title: Thinking inside and outside the envelopes of bone: dedicated to PDD.
abstract: BACKGROUND
  Bone modeling and remodeling is the final common pathway expressing all genetic and environmental factors that influence the attainment and maintenance of bone's material and structural strength. Modeling and remodeling require a surface, and during growth this cellular machinery fashions bone's external size, shape, and internal architecture by depositing bone on, and removing bone from, both its periosteal (external) and endosteal (internal) envelopes. Bone is distributed and redistributed to achieve strength commensurate with its loading requirements.
  
  
  METHODS
  Advancing age is associated with: (1) a reduction in the volume of bone resorbed by each basic multicellular unit (BMU); (2) an even greater reduction in the volume of bone formed by each BMU so that each remodeling event, whether adaptive or reparative, removes bone from the bone; (3) increased remodeling (number of BMUs) on the three (endocortical, intracortical, and trabecular) components of its endosteal envelope in midlife in women and late in life in both sexes; and (4) reduced bone formation on the periosteal envelope. The net effect is cortical thinning, increased intracortical porosity, trabecular thinning, and loss of connectivity.
  
  
  RESULTS
  While remodeling intensity on an envelope determines structure (e.g., trabecular perforations), the surface area of the envelope determines remodeling intensity, and, so, structure. High remodeling on trabecular surfaces decreases as trabeculae (with their surface) are lost. Conversely, remodeling on the endocortical and intracortical envelopes increases their surface area, so remodeling intensity increases and bone loss becomes predominantly cortical.
  
  
  CONCLUSIONS
  Understanding bone structural strength and its decay and the effects of genetic factors, exercise, nutrition, and drug therapy on bone requires thinking outside and inside these envelopes; their absolute and relative movements during growth and aging determine bone structure and its strength.
authors: INSERM 831 Research Unit, University of Lyon, Lyon, France.
authors: P
authors: P
authors: Szulc
authors: E
authors: E
authors: Seeman
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging / Q000502:physiology*; D016723:Bone Remodeling / Q000502:physiology*; D001862:Bone Resorption / Q000503:physiopathology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008593:Menopause / Q000502:physiology; D008875:Middle Aged; D010012:Osteogenesis / Q000502:physiology; D010521:Periosteum / Q000502:physiology; D012727:Sex Characteristics; D013499:Surface Properties; D055815:Young Adult