Bone Growth and Remodeling

Bones continue to grow and remodel themselves throughout life, changing size and shape to accommodate the changing forces applied to the skeleton. For example, in children the femurs become longer, the curvature of the cranium increases to accommodate a growing brain, and many bones develop surface bumps, spines, and ridges (described in chapter 8) as a child begins to walk and the muscles exert tension on the bones. The prominence of these surface features and the density of bone depend on the amount of stress to which a bone has been subjected. On average, the bones have greater density and mass in athletes and people engaged in heavy manual labor than they do in sedentary people. Anthropologists who study

Diaphysis

Epiphyseal plate

Epiphysis

Epiphyseal plates

Diaphysis

Epiphyseal plate

Epiphysis

Epiphyseal plates

Epiphyseal Plate

Figure 7.11 X Ray of a Child's Hand. The cartilaginous epiphyseal plates are evident at the ends of the long bones. These will disappear, and the epiphyses will fuse with the diaphyses, by adulthood.

Figure 7.10 The Fetal Skeleton at 12 Weeks. The red-stained regions are calcified at this age, whereas the elbow, wrist, knee, and ankle joints appear translucent because they are still cartilaginous. Why are the joints of an infant weaker than those of an older child?

Figure 7.11 X Ray of a Child's Hand. The cartilaginous epiphyseal plates are evident at the ends of the long bones. These will disappear, and the epiphyses will fuse with the diaphyses, by adulthood.

Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition skeletal remains can distinguish between members of different social classes by the degree of bone development— a reflection of the individual's nutritional status and history of manual labor. The skeleton also yields information about sex, race, height, weight, and medical history that can be useful to forensic pathology—study of the body to determine the identity of a person, cause and time of death, and so forth.

Cartilage grows by two mechanisms—interstitial22 growth (adding more matrix internally) and apposi-tional23 growth (adding more to the surface). Interstitial growth in the epiphyseal plate adds to the length of a bone. A mature bone, however, grows only by the appositional mechanism. Osteocytes have little room as it is and none to spare for the deposition of more matrix. The only way an adult bone can grow, therefore, is by adding more osseous tissue to the surface.

Appositional growth is similar to intramembranous ossification. The osteogenic cells in the inner layer of periosteum differentiate into osteoblasts. These deposit osteoid tissue on the bone surface, calcify it, and become trapped in it as osteocytes. At the bone surface, matrix is laid down in layers parallel to the surface, not in cylindrical osteons like those deeper in the bone. While deposition occurs at the outer surface of a bone, osteoclasts dissolve bone on the inner surface and thus enlarge the marrow cavity as the bone grows. There is a critical balance between bone deposition and removal. If one process outpaces the other, or if both of them occur too rapidly, various bone deformities and other developmental abnormalities occur (see table 7.4, especially osteitis deformans).

Chapter 7 Bone Tissue 229

Chapter 7 Bone Tissue 229

Figure 7.12 Achondroplastic Dwarfism. The student on the right, pictured with her roommate of normal height, is an achondroplastic dwarf with a height of about 122 cm (48 in.). Her parents were of normal height. Note the normal proportion of head to trunk but shortening of the limbs.

chance of exhibiting dwarfism, depending on the genotype of the other parent.

24 a = without + chondr = cartilage + plast = growth

Insight 7.2 Clinical Application

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Responses

  • NICOL GAUKROGERS
    How do mature bones continue to grow and remodel themselves?
    5 years ago
  • colin
    How do bone develop, grow, and remodel ?
    5 years ago

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