ALBERT

Notes: Summary It is proposed that there are two structurally different forms of bone loss with different rates, cellular mechanisms, and biomechanical effects. Rapid bone loss is the result of excessive depth of osteoclastic resorption cavities. This leads in trabecular bone to perforation of structural elements, increased size of marrow cavities, and discontinuity of the bone structure, and in cortical bone to subendosteal cavitation and conversion of the inner third of the cortex to a trabecularlike structure, which then undergoes the same changes as the trabecular bone originally present. These structural characteristics reduce the strength of the bones to a greater extent than the reduction in the amount of bone by itself would suggest. Slow bone loss results from incomplete refilling by osteoblasts of resorption cavities of normal or reduced size. This leads to simple thinning of residual structural elements in both trabecular and cortical bone, and reduces the strength of the bones in proportion to the reduction in the amount of bone. This concept, although derived mainly from an examination of postmenopausal bone loss, may be applicable to other osteopenic states. At the same time as bone loss is occurring on the endosteal surface, rapidly or slowly, bone is being added to the periosteal surface, but much more slowly than during growth. The cellular mechanism is the converse of that causing slow bone loss, consisting of slight overfilling of shallow resorption cavities. Slow periosteal gain serves to partly offset the structural weakness resulting from endosteal loss, but is not directly compensatory. Although less well established and of uncertain frequency and magnitude, it is likely that localized bone gain also occurs on some trabecular surfaces, especially the vertical trabecular plates in the spine that are subjected to compression. In contrast to periosteal gain, this may be a compensatory response to loss of horizontal trabeculae, but the cellular mechanism is unknown.

Notes: Summary The cellular basis of the normal bone remodeling sequence in the human adult is discussed in relation to a cycle of five stages—quiescence, activation, resorption, reversal, formation, and return to quiescence. Normally, 80% or more of free bone surfaces are quiescent with respect to remodeling. The structure of the quiescent surface comprises 5 layers; listed in order out toward the bone marrow these are: the lamina limitans (the electron dense outer edge of the mineralized bone matrix), unmineralized connective tissue that may be confused with osteoid by light microscopy, flattened lining cells of osteoblast lineage separated by narrow gaps, more unmineralized connective tissue, and finally either the squamous sac cells of red marrow or the cytoplasm of fat cells of yellow marrow. Activation requires the recruitment of new osteoclasts derived from precursor cells of the mononuclear phagocyte system (and so ultimately from the hematopoietic stem cell), a method for precursor cells to penetrate the cellular and connective tissue barrier of the quiescent surface, and so gain access to the bone mineral, and mechanisms for their attraction and binding to the mineralized surface, possibly in response to chemotactic signals released from bone matrix or mineral. Each of these three steps is probably mediated in some way by lining cells. Resorption is carried out by osteoclasts, most of which are multinucleated. The mean life span of individual nuclei is about 12.5 days; the additional nuclei needed to sustain resorption may be derived fromlocal as well as blood-bone precursors, but nothing is known of their fate. Mononuclear cells may participate not only as precursor cells but as additional resorbing cells, helper cells, and releasers of osteoclast-stimulating agents such as prostaglandins or OAF. It is not known how the size, shape and depth of resorption cavities are controlled, but termination of resorption may involve the release of a suppressor agent (such as prostacyclin) by osteocytes and/or lining cells. During the reversal period the resorption cavity is smoothed off and cement substance is deposited, but the responsible cells are unknown. Successful coupling of formation to resorption requires the proliferation and differentiation of osteoblast precursor cells, focal accumulation of the new osteoblasts within the resorption cavity, and their alignment as a continuous monolayer of uniform polarity. These processes are probably mediated by growth factors released during resorption, and by chemotactic agents present in the bone matrix or in the cement substance. Formation of new bone within the resorption cavity begins with rapid matrix apposition followed some days later by the onset of mineralization. Although the average rates of these two processes during the life span of the osteoid seam (the layer of unmineralized bone matrix) are the same, their instantaneous rates are systematically out of step, so that the osteoid seam width increases rapidly to a maximum of about 20 µm and then declines progressively. At each point on the surface a single osteoblast makes all the bone matrix that is formed, and how completely the cavities are refilled probably depends more on the number of osteoblasts initially assembled than on their individual activity. At the termination of matrix synthesis, mineralization continues more slowly until the osteoid seam eventually disappears and the cells remaining on the surface complete their morphologic and functional transformation to lining cells. The surface has now returned to its original state of quiescence except that the bone is younger. How the remodeling sequence just described is modified to accomplish structural change in response to altered mechanical load is unclear; in particular, it is not known whether there can be direct transformation of a quiescent to a forming surface without intervening resorption.

