Human fetal bone development: histomorphometric evaluation of the proximal femoral metaphysis

Quantitative data on metaphyseal bone histology during early human development are scarce. In the present study the proximal femoral metaphysis of 35 fetuses and newborns (gestational age 16-35 weeks) was analyzed by histomorphometry. Averaged over the entire metaphyseal area, the relative amount of bone and cartilage was higher in the third compared to the second trimester. Osteoid thickness increased with gestational age, whereas indices of bone resorption decreased. The relative amount of cartilage decreased with increasing distance from the growth plate, whereas the relative amount of bone increased. This was due to trabecular thickening, which occurred at an estimated rate of 3 microm/day in areas close to the growth plate. Despite this rapid rate of net bone gain, osteoid indices were relatively low, indicating that mineralization occurred very rapidly after bone deposition. These observations suggest that modeling, not remodeling, is the predominant mechanism responsible for the development of femoral metaphyseal cancellous bone in utero.

LDL receptor-related protein 5 (LRP5) affects bone accrual and eye development

In humans, low peak bone mass is a significant risk factor for osteoporosis. We report that LRP5, encoding the low-density lipoprotein receptor-related protein 5, affects bone mass accrual during growth. Mutations in LRP5 cause the autosomal recessive disorder osteoporosis-pseudoglioma syndrome (OPPG). We find that OPPG carriers have reduced bone mass when compared to age- and gender-matched controls. We demonstrate LRP5 expression by osteoblasts in situ and show that LRP5 can transduce Wnt signaling in vitro via the canonical pathway. We further show that a mutant-secreted form of LRP5 can reduce bone thickness in mouse calvarial explant cultures. These data indicate that Wnt-mediated signaling via LRP5 affects bone accrual during growth and is important for the establishment of peak bone mass.

Osteogenesis imperfecta at the beginning of bone and joint decade

Osteogenesis imperfecta (OI), or brittle bone disease, is a heritable disorder characterized by increased bone fragility. Four different types of the disease are commonly distinguished, ranging from a mild condition (type I) to a lethal one (type II). Types III and IV are the severe forms surviving the neonatal period. In most cases, there is a reduction in the production of normal type I collagen or the synthesis of abnormal collagen as a result of mutations in the type I collagen genes. These classic forms of OI are described in this review. There are instances, however, where alterations in bone matrix components, other than type I collagen, are the basic abnormalities of the OI. Recently, three such discrete types have been identified by histomorphometric evaluation (types V and VI) and linkage analysis (Rhizomelic OI). They provide evidence for the as yet poorly understood complexity of the phenotype-genotype correlation in OI. We also discuss bisphosphonates treatment as well as fracture management and surgical correction of deformities observed in the patients with OI. However, ultimately, strengthening bone in OI will involve steps to correct the underlying genetic mutations that are responsible for this disorder. Thus, we also describe different genetic therapeutic approaches that have been tested either on OI cells or on available OI murine models.

Targeted inactivation of the 25-hydroxyvitamin D(3)-1(alpha)-hydroxylase gene (CYP27B1) creates an animal model of pseudovitamin D-deficiency rickets

Pseudovitamin D-deficiency rickets is caused by mutations in the cytochrome P450 enzyme, 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-OHase). Patients with the disease exhibit growth retardation, rickets, and osteomalacia. Serum biochemistry is characterized by hypocalcemia, secondary hyperparathyroidism, and undetectable levels of 1alpha,25-dihydroxyvitamin D(3). We have inactivated the 1alpha-OHase gene in mice after homologous recombination in embryonic stem cells. Serum analysis of homozygous mutant animals confirmed that they were hypocalcemic, hypophosphatemic, hyperparathyroidic, and that they had undetectable 1alpha,25-dihydroxyvitamin D(3). Histological analysis of the bones from 3-week-old mutant animals confirmed the evidence of rickets. At the age of 8 weeks, femurs from 1alpha-OHase-ablated mice present a severe disorganization in the architecture of the growth plate and marked osteomalacia. These results show that we have successfully inactivated the 1alpha-OHase gene in mice and established a valid animal model of pseudovitamin D-deficiency rickets.

