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Biomedical subjects

A M Parfitt

Publications and source records attributed to A M Parfitt.

At least 55 records · Page 3Linked to original sources
PubMed1998

Rates of cell proliferation in adenomatous, suppressed, and normal parathyroid tissue: implications for pathogenesis.

In previous studies, the birth rate of new cells in parathyroid adenomas measured at the time of surgical excision was shown to be much too low to account for growth of the tumors from a single cell in the time available, but comparison with normal rates was not possible. We measured the prevalence of cells expressing the Ki-67 antigen, a cell cycle marker, in 55 parathyroid adenomas using the MIB-1 antibody and microwave antigen retrieval; in 22 cases, separate measurements were made in nonadenomatous tissue from the same glands. In 10 cases complete maps of the gland profile were reconstructed to study the distribution of labeled cells. The proportion of Ki-67-positive cells, estimated by systematic random sampling, was used to calculate cell birth rate assuming a duration of Ki-67 expression of 24 h; the results were compared to rates previously determined in normal parathyroid glands by the same method. The geometric mean cell birth rate was 9.97%/yr, about double the normal rate of 5.4%/yr, but less than a third of the cases had values above the normal range. The corresponding value in nonadenomatous tissue was 2.58%/yr, about half the normal rate. In 10 cases studied in more detail, the cell birth rate was 12.3%/yr in the peripheral regions and 6.2%/yr in the central regions, a value not significantly different from normal. The results in adenomas are in reasonable agreement with previous estimates of cell birth rate of 13.7%/yr using [3H]thymidine labeling and 6.4%/yr using prevalence of the mitotic karyotype. The proportion of Ki-67-positive cells using unbiased sampling was about 50 times smaller than that in previous studies using selective sampling. Cell birth rates at the time of excision were about 20-25 times lower than initial rates estimated from modeling tumor growth by the Gompertz function. We conclude that 1) cell birth rate in parathyroid adenomas has fallen substantially during the growth of the tumors and is only modestly greater than normal; 2) the fall in cell birth rate had been greater in the central and presumably older regions of the adenoma than in the peripheral and presumably younger regions; 3) nonadenomatous tissue was suppressed with respect to its proliferative as well as its secretory function, presumably as a result of hypercalcemia; and 4) the progressive fall in cell birth rate, despite the accumulation of mutations that are supposed to increase cell birth rate, is most readily explained by the set-point hypothesis.

Adenoma↗
PubMed1998

Osteoblast programmed cell death (apoptosis): modulation by growth factors and cytokines.

Once osteoblasts have completed their bone-forming function, they are either entrapped in bone matrix and become osteocytes or remain on the surface as lining cells. Nonetheless, 50-70% of the osteoblasts initially present at the remodeling site cannot be accounted for after enumeration of lining cells and osteocytes. We hypothesized that the missing osteoblasts die by apoptosis and that growth factors and cytokines produced in the bone microenvironment influence this process. We report that murine osteoblastic MC3T3-E1 cells underwent apoptosis following removal of serum, or addition of tumor necrosis factor (TNF), as indicated by terminal deoxynucleotidyl transferase-mediated dUTP-nick end labeling and DNA fragmentation studies. Transforming growth factor-beta and interleukin-6 (IL-6)-type cytokines had antiapoptotic effects because they were able to counteract the effect of serum starvation or TNF. In addition, anti-Fas antibody stimulated apoptosis of human osteoblastic MG-63 cells and IL-6-type cytokines prevented these changes. The induction of apoptosis in MG-63 cells was associated with an increase in the ratio of the proapoptotic protein bax to the antiapoptotic protein bcl-2, and oncostatin M prevented this change. Examination of undecalcified sections of murine cancellous bone revealed the presence of apoptotic cells, identified as osteoblasts by their proximity to osteoid seams and their juxtaposition to cuboidal osteoblasts. Assuming an osteoblast life span of 300 h and a prevalence of apoptosis of 0.6%, we calculated that the fraction that undergo this process in vivo can indeed account for the missing osteoblasts. These findings establish that osteoblasts undergo apoptosis and strongly suggest that the process can be modulated by growth factors and cytokines produced in the bone microenvironment.

Animals↗
PubMed1998

Mineralized bone loss at different sites in dialysis patients: implications for prevention.