Notes: Abstract. Assessment of the tubular reabsorption of calcium (Ca) by infusion is complicated by suppression of parathyroid hormone (PTH) secretion, and by activation of the serpentine Ca sensing receptor in the renal tubule, which inhibits Ca and sodium reabsorption, but little is known about the magnitude of the natriuretic effect of Ca in human subjects. Accordingly, we reanalyzed the relationship between serum Ca and urine Ca and sodium excretion expressed per unit of creatinine clearance (CaE and NaE), and per unit of time (UCa and UNa), during a standard Ca infusion, in 14 healthy volunteers and in 8 primary hyperparathyroid patients. In healthy subjects we observed a large effect of Ca infusion on NaE, which rose as high as 8 mmol/liter GFR. In patients with primary hyperparathyroidism both CaE and NaE during Ca infusion were significantly lower overall than in healthy subjects for comparable values of serum Ca (P 〈 0.05 by covariance analysis), due mainly to a decline or reversal of the slopes at the highest serum Ca levels. In both controls and primary hyperparathyroid subjects the variance of CaE as dependent variable was explained by both serum Ca and by NaE as independent variables (P 〈 0.001). We conclude that (1) The natriuretic effect of hypercalcemia was unexpected large and if maintained would lead to substantial depletion of extracellular fluid. (2) Patients with chronic hypercalcemia, including primary hyperparathyroidism, probably have mild sodium depletion, and are more susceptible to volume depletion. (3) Calcium reabsorption during Ca infusion is reduced by suppression of PTH secretion and increased by volume contraction due to sodium depletion. Discrimination between different basal levels of parathyroid function is successful because these effects usually cancel out. (4) The increase in tubular reabsorption of Ca due to volume contraction can initiate a vicious circle, of importance to the pathogenesis and treatment of severe hypercalcemia.

Notes: Summary The most plausible purpose for bone remodeling is to prevent excessive aging of bone, which can cause osteocyte death and increase susceptibility to fatigue microdamage. The age of any particular volume of bone depends on two factors: the probability of remodeling beginning on the nearest bone surface, which is given by the local activation frequency; and the probability of a particular remodeling event penetrating to a specified distance from the surface. These two probabilities can be combined in a mathematical model. According to the model, within about 40 μm from the surface, the rate of surface remodeling is the main determinant of bone age, but beyond 40 μm, the distance from the surface becomes progressively more important. Beyond 75 μm, the bone is essentially isolated from surface remodeling. Application of the model to subjects with and without vertebral fracture indicated that the proportion of iliac cancellous bone with a mean age greater than 20 years was less than 20% in all the control subjects without fracture, but was more than 20% in about one-third of the patients with fracture. Bone age is a major determinant of the degree of mineralization, so that osteoporotic patients with prolonged bone age should have bone of higher true mineral density. Accordingly, mineral density distribution was determined by scanning electron microscopy with backscattered electron imaging, calibrated in terms of atomic number. In osteoporotic patients, the mean atomic number was lower, the proportion of bone with high values was lower, and the proportion of bone with low values was higher than in control subjects, the opposite of what would be predicted by the bone age model just described. These data, together with our failure, to date, to detect osteocyte death and fatigue microdamage in iliac cancellous bone in patients with osteoporosis, cast doubt on the role of low bone turnover and increased bone age in the pathogenesis of vertebral fracture. Although conclusive data are still lacking, bone age, osteocyte death, and fatigue failure are more likely relevant to the pathogenesis of hip fracture. Nevertheless, enhanced bone conservation as a result of modest therapeutic inhibition of remodeling activation more than offsets the hypothetical risk of increasing bone age.