Bone Formation and Bone Remodeling September 25, 2007

Bone formation and remodeling Learning objectives - 1 Be able to describe, as well as recognize in section, the process of intramembranous bone formation, including the process whereby cancellous bone is converted into compact bone. Be able to recognize osteoblasts, osteocytes and osteoclasts and know their role in the process of intramembranous bone formation and conversion of cancellous bone to compact bone. Be able to recognize mature and immature (mottled or woven) bone. Understand the process of endochondral bone formation and know how a cartilage model is broken down and replaced by bone (e.g. formation of a bony collar, chondrocyte death, invasion of an osteogenic bud from the periosteum, etc.). Understand how the diameter of a long bone increases.

Intramembranous bone formation. #46-HE; #46-M; #120 Periosteum Bone trabecula(e) or spicules

Bone Cells Periosteum Osteoblasts Osteocytes Osteoclasts Osteoid

What is osteoid? Osteoid is the organic matrix of bone, not yet mineralized, that is laid down by osteoblasts (which also facilitate its mineralization).

What is the functional significance of lysosomes and phagocytic vacuoles present in osteoclasts? The degradation of mineralized bone matrix is considered to be due to focal decalcification and extracellular digestion by acid hydrolases and other enzymes contained in lysosomes.

Sharpey’s fibers;#115N

Conversion of trabecular bone to compact bone #115N

Beginning of osteon formation; #115N

Bone formation and remodeling Learning objectives - 1 Be able to describe, as well as recognize in section, the process of intramembranous bone formation, including the process whereby cancellous bone is converted into compact bone. Be able to recognize osteoblasts, osteocytes and osteoclasts and know their role in the process of intramembranous bone formation and conversion of cancellous bone to compact bone. Be able to recognize mature and immature (mottled or woven) bone. Understand the process of endochondral bone formation and know how a cartilage model is broken down and replaced by bone (e.g. formation of a bony collar, chondrocyte death, invasion of an osteogenic bud from the periosteum, etc.). Understand how the diameter of a long bone increases.

Immature (wooven/mottled) bone; #115N

Bone formation and remodeling Learning objectives - 1 Be able to describe, as well as recognize in section, the process of intramembranous bone formation, including the process whereby cancellous bone is converted into compact bone. Be able to recognize osteoblasts, osteocytes and osteoclasts and know their role in the process of intramembranous bone formation and conversion of cancellous bone to compact bone. Be able to recognize mature and immature (mottled or woven) bone. Understand the process of endochondral bone formation and know how a cartilage model is broken down and replaced by bone (e.g. formation of a bony collar, chondrocyte death, invasion of an osteogenic bud from the periosteum, etc.). Understand how the diameter of a long bone increases.

Bone formation and remodeling Learning objectives - 2 Understand how the epiphyseal growth mechanism results in elongation of a long bone. Be able to recognize the different zones of a cartilage growth plate and describe the processes of osteogenesis taking place in each zone (e.g. zone of resting cartilage, proliferation, hypertrophy, calcification and ossification). Be able to describe the process and types of bone remodeling and to recognize cells and structures involved in the process. Be able to describe how fracture repair resembles the process of endochondral bone formation.

Epiphyseal plate and endochondral bone formation #108 Resting cartilage Cartilage proliferation Cartilage hypertrophy Cartilage calcification Erosion of calicified cartilage and bone deposition (ossification) Tide mark

Why do chondrocytes die after they hypertrophy in the epiphyseal plate of the metaphysis? Because of calcification, the chondrocytes lose all sources of nutrients, etc. and die.

Epiphyseal plate and endochondral bone formation #49

Calcified cartilage core in trabecular bone #49

What will happen to the cartilage cores that persists in bone spicules (trabeculae) ? Some cartilage cores will remain as such for some time in boney trabeculae. Eventually, most trabeculae become resorbed and replaced by lamellar bone. The cartilage cores in bigger and thicker trabeculae will be invaded by blood vessels, osteoblasts and osteogenic cells that lay down concentric lamellae of bone replacing cartilage cores.

Bone formation and remodeling Learning objectives - 1 Be able to describe, as well as recognize in section, the process of intramembranous bone formation, including the process whereby cancellous bone is converted into compact bone. Be able to recognize osteoblasts, osteocytes and osteoclasts and know their role in the process of intramembranous bone formation and conversion of cancellous bone to compact bone. Be able to recognize mature and immature (mottled or woven) bone. Understand the process of endochondral bone formation and know how a cartilage model is broken down and replaced by bone (e.g. formation of a bony collar, chondrocyte death, invasion of an osteogenic bud from the periosteum, etc.). Understand how the diameter of a long bone increases.

Growth in diameter of a long bone #49 Periosteum

Long bone shaft:growth in diameter #48 (tibia and fibula) Osteoblasts Osteoid Periosteum

How does the diameter of the bone shaft increase with growth? The bone shaft increases in diameter by appositional growth. New bone is deposited on the outer surface of the diaphysis by successive generations of osteoblasts arising from osteogenic cells of the periosteum. To compensate this growth and prevent bone from becoming too thick and heavy, older bone on the inner surface of the shaft is being resorbed by osteoclasts so as to widen the marrow cavity.

Bone formation and remodeling Learning objectives - 2 Understand how the epiphyseal growth mechanism results in elongation of a long bone. Be able to recognize the different zones of a cartilage growth plate and describe the processes of osteogenesis taking place in each zone (e.g. zone of resting cartilage, proliferation, hypertrophy, calcification and ossification). Be able to describe the process and types of bone remodeling and to recognize cells and structures involved in the process. Be able to describe how fracture repair resembles the process of endochondral bone formation.

Bone Remodeling Erosion (resorption) Tunnel

Bone formation and remodeling Learning objectives - 2 Understand how the epiphyseal growth mechanism results in elongation of a long bone. Be able to recognize the different zones of a cartilage growth plate and describe the processes of osteogenesis taking place in each zone (e.g. zone of resting cartilage, proliferation, hypertrophy, calcification and ossification). Be able to describe the process and types of bone remodeling and to recognize cells and structures involved in the process. Be able to describe how fracture repair resembles the process of endochondral bone formation.

Repair of Fractured Bone

What is the correlation between woven (immature), spongy and compact bone and intramembranous and endochondral bone formation? There is very little correlation between these things. In terms of the three types of bone, the differences between spongy and compact bone was discussed earlier. Woven bone is rapidly formed, new bone and is a precursor to both of these types of bone. Intramembranous bone takes place in mesenchymal tissue and is the type of formation of most of the flat bones of the body (skull, mandible, etc.). It involves the direct formation of bone by osteoblasts and does not involve a cartilage precursor. Both compact and spongy bone can form in this way. Endochondrial bone formation begins with a cartilage precursor that proliferates, hypertrophies, and becomes imbedded in the core of bone. This method is most common in long bones of the body (like the shaft of the femur). An important point, here, is that cartilage does not become bone. Rather, bone is deposited on the surface of calcified cartilage. The type of bone formation does not correlate with the type of bone formed. The difference in the types of bone formation is the way in which the bone is initially formed, not in the end result.