bone. This is simply a continuation of laying down new layers of osteoid and osteocytes on the surface of the trabecula. The result is the slow build up of the trabecula until the edges of the adjacent trabecula meet to form primitive osteons (made up of woven bone). This osteon may have concentric rings of osteocytes like a mature osteon. However, the new bone tissue is woven bone (immature, primary bone). The collagen fibers (type I) are laid down in interwoven bundles, not in the layers of parallel fibers as in lamellar bone. The skull bones are spongy bone until about 8 years old. They are gradually remodeled into inner & outer tables of compact bone. These layers of dense cortical bone are separated by a middle layer, the diploe, which consists of spongy bone. 6. Define and describe an ossification center (1' and 2'). Relate their clinical importance.

In endochonrial ossification, the primary ossification center is where ossification first appears in the cartilage model. In the long bones of the body, this occurs at the center of diaphysis. A secondary ossification center is where ossification begins anew in the same cartilage model of bone, bu, in a different place after the primary ossification center has started. In long bones, this is in the epiphysis. Irregular bones may have multiple ossification centers. Clinically, formation of ossification centers may be used to determine “bone age” of small babies. The carpals and tarsals ossify at specific ages of the baby and do not depend on the size of the infant. 7. Describe the formation of flat bones in the skull. The skull bones are spongy bone until about 8 years old. Gradually they are remodeled into inner and outer tables of compact bone. These layers of cortical bone (dense) are separated by a middle layer, the diploe, which consists of spongy bone. 8. List common locations of intramembranous ossification. Intramembranous ossification occurs in flat bones of the skull and formed within the connective tissue proper: frontal, parietal, maxilla, palatine, lacrimal, nasal bones. 9. Relate how and when bone first forms in the body and when it stops. Bone first forms in the body a combination of intramembranous and endochondral ossification. Either the bone is formed off of a vascularized mesenchyme or out of an existing hyaline cartilage model of the bone. Remodeling stops through the process of closure. This occurs when the epiphyseal plate no longer functions to lengthen the bone. Long bones do not continue to grow indefinitely. In the growing child there is a balance between cartilage formation and cartilage erosion rate. Closure is initiated when this balance is changed. In late adolescent, the rate of proliferation slows down, while the rate of cartilage erosion remains the same. As a result there is a more rapid erosion of the plate until the erosion removes even the proliferation zone. When this occurs it is called closure. This closure is marked by the epiphyseal line in metaphysis which can be seen on X-rays. Closure usually occurs in some long bones about age 21-25. Other bones at the base of the skull don't close until the mid 30's. 10. List and describe the steps of endochondral ossification in the formation of a long bone. Endochondrial ossification occurs within a hyaline cartilage model of the bone. The hyaline cartilage model in the shape of the bone is formed prior to any bone tissue being laid down. The cartilage is removed and then replaced with bone tissue. The process occurs in two different places: during the initial development of the embryo/fetus and in the epiphyseal plate of a young child or adolescent. In the fetus, the process starts with a hyaline cartilage model of the bone. The first change occurs midshaft, where hypertrophy of chondrocytes

happens. The chondrocytes enlarge and secrete alkaline phosphatase. This causes the cartilage matrix to calcify. The cartilage cells begin to die because the flow of nutrients is cut off by the hydroxyapetite crystals. At the same time as the hyperatrophy of the chondrocytes, the perichondrium becomes active. The mesenchymal cells in the perichondrium differentiate into osteoblasts. The osteoid (unmineralized bone matrix) is laid down by the osteoblasts. Then mineralization of the osteoid can occur. This activity forms a periosteal collar (subperiosteal bone collar) around the entire midshaft of the cartilage model. The bony collar is formed outside of the cartilage. The bony collar is formed by intramembranous ossification within the periosteum. Next, comes the invasion of blood vessels in the form of periosteal buds, which enter into the cartilage matrix via holes in the forming bone collar. The periosteal buds erode away the uncalcified cartilage matrix. These blood vessels bring in osteoprogenitor cells into the forming cavities in the cartilage matrix. The osteoprogenitor cells differentiate into osteoblasts. In order for this to happen, the osteoprogenitor cells migrate from the blood vessel to the surface of the calcified cartilage matrix. The newly differentiated osteoblasts lay down osteoid on the surface of the remaining calcified cartilage matrix. The osteoblasts then lay down new bone osteoid, layer by layer. Calcification of the bone osteoid builds up. This is always on the free surface of the cavity so it is appositional growth of bone. Bone trabeculae are formed with a center of calcified cartilage matrix surrounded by bone tissue. The bone that is formed is spongy bone consisting of trabecula. This region is called primary spongiosa because it is the first area that trabecular bone is formed. The bone first laid down here is woven bone (immature) due to its random organization of collagen fibers. If compaction occurs (the spongy bone is converted into dense bone) the osteons formed will be primitive osteons. The next step is secondary bone formation. This is done by internal remodeling which removes the woven bone and replaces it with lamellar bone (mature). Endochondrial ossification is a continuous process that starts in the center of the bone and moves toward each end. The process starts at the primary ossification center where ossification first appears in cartilage model. At the long bones of the body, this occurs in the center of diaphysis. A secondary ossification center is where ossification begins anew in the same cartilage model of bone, but, in a different place after the primary ossification center has started. In long bones, this is in the epiphysis. Irregular bones have multiple ossification centers. 11. List and describe the steps (zones) of endochondral ossification in the epiphyseal plate of long bones. Ossification at the epiphyseal plate occurs at the plate between the diaphysis and epiphysis of a long bone. The plate is made up of hyaline cartilage between the bone tissue of the two centers (a synchondroses cartilage joint). The plate is organized into fibe different zones. The zone of reserve cartilage (resting zone) is located nearest to the epiphysis. It consists of normal hyaline cartilage and functions to attach cartilage to the bony epiphysis. The zone of