Mepe, the gene encoding a tumor-secreted protein in oncogenic hypophosphatemic osteomalacia, is expressed in bone

The MEPE (matrix extracellular phosphoglycoprotein) gene is a strong candidate for the tumor-derived phosphaturic factor in oncogenic hypophosphatemic osteomalacia (OHO). X-linked hypophosphatemia (XLH) is phenotypically similar to OHO and results from mutations in PHEX, a putative metallopeptidase believed to process a factor(s) regulating bone mineralization and renal phosphate reabsorption. Here we report the isolation of the murine homologue of MEPE, from a bone cDNA library, that encodes a protein of 433 amino acids, 92 amino acids shorter than human MEPE. Mepe, like Phex, is expressed by fully differentiated osteoblasts and down-regulated by 1,25-(OH)2D3. In contrast to Phex, Mepe expression is markedly increased during osteoblast-mediated matrix mineralization. Greater than normal Mepe mRNA levels were observed in bone and osteoblasts derived from Hyp mice, the murine homologue of human XLH. Our data provide the first evidence that MEPE/Mepe is expressed by osteoblasts in association with mineralization.

Thirty-three novel COL1A1 and COL1A2 mutations in patients with osteogenesis imperfecta types I-IV

Osteogenesis imperfecta (OI) is a heritable disease of bone characterized by low bone mass and bone fragility. Six different types of OI have been described to date, based on clinical phenotype and histological findings. The genetic defect in many patients with OI types I-IV is due to mutations in the genes encoding type I collagen, while patients with OI types V and VI show no evidence of mutations in the COL1A1/COL1A2 genes. Here we report thirty-three novel mutations in patients with types I-IV OI. Sixteen mutations were in COL1A1 and seventeen were in COL1A2. Most mutations resulted in substitutions for glycine: one of these, a doublet GG>CC transversion, created a unique Gly-->Pro missense mutation in the triple helical domain of COL1A2. Two rare triple helical Gly-->Glu substitutions in COL1A2 are also described. In addition, there were six single-base deletion mutations resulting in frameshifts, seven splice junction mutations, and a 9-bp triple helix insertion associated with a severe (OI II) phenotype. The variety of mutations described in the COL1A1/COL1A2 genes giving rise to an OI phenotype is in accordance with the clinical heterogeneity of the disease. Hum Mutat 17:434, 2001.

The use of bisphosphonates in children with osteogenesis imperfecta

Children with osteogenesis imperfecta (OI) suffer recurrent fractures resulting in pain, deformity and disability. There is no accepted medical therapy for the condition other than symptomatic pain relief, and surgical correction of the deformities. Recent experience with the bisphosphonate group of drugs suggests, however, that anti-resorptive therapy may reduce fracture frequency, increase bone density, promote remodeling of previously crush-fractured vertebrae, reduce chronic pain, and improve mobility in both children and infants. The prospects for preventing disability and deformity, and perhaps improving growth in affected children are good. Current studies are focusing on the evaluation of the efficacy of oral drug therapy, and protocols are under development to evaluate new, more efficient molecules. Until gene therapy becomes a reality, the use of bisphosphonates appears to be the most efficient way of altering the natural course of severe OI, and improving the quality of life of patients.

Structural and cellular changes during bone growth in healthy children

Normal postnatal bone growth is essential for the health of adults as well as children but has never been studied histologically in human subjects. Accordingly, we analyzed iliac bone histomorphometric data from 58 healthy white subjects, aged 1.5-23 years, 33 females and 25 males, of whom 48 had undergone double tetracycline labeling. The results were compared with similar data from 109 healthy white women, aged 20-76 years, including both young adult reference ranges and regressions on age. There was a significant increase with age in core width, with corresponding increases in both cortical width and cancellous width. In cancellous bone there were increases in bone volume and trabecular thickness, but not trabecular number, wall thickness, interstitial thickness, and inferred erosion depth. Mineral apposition rates declined on the periosteal envelope and on all subdivisions of the endosteal envelope. Because of the concomitant increase in wall thickness, active osteoblast lifespan increased substantially. Bone formation rate was almost eight times higher on the outer than on the inner periosteum, and more than four times higher on the inner than on the outer endocortical surface. On the cancellous surface, bone formation rate and activation frequency declined in accordance with a fifth order polynomial that matched previously published biochemical indices of bone turnover. The analysis suggested the following conclusions: (1) Between 2 and 20 years the ilium grows in width by periosteal apposition (3.8 mm) and endocortical resorption (3.2 mm) on the outer cortex, and net periosteal resorption (0.4 mm) and net endocortical formation (1.0 mm) on the inner cortex. (2) Cortical width increases from 0.52 mm at age 2 years to 1.14 mm by age 20 years. To attain adult values there must be further endocortical apposition of 0.25 mm by age 30 years, at a time when cancellous bone mass is declining. (3) Lateral modeling drift of the outer cortex enlarges the marrow cavity; the new trabeculae filling this space arise from unresorbed cortical bone and represent cortical cancelization; (4) Lateral modeling drift of the inner cortex encroaches on the marrow cavity; some trabeculae are incorporated into the expanding cortex by compaction. (5) The net addition of 37 microm of new bone on each side of a trabecular plate results from a <5% difference between wall thickness and erosion depth and between bone formation and bone resorption rates; these small differences on the same surface are characteristic of bone remodeling. (6) Because the amount of bone added by each cycle of remodeling is so small, the rate of bone remodeling during growth must be high to accomplish the necessary trabecular hypertrophy.