To characterize the magnitude and location of mineralized bone loss, 40 patients (20 men, 20 women, 29 white, 11 black) with clinically significant renal osteodystrophy who could be unambiguously classified based on histologic criteria as having osteitis fibrosa (OF; 20 cases) or osteomalacia (OM; 20 cases) were studied; they had been on maintenance hemodialysis for 4.6 +/- 3.0 yr. One hundred forty-two healthy women of similar age and ethnic composition served as control subjects. In all subjects, the proportions of mineralized bone, osteoid, and porosity (nonbone soft tissue) were measured separately in cortical and cancellous bone tissue, from intact full-thickness biopsies of the ilium, representative of the axial skeleton. The results were related to the volumes of cortical and cancellous bone tissue separately and to the volume of the entire biopsy core. Approximately three-quarters of the patients had measurements in the appendicular skeleton by single photon absorptiometry of the radius and morphometry of the metacarpal. Disease effects did not differ significantly between ethnic groups. Mineralized cortical bone volume (per unit of core volume) was reduced by approximately 45% in both patient groups. Mineralized cancellous bone volume was significantly increased by 36% in the patients with OF and nonsignificantly reduced by 9% in the patients with OM; however, the reduction in the latter patients was significant in relation to tissue volume. The combined total deficit for both types of iliac bone was approximately 20% in the patients with OF and approximately 40% in the patients with OM. Significant reductions in appendicular cortical bone were demonstrated in both patient groups at both measurement sites. Regardless of the current histologic classification, the major structural abnormality in the skeleton is generalized thinning of cortical bone due to increased net endocortical resorption, the most characteristic effect on bone of hyperparathyroidism. Protection of the skeleton from the adverse consequences of renal failure will require therapeutic intervention in patients with no symptoms of either renal or bone disease.

Absorptiometry, Photon↗
PubMed1998

Biologic and therapeutic determinants of bone mineral density in multiple myeloma.

The net impact of malignancy and anti-tumor therapy on bone resorption in myeloma is poorly understood because conventional skeletal radiographs are relatively insensitive for the diagnosis and monitoring of bone disease. We performed determinations of bone mineral density (BMD) at the lumbar spine, femoral neck and radial diaphysis by dual energy X ray absorptiometry (DEXA) in 168 consecutive patients with myeloma seen at our institution. Follow up studies were performed in 41 of these patients. A detailed analysis of patient and disease characteristics was performed to identify the determinants of BMD. Compared to normal age and sex matched controls, mean (+/- SE) BMD was significantly decreased at the lumbar spine (Z score -0.4 +/- 0.10) and femoral neck (Z score -1.0 +/- 0.10), but was surprisingly above normal at the radial diaphysis (Z score +0.35 +/- 0.10), a cortical bone site devoid of hematopoietic marrow, suggesting a differential bone preserving effect at this site. Lack of correlation between the BMD findings and the presence or extent of radiographically evident osteolytic lesions suggested the presence of a systemic bone disease. On multivariate analysis, duration of disease >12 months (p = 0.003) and female sex (p = 0.01) were independently associated with a lower BMD at the femoral neck/lumbar spine. On follow up DEXA (n = 41), BMD increased at > or = 1 site in 9 of 20 patients receiving bisphosphonates and in only 2 of 21 patients not receiving such therapy (p = 0.02). Similarly a decline in BMD at > or = 1 site was seen in 9 of 21 patients not receiving bisphosphonates, irrespective of the disease response status. Interval pamidronate therapy (p = 0.0007) and a low serum beta-2-microglobulin (< 2.5 mg/l) (p = 0.04) were the two most significant variables associated with an increase in BMD on multivariate analysis. These data suggest that myeloma is associated with a systemic bone disease with progressive generalized cancellous bone loss and a bone preserving effect on the radial cortical bone. The early use of bisphosphonates may improve myeloma related bone disease.

Absorptiometry, Photon↗
PubMed1997

The basal rate of cell proliferation in normal human parathyroid tissue: implications for the pathogenesis of hyperparathyroidism.

