6-5 Bone Formation and Growth Human bones grow until about age 25 Bone formation and growth © 2015 Pearson Education, Inc.

6-5 Bone Formation and Growth Bone Development The process of depositing calcium salts Occurs during bone ossification and in other tissues Ossification Two main forms of ossification ossification © 2015 Pearson Education, Inc.

6-5 Bone Formation and Growth Endochondral Ossification Ossifies bones that originate as Most bones originate as hyaline cartilage There are in endochondral ossification © 2015 Pearson Education, Inc.

Figure 6-11 Endochondral Ossification (Part 5 of 11). As the cartilage enlarges, chondrocytes near the center of the shaft increases greatly in size. The matrix is reduced to a series of small struts that soon begin to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage. Enlarging chondrocytes within calcifying matrix Hyaline cartilage Disintegrating chondrocytes of the cartilage model

Figure 6-11 Endochondral Ossification (Part 6 of 11). 2 Blood vessels grow around the edges of the cartilage, and the cells of the perichondrium convert to osteoblasts. The shaft of the cartilage then becomes ensheathed in a superficial layer of bone. Perichondrium Epiphysis Bone collar Diaphysis Blood vessel Periosteum formed from perichondrium

Figure 6-11 Endochondral Ossification (Part 7 of 11). 3 Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrating with the blood vessels differentiate into osteoblasts and begin producing spongy bone at a primary ossification center. Bone formation then spreads along the shaft toward both ends of the former cartilage model. Medullary cavity Primary ossification center Superficial bone Spongy bone

Figure 6-11 Endochondral Ossification (Part 8 of 11). 4 Remodeling occurs as growth continues, creating a medullary cavity. The osseous tissue of the shaft becomes thicker, and the cartilage near each epiphysis is replaced by shafts of bone. Further growth involves increases in length and diameter. Medullary cavity Metaphysis

Figure 6-11 Endochondral Ossification (Part 9 of 11). 5 Capillaries and osteoblasts migrate into the epiphyses, creating secondary ossification centers. Hyaline cartilage Epiphysis Metaphysis Periosteum Compact bone Secondary ossification center

Figure 6-11 Endochondral Ossification (Part 10 of 11). The epiphyses eventually become filled with spongy bone. The metaphysis, a relatively narrow cartilaginous region called the epiphyseal cartilage, or epiphyseal plate, now separates the epiphysis from the diaphysis. On the shaft side of the metaphysis, osteoblasts continuously invade the cartilage and replace it with bone. New cartilage is produced at the same rate on the epiphyseal side. Articular cartilage Spongy bone Epiphyseal cartilage Diaphysis Within the epiphyseal cartilage, the chondrocytes are organized into zones. Chondrocytes at the epiphyseal side of the cartilage continue to divide and enlarge. Chondrocytes degenerate at the diaphyseal side. Osteoblasts migrate upward from the diaphysis and cartilage is gradually replaced by bone.

Figure 6-11 Endochondral Ossification (Part 11 of 11). 7 At puberty, the rate of epiphyseal cartilage production slows and the rate of osteoblast activity accelerates. As a result, the epiphyseal cartilage gets narrower and narrower, until it ultimately disappears. This event is called epiphyseal closure. The former location of the epiphyseal cartilage becomes a distinct epiphy- seal line that remains after epiphy- seal growth has ended. Articular cartilage Epiphyseal line Spongy bone Medullary cavity A thin cap of the original cartilage model remains exposed to the joint cavity as the articular cartilage. This cartilage prevents damaging the joint from bone-to-bone contact.

6-5 Bone Formation and Growth Compact bone thickens and strengthens long bone with layers of circumferential lamellae © 2015 Pearson Education, Inc.

6-5 Bone Formation and Growth Epiphyseal Lines When long bone , after puberty: Epiphyseal cartilage disappears Is visible on x-rays as an Mature Bones As long bone matures: Osteoclasts form around blood vessels in compact bone © 2015 Pearson Education, Inc.

Figure 6-10a Bone Growth at an Epiphyseal Cartilage. An x-ray of growing epiphyseal cartilages (arrows)

Figure 6-10b Bone Growth at an Epiphyseal Cartilage. Epiphyseal lines in an adult (arrows)

6-5 Bone Formation and Growth Intramembranous Ossification Also called Because it occurs in the Produces such as mandible (lower jaw) and clavicle (collarbone) There are in intramembranous ossification © 2015 Pearson Education, Inc.

Figure 6-12 Intramembranous Ossification (Part 1 of 5). Parietal bone 1 Mesenchymal cells cluster together, differentiate into osteoblasts, and start to secrete the organic components of the matrix. The resulting osteoid then becomes mineralized with calcium salts forming bone matrix. Frontal bone Bone matrix Osteoid Occipital bone Mesenchymal cell Ossification center Blood vessel Osteoblast Mandible Intramembranous ossification starts about the eighth week of embryonic development. This type of ossification occurs in the deeper layers of the dermis, forming dermal bones.

Figure 6-12 Intramembranous Ossification (Part 2 of 5). As ossification proceeds, some osteoblasts are trapped inside bony pockets where they differentiate into osteo- cytes. The developing bone grows outward from the ossification center in small struts called spicules. Spicules Osteocyte

Figure 6-12 Intramembranous Ossification (Part 3 of 5). Blood vessels begin to branch within the region and grow between the spicules. The rate of bone growth accelerates with oxygen and a reliable supply of nutrients. As spicules interconnect, they trap blood vessels within the bone. Blood vessel trapped within bone matrix

Figure 6-12 Intramembranous Ossification (Part 4 of 5). Continued deposition of bone by osteoblasts located close to blood vessels results in a plate of spongy bone with blood vessels weaving throughout.

Figure 6-12 Intramembranous Ossification (Part 5 of 5). Subsequent remodeling around blood vessels produces osteons typical of compact bone. Osteoblasts on the bone surface along with connective tissue around the bone become the periosteum. Fibrous periosteum Blood vessels trapped within bone matrix Areas of spongy bone are remodeled forming the diploë and a thin covering of compact (cortical) bone. Cellular periosteum

6-5 Bone Formation and Growth Blood Supply of Mature Bones Nutrient A single pair of large blood vessels Enter the diaphysis through the nutrient foramen Femur has more than one pair vessels Supply the epiphyseal cartilage Where bone growth occurs Blood to superficial osteons Secondary ossification centers © 2015 Pearson Education, Inc.

Figure 6-13 The Blood Supply to a Mature Bone. Vessels in Bone Articular cartilage Epiphyseal artery and vein Metaphyseal artery and vein Branches of nutrient artery and vein Periosteum Compact bone Nutrient artery and vein Medullary cavity Periosteal arteries and veins Nutrient foramen Periosteum Connections to superficial osteons Metaphysis Metaphyseal artery and vein Epiphyseal line

6-5 Bone Formation and Growth Lymph and Nerves The also contains: Networks of Sensory nerves © 2015 Pearson Education, Inc.

6-6 Bone Remodeling Process of The adult skeleton: Maintains itself Replaces reserves Recycles and Involves osteocytes, osteoblasts, and osteoclasts © 2015 Pearson Education, Inc.

6-6 Bone Remodeling Process of Bone continually remodels, recycles, and replaces Turnover rate varies: If deposition is greater than removal, bones get stronger If removal is faster than replacement, bones get weaker © 2015 Pearson Education, Inc.