Type V osteogenesis imperfecta: a new form of brittle bone disease

Osteogenesis imperfecta (OI) is commonly subdivided into four clinical types. Among these, OI type IV clearly represents a heterogeneous group of disorders. Here we describe 7 OI patients (3 girls), who would typically be classified as having OI type IV but who can be distinguished from other type IV patients. We propose to call this disease entity OI type V. These children had a history of moderate to severe increased fragility of long bones and vertebral bodies. Four patients had experienced at least one episode of hyperplastic callus formation. The family history was positive for OI in 3 patients, with an autosomal dominant pattern of inheritance. All type V patients had limitations in the range of pronation/supination in one or both forearms, associated with a radiologically apparent calcification of the interosseous membrane. Three patients had anterior dislocation of the radial head. A radiodense metaphyseal band immediately adjacent to the growth plate was a constant feature in growing patients. Lumbar spine bone mineral density was low and similar to age-matched patients with OI type IV. None of the type V patients presented blue sclerae or dentinogenesis imperfecta, but ligamentous laxity was similar to that in patients with OI type IV. Levels of biochemical markers of bone metabolism generally were within the reference range, but serum alkaline phosphatase and urinary collagen type I N-telopeptide excretion increased markedly during periods of active hyperplastic callus formation. Qualitative histology of iliac biopsy specimens showed that lamellae were arranged in an irregular fashion or had a meshlike appearance. Quantitative histomorphometry revealed decreased amounts of cortical and cancellous bone, like in OI type IV. However, in contrast to OI type IV, parameters that reflect remodeling activation on cancellous bone were mostly normal in OI type V, while parameters reflecting bone formation processes in individual remodeling sites were clearly decreased. Mutation screening of the coding regions and exon/intron boundaries of both collagen type I genes did not reveal any mutations affecting glycine codons or splice sites. In conclusion, OI type V is a new form of autosomal dominant OI, which does not appear to be associated with collagen type I mutations. The genetic defect underlying this disease remains to be elucidated.

Temporal and spatial expression of bone morphogenetic protein-2, -4, and -7 during distraction osteogenesis in rabbits

The Ilizarov method of limb lengthening makes use of the fact that osteogenesis is induced at an osteotomy site when distraction is applied. It is unknown at present how the mechanical forces created by distraction are translated into biological signals. Because bone morphogenetic proteins (BMPs) are potent inducers of osteogenesis in many experimental systems, they are obvious candidates for playing a role in this process. In this study, we investigated the temporal and spatial expression of BMP-2, -4, and -7 proteins during distraction osteogenesis using immunohistochemistry. An osteotomy was performed on the right tibiae of white New Zealand rabbits. After a delay of 7 days, distraction was started at a rate of 0.25 mm/12 h for 3 weeks, followed by a 3 week consolidation phase. Each week after osteotomy one rabbit was killed for immunohistochemical studies. Staining for BMP-2, -4, and -7 was evident before distraction was applied and was mainly localized to mesenchymal cells and osteoblastic cells in the periosteal region. After distraction was started, the typical fibrous interzone developed between the osteotomy fragments, where both intramembranous and endochondral ossification were noted. In this area, cells resembling fibroblasts and chondrocytes, but not mature osteoblasts, showed intense staining for all three BMPs. This high level of expression was maintained during the entire distraction phase and then gradually disappeared during the consolidation phase. These results are compatible with the hypothesis that BMPs play an important role in the signaling pathways that link the mechanical forces created by distraction to biological responses.