BACKGROUND: The basal rate of cell proliferation in the human parathyroid gland is generally believed to be low, but has never previously been measured directly, although, as in other tissues, it is relevant to the pathogenesis of neoplasia. METHODS: We retrieved embedded tissue blocks of parathyroids removed at autopsy from 39 patients without hypercalcaemia or abnormal renal function, aged 18 to 76 years (mean 47.6). New sections were cut and examined for expression of Ki-67, a cell-cycle marker, using the MIB-1 antibody with microwave antigen retrieval, and tonsil as positive control. Using an eyepiece graticule with unbiased square-counting frame, positively labelled chief cells were counted in large squares and total chief cells in small squares in fields selected by systematic random sampling from multiple regions. A minimum of 15 x 10(3) chief cells were accumulated in each case. The prevalence of Ki-67 positive cells per 10(4) cells (label index or LI) was converted to cell birth rate assuming that the duration of Ki-67 expression was 24 hours. In ten cases, the entire section was examined; a map of the distribution of the positive cells was reconstructed, and divided into outer and inner regions. RESULTS: The geometric mean value for LI was 1.44/ 10(4), and multiplicative SD 2.54. The corresponding geometric mean cell-birth rate was 5.24%/year and 95% confidence interval 0.81-33.8%/year. We found no significant effect of age, sex, race, or duration of tissue storage. The distribution of cells conformed to a Poisson distribution, and there was no difference between central and peripheral regions. CONCLUSIONS: (1) Our results establish the human parathyroid gland as a conditionally renewing tissue with very low basal cell-birth rate; other reports of much higher LI values are probably due to selective and consequently biased sampling. (2) Since the total number of cell divisions is a major determinant of the total number of mutations, our results place some constraints of possible mechanisms for parathyroid neoplasia.

Adolescent↗
PubMed1997

Genetic effects on bone mass and turnover-relevance to black/white differences.

The mass of a bone is given by its volume and its apparent density--mass per unit external volume. Most measurements of so-called density are of mass incompletely normalized by some index of bone size. Genes control about 60% to 75% of the variance of peak bone mass/density and a much smaller proportion of the variance in rate of loss. Genetic influence on bone mass/density are mediated in large part by body size, bone size, and muscle mass. Most of the fifty-fold increase in bone mass from birth to maturity is due to bone growth, which is linked to muscle growth and bodily growth. Three-D apparent bone density in the vertebrae increases about 15% during the pubertal growth spurt. The genetic potential for bone accumulation can be frustrated by insufficient calcium intake, disruption of the calendar of puberty and inadequate physical activity. The growing skeleton is much more responsive than the mature skeleton to the osteotrophic effect of exercise, which is mediated by the detection of deviations from a target value for strain, and orchestration of cellular responses that restore the target value, processes collectively termed the mechanostat. Production of metaphyseal cancellous bone and growth in length are both linked to endochondral ossification, which is driven by growth plate cartilage cell proliferation. Production of diaphyseal cortical bone and growth in width are both linked to periosteal apposition, which is driven by osteoblast precursor proliferation. During adolescence trabeculae and cortices become thicker by net endosteal apposition, which increases apparent density. Two lines of evidence support a genetic basis for black/white differences in bone mass. First, the magnitude (10% to 40%) is incommensurate with known nongenetic factors. Second, the difference is already evident in the fetus and increases progressively during growth, especially in adolescence; the difference in peak bone mass persists throughout life. The genetic determination of bone mass is mediated by two classes of gene. The first regulates growth of the body, including muscles and bones, under the control of a master gene or set of genes whose products function as the sizostat. The second regulates the increase in apparent bone density in response to load bearing, under the control of a master gene or set of genes whose products function as the mechanostat.

Black People↗
PubMed1997

The effects of androgen deficiency on murine bone remodeling and bone mineral density are mediated via cells of the osteoblastic lineage.

Both estrogens and androgens act on bone marrow stromal/osteoblastic cells to inhibit the production of local factors that promote osteoclast development. Based on this and the evidence that loss of sex steroids up-regulates not only osteoclastogenesis but also osteoblastogenesis, we have hypothesized that cells of the osteoblastic lineage are the mediators of the adverse effects of sex steroid deficiency on bone. To test this hypothesis, we used the senescence-accelerated mouse (SAMP6), a model of defective osteoblast development, and examined the effects of orchidectomy on static and dynamic histological features of bone remodeling and on bone mineral density. After orchidectomy in SAMP6 mice, the expected increases in osteoblast precursors, cancellous osteoclasts and osteoblasts, frequency of remodeling events, trabecular spacing, and rate of bone formation were absent or greatly attenuated. Moreover, whereas bone mineral density decreased in orchidectomized controls, it did not change in SAMP6. Our data indicate that when osteoblast development is defective, orchidectomy fails to result in bone loss. This evidence suggests that cells of the osteoblastic lineage are essential mediators of the changes in the rate of bone remodeling and loss of bone mass that ensue following loss of androgens.

