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The in vitro formation of 24, 25-(OH) 2D3 during incubation of rat calvarium with 25-(OH) D3 could be demonstrated. The in vitro synthesis of 24, 25-(OH) 2D3 has been localized in the mitochondrial fraction of the calvarium tissue. It could be detected also in calvarium cells in culture, but results indicate that the ability to synthetize 24, 25-(OH) 2D3 may be specific of some cell types only. The in vitro formation of 24, 25-(OH) 2D3 was observed with calvarium from rat neonates as well as from younger fetal rats, yet, in this latter period the in vitro formation of 24, 25-(OH) 2D3 was not found at all days of pregnancy tested in our experimental conditions (17th to 21st day).

Aluminum (Al) accumulation in bone is associated with low bone formation and mineralization rates; resorption may also be reduced. The mechanism of these Al-induced changes was investigated using cultured mouse osteoblast-like (OB) and osteoclast-like (OC) cells. The Al effect on bone resorption was measured by the in vitro release of 45Ca and beta-glucuronidase from mouse fetal limb-bones. Al had a biphasic effect. High concentrations (greater than 1.5 X 10(-6) M) of Al inhibited collagen and DNA synthesis, ornithine decarboxylase and alkaline phosphatase activity in OB, and depressed tartrate-resistant acid phosphatase activity in OC. Lower Al concentrations stimulated these cellular activities and 45Ca and beta-glucuronidase release from fetal bones. Al had no effect on basal cAMP levels in OB but inhibited the stimulating effect of bPTH on cAMP content. Al also altered the 1,25(OH)2D3 effects on the ornithine decarboxylase activity of OB cells. These data suggest that: (i) the low bone formation observed in vivo during Al intoxication may be due to the inhibition of collagen synthesis and to depressed cell proliferation; and (ii) Al may indirectly influence bone remodeling by interfering with the actions of bPTH and 1,25(OH)2D3 on bone cells.

In order to evaluate the role of intrinsic defects in osteoblast function in the pathogenesis of diseases of skeletal development, we developed techniques which permit the evaluation of the metabolic properties of bone-derived cells in vitro. Cells from control children demonstrated a variety of properties classically attributed to osteoblasts (presence of alkaline phosphatase positive cells and synthesis of bone gla protein) and responded to PTH (cAMP production) and to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) ([3H]25-hydroxyvitamin D3 conversion into [3H]24,25-dihydroxyvitamin D3 and bone gla protein secretion). Using these techniques we evaluated the function of cultured bone cells from patients with three rare diseases of skeletal development. Cells from a patient with rickets resistant to 1,25(OH)2D3 were resistant to 1,25(OH)2D3 but responded normally to PTH. Cells from a patient with acroosteolysis with osteoporosis responded normally to PTH and 1,25(OH)2D3. Cells from a patient with hyperphosphatasia with osteoectasia responded normally to 1,25(OH)2D3 but did not respond to PTH. The results demonstrate that bone cell cultures can provide information about the role of osteoblast dysfunction in such diseases.