Bisphosphonate therapy for severe osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous group of disorders principally affecting type I collagen. Children with the severe forms of the condition suffer recurrent fractures resulting in limb and spine deformities, and restricted ambulation. Recently, cyclical intravenous administration of pamidronate has proven of benefit to children with the severe forms of OI. Bone mineral density increased, and the incidence of fractures decreased. The treatment does not alter fracture healing, growth rate, or growth plate appearances. Dependence on mobility aids is reduced and there is substantial relief of chronic pain and fatigue. No significant adverse side effects have been noted. New bisphosphonates are under investigation to compare their effects to those of pamidronate. Although the use of bisphosphonates does not address the basic abnormalities that underlie the OI syndromes, it represents the first therapy to significantly alter the natural course of the disease and improve patients' clinical status and quality of life.

Deficient mineralization of intramembranous bone in vitamin D-24-hydroxylase-ablated mice is due to elevated 1,25-dihydroxyvitamin D and not to the absence of 24,25-dihydroxyvitamin D

The 25-hydroxyvitamin D-24-hydroxylase enzyme (24-OHase) is responsible for the catabolic breakdown of 1,25-dihydroxyvitamin D [1,25(OH)2D], the active form of vitamin D. The 24-OHase enzyme can also act on the 25-hydroxyvitamin D substrate to generate 24,25-dihydroxyvitamin D, a metabolite whose physiological importance remains unclear. We report that mice with a targeted inactivating mutation of the 24-OHase gene had impaired 1,25(OH)2D catabolism. Surprisingly, complete absence of 24-OHase activity during development leads to impaired intramembranous bone mineralization. This phenotype was rescued by crossing the 24-OHase mutant mice to mice harboring a targeted mutation in the vitamin D receptor gene, confirming that the elevated 1,25(OH)2D levels, acting through the vitamin D receptor, were responsible for the observed accumulation of osteoid. Our results confirm the physiological importance of the 24-OHase enzyme for maintaining vitamin D homeostasis, and they reveal that 24,25-dihydroxyvitamin D is a dispensable metabolite during bone development.

Temporal and spatial expression of bone morphogenetic protein-2, -4, and -7 during distraction osteogenesis in rabbits

The Ilizarov method of limb lengthening makes use of the fact that osteogenesis is induced at an osteotomy site when distraction is applied. It is unknown at present how the mechanical forces created by distraction are translated into biological signals. Because bone morphogenetic proteins (BMPs) are potent inducers of osteogenesis in many experimental systems, they are obvious candidates for playing a role in this process. In this study, we investigated the temporal and spatial expression of BMP-2, -4, and -7 proteins during distraction osteogenesis using immunohistochemistry. An osteotomy was performed on the right tibiae of white New Zealand rabbits. After a delay of 7 days, distraction was started at a rate of 0.25 mm/12 h for 3 weeks, followed by a 3 week consolidation phase. Each week after osteotomy one rabbit was killed for immunohistochemical studies. Staining for BMP-2, -4, and -7 was evident before distraction was applied and was mainly localized to mesenchymal cells and osteoblastic cells in the periosteal region. After distraction was started, the typical fibrous interzone developed between the osteotomy fragments, where both intramembranous and endochondral ossification were noted. In this area, cells resembling fibroblasts and chondrocytes, but not mature osteoblasts, showed intense staining for all three BMPs. This high level of expression was maintained during the entire distraction phase and then gradually disappeared during the consolidation phase. These results are compatible with the hypothesis that BMPs play an important role in the signaling pathways that link the mechanical forces created by distraction to biological responses.