Androgens↗
PubMed1997

Increased adipogenesis and myelopoiesis in the bone marrow of SAMP6, a murine model of defective osteoblastogenesis and low turnover osteopenia.

Bone formation and hematopoiesis are anatomically juxtaposed and share common regulatory mechanisms. However, little is known about the interrelationship between these two processes. We have previously shown that the senescence accelerated mouse-P6 (SAMP6) exhibits decreased osteoblastogenesis in the bone marrow that is temporally linked with a low rate of bone formation and decreased bone mineral density. Here we report that in contrast to decreased osteoblastogenesis, ex vivo bone marrow cultures from SAMP6 mice exhibited an increase in the number of colony-forming unit adipocytes, as well as an increase in the number of fully differentiated marrow adipocytes, compared with SAMR1 (nonosteopenic) controls. Further, long-term bone marrow cultures from SAMP6 produced an adherent stromal layer more rapidly, generated significantly more myeloid progenitors and produced more IL-6 and colony-stimulating activity. Consistent with this, the number of myeloid cells in freshly isolated marrow from SAMP6 mice was increased, as was the number of granulocytes in peripheral blood. The evidence that SAMP6 mice exhibit decreased osteoblastogenesis, and increased adipogenesis and myelopoiesis, strongly suggests that a switch in the differentiation program of multipotential mesenchymal progenitors may underlie the abnormal phenotype manifested in the skeleton and other tissues of these animals. Moreover, these observations support the contention for the existence of a reciprocal relationship between osteoblastogenesis and adipogenesis that may explain the association of decreased bone formation and the resulting osteopenia with the increased adiposity of the marrow seen with advancing age in animals and humans.

Adipocytes↗
PubMed1997

Effects of ethnicity and age or menopause on osteoblast function, bone mineralization, and osteoid accumulation in iliac bone.

We measured histologic indices of osteoblast function, bone mineralization, and osteoid accumulation separately on the cancellous, endocortical, and intracortical subdivisions of the endosteal envelope and on the combined total surface in transiliac biopsies obtained after double tetracycline labeling in 142 healthy women, aged 20-74 years, 34 who were black (19 pre- and 15 postmenopausal) and 108 white (42 pre- and 66 postmenopausal). The data were subjected to two-way analysis of variance of the four groups defined by age/menopause and ethnicity. Also, linear regressions of selected variables on age and between functionally related but independently measured variables were examined. None of the interaction terms was significant, and none of the regression slopes on age differed between blacks and whites, indicating that, as for the previously reported structural and remodeling indices, the effects of ethnicity and of age/menopause are independent. Accordingly, the data were analyzed separately for the effects of ethnicity (pre- and postmenopausal combined) and age/menopause (blacks and whites combined). The analyses led to the following conclusions (1) Osteoid surface and volume were higher and adjusted apposition rate and osteoid mineralization rate lower in postmenopausal than in premenopausal subjects, but none of the indices of osteoid accumulation differed between blacks and whites. (2) Each index of osteoid accumulation was significantly correlated with its primary independently measured kinetic determinant (osteoid thickness with adjusted apposition rate, osteoid surface/bone surface with activation frequency, and osteoid volume/bone volume with bone formation rate/bone volume). None of the regression parameters differed significantly between blacks and whites. (3) The ratio of mineralizing surface to osteoid surface (MS/OS) was substantially lower in all demographic groups than could be accounted for by the later onset of mineralization than of matrix apposition at individual bone forming sites. (4) The low values for MS/OS can be explained by terminal mineralization being too slow to trap enough tetracycline molecules to produce detectable fluorescence, and do not require that mineralization be interrupted. (5) MS/OS was about 25% lower in blacks than in whites on all surfaces with corresponding differences in derived indices based on MS/OS, including adjusted apposition rate, mineralization lag time, and formation period. (6) The lower values for MS/OS in blacks are most likely due to slower terminal mineralization. This could not be accounted for by a lower serum level of calcidiol, but is consistent with the reported effect of reduced bone blood flow. (7) All differences in bone cell function between blacks and whites that we have observed could be the result of the ethnic, and presumably genetic, difference in bone accumulation during growth. Higher bone mass would result in less fatigue microdamage, less need for repair by directed bone remodeling, lower bone turnover, lower bone blood flow, and slower terminal mineralization. (8) If this explanation is correct, there are no fundamental differences in the biology of bone remodeling between ethnic groups.

