Morphological and Functional Features of Clasts in Low Phosphate, Vitamin D-Deficiency Rickets

Abstract

Focusing on resorption processes, we have extended our previous studies on chondroclasts and osteoclasts in normally developing tissues, using a model of nutritionally induced vitamin D-deficiency rickets. To analyze the resorption process, we investigated the matrix-resorbing cells in this modified and poorly mineralized tissue regarding morphological features and expression of tartrate-resistant acid phosphatase (TRAP) at the subcellular level. Our goal was to test the hypotheses that initiation of resorption is impaired with unmineralized matrix, and that such alterations involve changes in the subcellullar distribution of TRAP, implicating a role for this enzyme in the resorption process. Our results reveal distinctly different morphological appearances of clast-like cells in rickets compared with normal osteoclasts and chondroclasts. Ordinary resorption structures of osteoclasts and chondroclasts at the cell-matrix border, i.e., ruffled borders and clear zones, are profoundly altered in favor of a less well-defined intermediate zone. TRAP distribution at the subcellullar level is also clearly different from that in osteoclasts and chondroclasts from normal rodents, with impaired secretion; consequently, the enzyme is unable to function in the matrix outside the ruffled border. Our ultrastructural observations demonstrate that in rickets, the clasts are incapable of degrading the poorly mineralized cartilage and bone efficiently. Rachitic clasts seem to be recruited to the matrix surface and interaction between cell and matrix is also initiated, but definitive resorption structures at the cell-matrix border are not normally developed. Whether resorption is inhibited by the mere lack of mineral or mineral-associated proteins, or by other mechanisms remains to be settled.

Glucocorticoids maintain human osteoclasts in the active mode of their resorption cycle

“…This arrest is consistent with prevention of OC resorptive activity when bone surfaces are covered with collagen (30) and with the inability of OCs to resorb poorly mineralized bone in rickets. (32) Interestingly, this shift in cellmatrix contact also results in making OCs moving to a new resorption site, in accordance with the alternating resorption/ migration episodes of the resorption cycle (Fig. 4).…”

“…The generation of demineralized collagen in the resorption pit may well contribute in itself to making OCs moving away because OCs shifting from a mineralized surface to a pure collagen surface undergo a number of transformations reminiscent of an epithelial-mesenchymal transition (29) and contributing to a migratory phenotype (33) instead of a secretory phenotype. These transformations include the loss of extended ruffled border, (32,34) changes in expression of specific cell surface proteins, (29,35) the switch from cathepsin K to matrix metalloproteinases activity, (33) and reorganization of the cytoskeleton with loss of actin ring and polarization and, instead, development of podosomes characteristic of migratory activity. (24,25,29) The hypothesis that collagen signals to migrate away from the pit so that a new pit can be made nearby is also supported by the observation that OCs make rather continuous resorption tracks on pure mineral compared with distinct round pits on bone or dentine, where collagen is part of the matrix.…”

Increased Tartrate-Resistant Acid Phosphatase Expression in Osteoblasts and Osteocytes in Experimental Osteoporosis in Rats

“…Osteoclastic TRAP has been demonstrated in transcytotic intracellular vesicles as well as in the ruffled border beneath the osteoclast during active bone resorption [7, 23, 29, 35, 36]. The secretion of TRAP from the osteoclast to the resorption lacuna makes TRAP available for other bone cells, and TRAP has been suggested as one of the “coupling factors” between bone resorption and bone formation [37].…”

Transiently Increased Bone Density After Irradiation and the Radioprotectant Drug Amifostine in a Rat Model

“…TRAP-positive osteoclasts and chondroclasts were counted in a constant area within the juxtaphyseal region from the chondro-osseous junction to the secondary spongiosa. 7 The constant area was a 0.7-mm wide by 0.35-mm high rectangle constrained by the distal chondroosseous junction and the secondary spongiosa within the midportion of the bone and with the longer dimension oriented perpendicular to the long axis of bone.…”