Static and dynamic bone histomorphometry in children with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic disorder characterized by increased bone fragility and low bone mass. Four clinical types are commonly distinguished. Schematically, type I is the mildest phenotype, type II is usually lethal, type III is the most severe form compatible with postnatal survival, and type IV is moderately severe. Although mutations affecting collagen type I are responsible for the disease in most patients, the mechanisms by which the genetic defects cause abnormal bone development have not been well characterized. Therefore, we evaluated quantitative static and dynamic histomorphometric parameters in tetracycline-labeled iliac bone biopsies from 70 children, aged 1.5 to 13.5 years, with OI types I (n = 32), III (n = 11), and IV (n = 27). Results were compared with those of 27 age-matched controls without metabolic bone disease. Biopsy core width, cortical width, and cancellous bone volume were clearly decreased in all OI types. Decreased cancellous bone volume was due to a 41%-57% reduction in trabecular number and a 15%-27% lower trabecular thickness. Regression analyses revealed that trabecular number did not vary with age in either controls or OI patients, indicating that no trabecular loss occurred. The annual increase in trabecular thickness was 5.8 microm in controls and 3.6 microm in type I OI, whereas no trabecular thickening was evident in type III and IV OI. Wall thickness, which reflects the amount of bone formed during a remodeling cycle, was decreased by 14% in a subgroup of 17 type I OI patients, but was not determined in the other OI types. The remodeling balance was less positive in type I OI than in controls, and probably close to zero in types III and IV. Surface-based parameters of bone remodeling were increased in all OI types, indicating increased recruitment of remodeling units. No defect in matrix mineralization was found. In conclusion, there was evidence of defects in all three mechanisms, which normally lead to an increase in bone mass during childhood; that is, modeling of external bone size and shape, production of secondary trabeculae by endochondral ossification, and thickening of secondary trabeculae by remodeling. Thus, OI might be regarded as a disease in which a single genetic defect in the osteoblast interferes with multiple mechanisms that normally ensure adaptation of the skeleton to the increasing mechanical needs during growth.

Perinatal metabolism of vitamin D

During pregnancy, maternal serum concentrations of 25-hydroxyvitamin D, the circulating form of vitamin D, correlate with dietary vitamin D intake. Maternal serum concentrations of 1,25-dihydroxyvitamin D, the hormonal circulating and active form of vitamin D, are elevated during pregnancy; 1,25-dihydroxyvitamin D is synthesized mainly by the decidual cells of the placenta and allows for increased calcium absorption. The fetus is entirely dependent on the mother for its supply of 25-hydroxyvitamin D, which is believed to cross the placenta. Hypocalcemia and increased parathyroid hormone secretion induce synthesis of 1,25-dihydroxyvitamin D after birth in both full-term and preterm neonates. Nevertheless, serum concentrations of 25-hydroxyvitamin D are a rate-limiting factor in the synthesis of 1,25-dihydroxyvitamin D. In vitamin D-replete infants, circulating 1,25-dihydroxyvitamin D concentrations are higher than those observed in older infants. In countries where dairy products are not routinely supplemented with vitamin D, maternal vitamin D supplementation during pregnancy is necessary. However, there is no indication for the use of pharmacologic doses of vitamin D or its metabolites in the perinatal period.

Deficient bone formation in idiopathic juvenile osteoporosis: a histomorphometric study of cancellous iliac bone

Idiopathic juvenile osteoporosis (IJO), a rare cause of osteoporosis in children, is characterized by the occurrence of vertebral and metaphyseal fractures. Little is known about the histopathogenesis of IJO. We analyzed by quantitative histomorphometry iliac crest biopsies from 9 IJO patients (age, 10.0-12.3 years; 7 girls) after tetracycline labeling. Results were compared with identically processed samples from 12 age-matched children without metabolic bone disease and 11 patients with osteogenesis imperfecta type I. Compared with healthy controls, cancellous bone volume (BV) was markedly decreased in IJO patients (mean [SD]: 10.0% [3.1%] vs. 24.4% [3.8%]), because of a 34% reduction in trabecular thickness (Tb.Th) and a 37% lower trabecular number (Tb.N; p < 0.0001 each; unpaired t-test). Bone formation rate (BFR) per bone surface was decreased to 38% of the level in controls (p = 0.0006). This was partly caused by decreased recruitment of remodeling units, as shown by a trend toward lower activation frequency (54% of the control value; p = 0.08). Importantly, osteoblast team performance also was impaired, as evidenced by a decreased wall thickness (W.Th; 70% of the control value; p < 0.0001). Reconstruction of the formative sites revealed that osteoblast team performance was abnormally low even before mineralization started at a given site. No evidence was found for increased bone resorption. Compared with children with osteogenesis imperfecta (OI), IJO patients had a similarly decreased cancellous BV but a much lower bone turnover. These results suggest a pathogenetic model for IJO, in which impaired osteoblast team performance decreases the ability of cancellous bone to adapt to the increasing mechanical needs during growth. This will finally result in load failure at sites where cancellous bone is essential for stability.