proliferation is a region of rapid proliferation of chondroblasts. It is recognized as columns of flattened cells that are really isogenous groups of chondroblasts originating from one stem cell. The ECM is slowly laid down to force these cells apart. The zone of proliferation is the interstitial growth of the cartilage. This is key to the function of the plate. The zone of maturation and hypertrophy is where the chondrocytes round up and enlarge. This is due to the increasing cell size as the matrix between cells in the column decreases. The cells here begin to secrete alkaline phosphatase. The zone of calcification is a place where the cartilage matrix becomes mineralized. The amount of mineral being deposited in the matrix cuts off the nutrients and the cells die. It takes special calcium stains to visualize this zone so we can identify it by location. It is the last two rows of enlarged cells adjacent to the resorption front. The zone of ossification is where the blood vessels invade into the cartilage matrix and erode it away. The resorption front is the same thing as the invasion zone. The dead chondrocytes and uncalcified cartilage matrix is removed. Osteoprogenitor cells are brought in with the invading blood vessels. They migrate across the space to the surface of the remaining cartilage matrix. Osteoblasts then differentiate on the surface of calcified cartilage matrix, laying down bone osteoid on the cartilage surface. The osteoid is mineralized and the new deposited bone is woven bone. The trabeculae tha tare formed are mostly longitudinal in orientation. The bone typed formed here is spongy bone and it is called the primary spongiosa or zone of primary ossification. The zone of secondary ossification is further away from the epiphyseal plate and not part of it. This is where internal remodeling takes place and the woven bone is replaced by lamellar bone. 12. Integrate the morphology of the zones of ossification at the epiphyseal plate with the process of endochondral ossification. See 11 13. List and describe the function of the epiphyseal plate. The plate is found between the daiphysis and the epiphysis of a long bone. It consists of a plate of hyaline cartilage between the bone tissue of the two centers. The plate is organized into the zones (see 11). When closure occurs, an epiphyseal line is seen in the metaphysis on X-rays. The function of the epiphyseal plate is to increase the length of the bone. The thickness of the plate remains the same throughout the life of the plate. 14. List and describe how substances can affect the epiphyseal plate and bone remodeling in young and adult. Sex hormones such as androgens and estrogen can stimulate bone formation. These accelerate the closure of epiphyseal plate. Puberty, with its increased sex hormones, causes closure of the major long bones in females about 2 years ahead of males. Hence, females are typically shorter than males. Growth hormone (somatotropin) is secreted by the anterior pituitary gland and stimulates bone formation at the epiphyseal plate. Gigantism results if there is an excess of GH in a growing child. Pituitary dwarfism results if there