Adult↗
PubMed1997

Effects of ethnicity and age or menopause on the remodeling and turnover of iliac bone: implications for mechanisms of bone loss.

We measured histologic indices of bone remodeling and turnover separately on the cancellous, endocortical, and intracortical subdivisions of the endosteal envelope, and on the combined total surface, in transiliac bone biopsies obtained after double tetracycline labeling in 142 healthy women, aged 20-74 years, 34 black and 108 white, 61 premenopausal and 81 postmenopausal. The data were analyzed by two-way analysis of variance of the four groups defined by age/menopause and ethnicity and by linear regression of the major variables on age. None of the interaction terms was significant and none of the regression slopes on age differed between blacks and whites, indicating that, as for the previously reported structural indices, the effects of ethnicity and of age/menopause are independent. Accordingly, the data were also analyzed separately for the effect of ethnicity (pre- and postmenopausal combined) and age/menopause (blacks and whites combined). The analyses led to the following conclusions. (1) The geometric mean bone formation rate on the combined total surface was 25% lower in blacks than in whites; other histologic differences between ethnic groups were inconsistent between surfaces. (2) Serum osteocalcin (OC) but not bone-specific alkaline phosphatase (BSAP) was lower by about 15% in blacks than in whites. (3) The lower bone turnover in blacks is most likely in the directed rather than in the stochastic component because of a higher bone mass and consequent reduced susceptibility to fatigue damage. (4) All Class 1 bone formation variables and the three resorption indices were significantly higher in the postmenopausal compared with the premenopausal subjects, reflecting a 33% increase in activation frequency. (5) BSAP, but not OC, was increased relatively more (66%) than the bone formation rate (BFR). Consequently, BSAP is more sensitive to the effects of menopause than OC, but OC is more sensitive to the effects of ethnicity than BSAP. (6) There were highly significant differences between the three subdivisions of the endosteal envelope for every non-cell-related variable. All Class 1 formation variables were highest on the endocortical surface, but the magnitude and pattern of the differences otherwise was inconsistent between variables. The contributions of the different subdivisions to the total bone formation rate were cancellous 54%, endocortical 13%, and intracortical 33%. (7) The previously reported changes in bone surface location, together with the presently reported changes in activation frequency and wall thickness indicated that there was no significant effect of age/menopause on erosion depth on the cancellous and intracortical surfaces but a large increase in erosion depth on the endocortical surface. (8) The increase in bone turnover that results from hormonal changes is most likely in the stochastic rather than in the directed component because it serves no purpose but has harmful effects on skeletal integrity.

Adult↗
PubMed1997

Quantification of vitamin D receptor mRNA by competitive polymerase chain reaction in PBMC: lack of correspondence with common allelic variants.

It has been recently claimed that polymorphism for the vitamin D receptor (VDR) influences several aspects of calcium and bone metabolism. To evaluate the physiologic plausibility of these claims, we compared the abundance of the VDR mRNA in peripheral blood mononuclear cells (PBMCs) between different VDR genotypes using a quantitative reverse transcribed polymerase chain reaction-based method. The method is based on the coamplification of VDR cDNA and an internal standard consisting of known concentrations of a human VDR CDNA mutated at a BglII restriction site; the interassay coefficient of variation is 11%. To validate the method, we made use of earlier receptor binding studies indicating that normal human monocytes and activated, but not resting, lymphocytes expressed the VDR. The concentration of the VDR mRNA was 10(-8) to 10(-7) g/g of total RNA in cell-sorted monocytes and in in vitro activated lymphocytes, but only 10(-12) g/g of total mRNA in resting lymphocytes, establishing that the VDR mRNA determined by our method in PBMCs is due to constitutive expression in monocytes. Following an initial genotype screening of 85 normal volunteers by polymerase chain reaction or restriction fragment length polymorphism analysis, 14 individuals with the Bb genotype, 12 with the bb genotype, and 12 with the BB genotype were selected. The concentration of the VDR mRNA, corrected for the number of monocytes, was similar among the three genotype groups, as were the other variables examined: serum calcitriol, serum osteocalcin, and vertebral and hip bone density. We conclude that VDR polymorphism does not affect the abundance of the VDR mRNA.

Aged↗