Pamidronate treatment of severe osteogenesis imperfecta in children under 3 years of age

Severe osteogenesis imperfecta (OI) is a hereditary disorder characterized by increased bone fragility and progressive bone deformity. Cyclical pamidronate infusions improve clinical outcome in children older than 3 yr of age with severe OI. Because earlier treatment may have potential to prevent deformities and improve functional prognosis in young children, we studied nine severely affected OI patients under 2 yr of age (2.3-20.7 months at entry) for a period of 12 months. Pamidronate was administered i.v. in cycles of 3 consecutive days. Patients received four to eight cycles during the treatment period, with cumulative doses averaging 12.4 mg/kg. Clinical changes were evaluated regularly during treatment, and radiological changes were assessed after 6-12 months of treatment. The control group consisted of six age-matched, severely affected OI patients, who had not received pamidronate treatment. During treatment bone mineral density (BMD) increased between 86-227%. The deviation from normal, as indicated by the z-score, diminished from -6.5 +/- 2.1 to -3.0 +/- 2.1 (P < 0.001). In the control group the BMD z-score worsened significantly. Vertebral coronal area increased in all treated patients (11.4 +/- 3.4 to 14.9 +/- 1.8 cm2; P < 0.001), but decreased in the untreated group (P < 0.05). In the treated patients, fracture rate was lower than in control patients (2.6 +/- 2.5 vs. 6.3 +/- 1.6 fractures/year; P < 0.01). No adverse side-effects were noted, apart from the well known acute phase reaction during the first infusion cycle. Pamidronate treatment in severely affected OI patients under 3 yr of age is safe, increases BMD, and decreases fracture rate.

Normative data for iliac bone histomorphometry in growing children

Many insights into normal and pathologic bone development can only be gained by bone histomorphometry. However, the use of this technique in pediatrics has so far been hampered by the lack of reference data. Therefore, we obtained transfixing iliac bone samples from 58 individuals between 1.5 and 22.9 years of age (25 male; tetracycline labeling performed in 48 subjects), who underwent surgery for reasons independent of abnormalities in bone development and metabolism. The results of histomorphometric analyses of cancellous parameters and cortical width are presented as means and standard deviations, as well as medians and ranges in five age groups. In addition, the original data are available from the authors. There were significant age-dependent increases in both cortical width and cancellous bone volume, the latter being due to an increase in trabecular thickness. Osteoid thickness did not vary significantly with age. Bone surface-based indicators of bone formation showed an age-dependent decline, reflecting similar changes in activation frequency. Mineral apposition rate decreased continuously with age. Parameters of bone resorption did not vary significantly between age groups. Paired biopsies from adjacent sites, obtained in eight subjects, were used to examine the reproducibility of histomorphometric parameters in children. The lowest coefficients of variation (<10%) were found for structural measures, as well as mineral apposition rate and wall thickness. The highest variability was found for cellular parameters. The availability of reference material will greatly facilitate the use of histomorphometry in pediatrics.

The mineralization density of iliac crest bone from children with osteogenesis imperfecta

We studied iliac crest biopsy cores taken from young individuals with osteogenesis imperfecta of several types, and from age-matched normals; the same samples had been used in prior studies using conventional light microscopic histomorphometric procedures. The PMMA blocks were micro-milled to a fine finish, carbon coated, and imaged using backscattered electrons (BSE) in an automated digital scanning electron microscope (SEM). For comparison of BSE signal levels between samples, microscope operation parameters were standardized by reference to halogenated dimethacrylate standards, and recording data from stereological arrays of 512*512 nonoverlapping pixels at 3.5 micrometer separation. All OI types showed higher average mineralization densities than age- and site-matched normals. This is interpreted as the result of the failure in matrix assembly, such that it has a higher water volume fraction available for mineral deposition. Added to the net deficit in bone quantity, the predicted higher stiffness of the more mineralized bone will account for much of the observed 'brittleness' that characterizes this class of genetic disease. The mean mineralization density, which was higher in types III, IV, and V than in type I, appears to be correlated with disease severity.