is a deficiency of GH in a child. Acromegaly results if there is an excess of GH in adults. This is characterized by thickening of bones, especially facial bones by the eyebrows and enlargement of hands and feet. Vitamin A normally acts to balance bone deposition with bone degradation (like in remodeling). Vitamin A coordinates the activity between th eosteoblasts and the osteoclasts. If there is excess vitamin A, there is more rapid erosion of cartialge resulting in a premature closure of the epiphyseal plate. This would result in dwarfism. If there is a deficiency, there would be a decrease in the growth rate as well. This would lead to a lack of bone resorption during normal remodeling. Bone growth would not keep up with the growth rate of the rest of the body and the cranial cavity and spinal column would fail to enlarge fast enough to accommodate the growing bran and spinal cord. The skull needs to remodel fast for brain expansion and spinal cord, if not retardation and death can result. Vitamin D controls the normal absorption of Ca++ from the gut. Rickets is caused by a deficiency in children. Due to a lack of Ca++ there is no mineralization of the cartilage in the epiphyseal plate and bone osteoid. Boens lack structural strength and bend due to enlarged zone of calcification. This is characterized by “bow legs”. Osteomalacia (adult rickets) is caused by a deficiency of Ca++ in adults. During remodeling, newly formed osteons do not calcify sufficiently. As a result, the bones are greatly weakend and break easily. Vitamin C (scurvy) is a deficiency of vitamin C in adults. Vitamin C is a cofactor for the hydroxylation of proline in alpha chains of forming procollagen. The lack of hydrogen bonding between the hydroxyproline of adjacent alpha chains results in weak tropocollagen. As a result, collagen in the bone matrix is weak and strong osteoid is not formed. The bone is weak due to thinner cortical bone. Growth and repair of fractures is also retarded. Age-related bone loss is the normal loss of bone from all parts of the skeleton with aging. This is due to the rate of bone resorption in remodeling remaining unchanged while the rate of bone deposition decreases. The result is the thinning of cortical bone and the loss of trabecula throughout the bone mass. Osteoporosis is the severe bone loss of bone mass with aging. The reduction of bone mass of the skeleton is so great that it is no longer able to maintain the mechanical support of body weight. It afflcits 25% of all postmenopausal women. This disease is characterized by fractures, backaches, and vertebral deformities. 15. Describe how the shape of a long and flat bone (skull) is maintained as the individual increases in size (modeling). The long bones increase in length by endochondral ossification at the epiphyseal plate. They increase in diameter by intramembranous ossification in the periosteum. Both ossificatin and reabsorption occur simultaneously but must be balanced. In the conical region of bone, the bone is trimmed down from the thick wide epiphysis to the narrow shaft. At the top of the cone, the bone is resorbed on the ouside of the cortical bone by osteoclasts. At the same time, bone is being laid down on inside of cortical bone by osteoblasts. The rates of

these two processes is balanced so that the cortical bone remains the same thickness, but the diameter of shaft is smaller. At the bottom of the cone, bone is added to the outside of cortical bone by the osteoblasts. At the same time, bone is removed from inside of the cortical bone. Again, cortical bone remains the same thickness but diameter increases. The skull bones (flat bones) are enlarged by adding bone tissue at the suture lines. Bone is added to the outer surface of the inner and outer tableau of the cortical bone. At the same time, bone is removed from the inner surface of inner and outer tableau. The rate at which the bone is added and removed is not equal across the entire extent of bone. This non equal process decreases the curvature of the cranial vault so that the midline bone moves the least and the outside bone is added further out. 16. List the hormones which affect the remodeling processes and describe how they work in bones. Sex hormones such as androgens and estrogen can stimulate bone formation. These accelerate the closure of epiphyseal plate. Puberty, with its increased sex hormones, causes closure of the major long bones in females about 2 years ahead of males. Hence, females are typically shorter than males. Growth hormone (somatotropin) is secreted by the anterior pituitary gland and stimulates bone formation at the epiphyseal plate. Gigantism results if there is an excess of GH in a growing child. Pituitary dwarfism results if there is a deficiency of GH in a child. Acromegaly results if there is an excess of GH in adults. This is characterized by thickening of bones, especially facial bones by the eyebrows and enlargement of hands and feet. Two principal hormones are involved in the process of internal remodeling: parathyroid hormone and calcitonin. Parathyroid hormone is secreted due to low peripheral blood Ca++ levels. Parathyroid hormone stimulates the osteoclasts to increase their resorption activity. This is done by: removing bone matrix and releasing Ca++ and by decreasing osteoblastic activity. The net effect is to increase blood calcium ion levels. The receptors for parathyroid hormone are on the osteoblasts. The osteoblasts secrete osteoclastic stimulating factor, which goes to the osteoclast. Calcitonin is a hormone secreted when the Ca++ level in the blood is too high. The receptors for calcitonin are on the osteoclasts. Calcitonin acts to decrease osteoclastic activity and increase osteoblastic activity and deposit Ca++. The net effect is to decrease blood Ca++ levels. 17. Show how remodeling processes (internal and trabecular) are thought to be related to osteoporosis. Osteoporosis occurs when the break down process is occuring faster than the rebuilding process. Basically the osteoclast activity is greater than the osteoblast activity. 18. Describe the difference of primitive and definitive Haversian systems and their respective locations.

The Haversian system is the osteon. This is the basic structural unit of compact bone. The Haversian canal contains blood vessels, making the bone quite vascular. Primitive osteons are made up of woven bone. They are the result of the slow buld up of the trabecula until the edges of adjacent trabecula meet. 19. List and describe the steps and processes that occur during fracture repair. A simple fracture is when the bone breaks into two pieces. The periosteum is disrupted and there are two separate fragments. Reduction of a fracture is when the two fragments are moved back into proper alignment.