EXTENT OF ALKALINE-PHOSPHATASE CYTOCHEMISTRY VS EXTENT OF TETRACYCLINE FLUORESCENCE IN THE EVALUATION OF HISTODYNAMIC VARIABLES OF BONE-FORMATION

Alkaline phosphatase (ALP) activity is a new histomorphometric index o f the extent of osteoblastic surfaces involved in mineralization. To a ssess its validity in the evaluation of bone formation, we carried out a comparative study between histomorphometric values obtained on the basis of the extent of tetracycline labeling and of the length of ALP- positive endosteal surfaces. The following variables were compared (in dicated by ALP when based on the extent of ALP positivity): trabecular mineralizing surface (MS/BS vs, ALP.S/BS); osteoid mineralizing surfa ce (MS/OS vs. ALP.S/OS); bone formation rate (BFW/BS vs. ALP.BFR/BS); and adjusted appo sitional rate (Aj.AR vs. ALP.Aj.AR). Bone biopsies f rom 39 patients with chronic renal failure and different types of rena l osteodystrophy were considered (48 +/- 12 years of age; 19 men and 2 0 women). Patients were double labeled with tetracycline and biopsies were embedded in glycol-methacrylate at +4 degrees C. Patients showed various types of renal osteodystrophy and were assigned to different g roups of pathologies. Although it differed in incidence according to t he different groups, ALP activity was found in typical plump osteoblas ts bordering osteoid seams and in flat cells, either in contact with o steoid or along the quiescent surfaces of bone in continuity with it. Tetracycline codistributed with all these features to variable extents , according to groups. In all patients, however, ALP.S/BS and ALP.S/OS respectively exceeded MS/BS and MS/OS. In consequence of this, ALP.BF R/BS and ALP.Aj.AR were greater than BFR/BS and Aj.AR, respectivety. F or each of the variable considered, differences among groups of patien ts with different types of renal osteodystrophy were highly significan t. Good correlations were found between the variables measured with th e two methods, Correlations of ALP.BFR/BS and ALP.Aj.AR with serum ind ices of bone metabolism (calcium, phosphate, osteocalcin, ALP, and C-t erminal PTH) were comparable with those of BFR/BS and Aj.AR, respectiv ely. On the basis of the results obtained in our population study, the length of bone surfaces covered by ALP-positive osteoblastic cells gr eatly overestimates the extent of mineralizing surface as assessed by tetracycline labeling and cannot be assumed to be a reliable indicator of the true extent of the mineralizing surface. Histodynamic variable s evaluated by considering bone surfaces positive to ALP to be mineral izing surfaces do not give more accurate information on bone metabolis m compared with those derived by taking into account the extent of tet racycline labeling.