Engineering bone regeneration with bioabsorbable scaffolds with novel microarchitecture

Critical-sized defects (CSDs) were introduced into rat calvaria to test the hypothesis that absorption of surrounding blood, marrow, and fluid from the osseous wound into a bioabsorbable polymer matrix with unique microarchitecture can induce bone formation via hematoma stabilization. Scaffolds with 90% porosity, specific surface areas of approximately 10 m2/g, and median pore sizes of 16 and 32 microm, respectively, were fabricated using an emulsion freeze-drying process. Contact radiography and radiomorphometry revealed the size of the initial defects (50 mm2) were reduced to 27 +/- 11 mm2 and 34 +/- 17 mm2 for CSDs treated with poly(D,L-lactide-co-glycolide). Histology and histomorphometry revealed scaffolds filled with significantly more de novo bone than negative controls (p < 0. 007), more osteoid than both the negative and autograft controls (p < 0.002), and small masses of mineralized tissue (< 15 mm in diameter) observed within the scaffolds. Based on these findings, we propose a change in the current paradigm regarding the microarchitecture of scaffolds for in vivo bone regeneration to include mechanisms based on hematoma stabilization.

The microscopic structure of bone in normal children and patients with osteogenesis imperfecta: a survey using backscattered electron imaging

The microstructure of iliac crest biopsies from normal children or from those afflicted with osteogenesis imperfecta (OI) has not previously been studied to determine the tissue histology in the context of the degree of mineralization. The material in this study comprised 112 iliac crest biopsies from children aged 1.9-22.9 years. Fifty-eight were reference biopsies taken from children with no bone disease and the remainder were biopsies from children diagnosed as having OI (23 were Type I, 8 Type III, 18 Type IV, and 5 Type V). The specimens, which had been embedded in polymethylmethacrylate (PMMA), were micromilled and carbon coated to permit backscattered electron imaging. Reference biopsies from very young children often contained densely mineralized cartilage, and evidence of rapid cortical drift. Circumferential lamellae became a prominent feature after the toddler stage, and active remodeling and slower cortical drift continued through childhood. The biopsies from older teenagers and young adults were indistinguishable. Occasional mineralized osteocyte lacunae were detected in even the youngest children. Bone from children with OI Type I often appeared normal in microstructure and amount, but in some there was a dearth of bone and an abundance of osteocytes. Compared with age-matched controls, cortical and trabecular bone from children with OI Types III and IV were markedly sparse and very cellular, and primary osteonal systems continued to be formed later than expected. A distinguishing feature of the bone from OI Type V patients was the failure of patches of bone to mineralize, especially adjoining a reversal line. Packets of bone tissue exhibiting either considerably higher than normal or deficient mineralization would contribute to the characteristic trait of mechanical weakness.

Molecular cloning of (25-OH D)-1 alpha-hydroxylase: an approach to the understanding of vitamin D pseudo-deficiency

Pseudovitamin D-deficiency rickets (PDDR) is the first identified inborn error of vitamin D metabolism. Its clinical course is similar to that of nutritional rickets due to simple vitamin D deficiency. The treatment of choice is replacement therapy with calcitriol [1,25(OH)2D3]. PDDR is inherited as a simple autosomal recessive trait. The PDDR locus has been mapped to chromosome 12q13-q14. The molecular defect underlying the 25-hydroxyvitamin D-1 alpha-hydroxylase enzyme dysfunction has remained elusive due to the lack of sequence information for the gene encoding the cytochrome P450 moiety of the enzyme. We have used a probe derived from the rat 25-hydroxyvitamin D-24-hydroxylase sequence to identify and clone the 1 alpha-OHase cDNA. The candidate gene was transiently expressed in P19 embryonal carcinoma cells. Only those cells that were transfected with the candidate cDNA in the sense orientation were able to produce a compound that co-eluted with the 1 alpha, 25 vitamin D3 standard. Mass spectrometry analysis confirmed the identity of the produced metabolite. A human genomic clone was isolated from a chromosome 12 cosmid library and subsequently mapped to human chromosome 12q13.1-q13.3. To address the putative biological function of 24,25-dihydroxyvitamin I) 24,25(OH)2D, we also engineered a null mutation in the 24-OHase gene in embryonic stem cells (ES). Animals heterozygous for the engineered mutation are normal and fertile. One half of the homozygous animals die before weaning. Breeding of surviving females gives an F2 generation in which bone development is abnormal at sites of intramembranous ossification. Growthplate maturation and endochondral ossification appeared to proceed normally. The results show that a complete absence of vitamin D metabolites hydroxylated in position 24 during embryogenesis leads to abnormal bone structure and suggests a key role for 24,25(OH)2D in the developmental regulation of intramembranous ossification.