Influence of muscular activity on local mineralization patterns in metatarsals of the embryonic mouse.

This study addressed the theory that local mechanical loading may influence the development of embryonic long bones. Embryonic mouse metatarsal rudiments were cultured as whole organs, and the geometry of the primary ossification center was compared with that of rudiments that had developed in utero. The mineralization front in vivo was found to be nearly straight, whereas in vitro it acquired a more convex shape due to a slower mineralization rate at the periphery of the mineralized cylinder. A poroelastic finite element analysis was performed to calculate the local distributions of distortional strain and fluid pressure at the mineralization front in the metatarsal during loading in vivo as a result of muscle contractions in the embryonic hindlimbs. The distribution of fluid pressure from the finite element analysis could not explain the difference in mineralization shape. The most likely candidate for the difference was the distortional strain, resulting from muscle contraction, which is absent in vitro, because its value at the periphery was significantly higher than in the center of the tissue. Without external loads, the mineralization process may be considered as pre-programmed, starting at the center of the tissue and resulting in a spherical mineralization front. Strain modulates the rate of the mineralization process in vivo, resulting in the straight mineralization front. These results confirm that disturbances in muscle development are likely to produce disturbed mineralization patterns, resulting in a disordered osteogenic process.

Correlation between ALK-6 (BMPR-IB) distribution and responsiveness to osteogenic protein-1 (BMP-7) in embryonic mouse bone rudiments.

Osteogenic protein-1 (OP-1) or bone morphogenetic protein-7 (BMP-7) stimulates cartilage formation in mouse bone rudiments in vitro but arrests terminal differentiation of prehypertrophic chondrocytes into hypertrophic chondrocytes. In this study we report that these effects of OP-1 depend on the developmental stage of the bone rudiment, early stages (E14 and E15 metatarsals) being most responsive. E17 metatarsals that already contained a hypertrophic area that had initiated mineralization were no longer affected by OP-1. We then investigated whether the sensitivity of the early long bone rudiments to OP-1 correlated with high expression of the OP-1 binding type I serine/threonine kinase receptors (activin receptor-like kinase: ALK-2/ActR-I, ALK-3/BMPR-IA or ALK-6/BMPR-IB) at this early stage. We did not find any significant difference in overall mRNA levels of these ALKs between stages E14 through E17 as assessed by RNase protection assays. However, by immunohistochemistry we found that ALK-6 staining was strong in E14 early cartilage primordium and its future perichondrium but dropped sharply to low levels in these cell types until onset of chondrocyte (pre)hypertrophy at E16. By contrast, ALK-2 and ALK-3 immunostainings in E14 were barely detectable. We also examined by immunohistochemistry the local synthesis of OP-1. OP-1 was present in E14 early chondrocytes and forming perichondrium but in low amounts; however, production of OP-1 increased in these cell types with age. All three receptor types as well as OP-1 were present in significant amounts in prehypertrophic chondrocytes and late hypertrophic chondrocytes including those undergoing mineralization. The temporary high immunostaining for ALK-6 in the early proliferating chondrocytes and future perichondrium of E14 bone rudiments, and its absence in older bones correlated with the sensitivity of chondrocytes and perichondrium to (exogenous) OP-1. We therefore propose that the effects of OP-1 on these cells in vitro are mediated by ALK-6/BMPR-IB. We furthermore conclude that locally produced OP-1 is a potential autocrine/paracrine growth factor. Increased local production of OP-1 may be partially responsible for the age-related decrease in responsiveness to exogenous OP-1 with respect to hypertrophy and mineralization of cartilage.

Inhibition of terminal chondrocyte differentiation by bone morphogenetic protein 7 (OP-1) in vitro depends on the periarticular region but is independent of parathyroid hormone-related peptide.

Bone morphogenetic protein-7, or BMP-7 (OP-1), is highly expressed in the perichondrium of embryonic long bones and is thought to play a role in endochondral ossification. Previously we have shown that BMP-7 inhibits terminal chondrocyte differentiation; that is, chondrocyte hypertrophy and mineralization in cultured explants of embryonic mouse metatarsals. However, the mechanism of this inhibition and the target cells of BMP-7 are still unknown. In this study we show that BMP-7 inhibits terminal chondrocyte differentiation indirectly, via an interaction with the periarticular region of the explants. This region also expresses parathyroid hormone-related peptide (PTHrP). PTHrP regulates terminal chondrocyte differentiation by inhibiting hypertrophic differentiation of prehypertrophic chondrocytes. The differentiating center in turn regulates PTHrP expression via a feedback loop involving Indian hedgehog (Ihh), which is expressed in the prehypertrophic chondrocytes. Ihh is thought to act on perichondrial cells, which in turn start to express an as yet unknown mediator that stimulates PTHrP expression in the periarticular region. It has been suggested that this factor belongs to the BMP-family. We investigated whether the inhibition of terminal chondrocyte differentiation by BMP-7 was due to upregulation of the PTHrP-Ihh feedback loop and whether BMP-7 was the unknown factor in the loop. Here we show that exogenous BMP-7 did not upregulate the mRNA expression of PTHrP, Ihh, or the PTH/PTHrP receptor in cultured wild-type embryonic metatarsals. Furthermore, BMP-7 could still inhibit terminal chondrocyte differentiation in the metatarsals of PTHrP-deficient (PTHrP-/-) mouse embryos. These data indicate that the BMP-7-mediated inhibition of terminal chondrocyte differentiation in vitro is independent of the PTHrP-Ihh feedback loop. We concluded that BMP-7 modulates terminal chondrocyte differentiation and cartilage mineralization of fetal bone explants in vitro via as yet unknown inhibitory factor(s) produced in the periarticular region.

OP-1 (BMP-7) affects mRNA expression of type I, II, X collagen, and matrix Gla protein in ossifying long bones in vitro.

In long bone development, a regulating role of OP-1 is suggested by the local correlated expression of both OP-1 ligand and OP-1 binding receptors in developing mouse hind limbs. OP-1 is expressed in the interdigital mesenchyme, whereas OP-1 binding receptors are found in the bordering perichondrium, and both OP-1 ligand and receptors are present in the zone of (pre)hypertrophic chondrocytes. We investigated the role of OP-1 in long bone development experimentally by treating organ cultures of embryonic mouse metatarsals with rhOP-1. The mRNA expression patterns of type I, II, X collagen, and matrix Gla protein (MGP) were studied using in situ hybridization and cell proliferation using [3H]thymidine and BrdU labeling. In the epiphyseal perichondrium, treatment with 40 ng/ml OP-1 enhanced cell proliferation after day 2, while 6-day treatment caused a shift in expression from type I collagen to type II collagen mRNA. This supports previous histochemical findings that OP-1 induced the transition of perichondrium into cartilage. In the center of the rudiment, OP-1 inhibited the expression of type X collagen mRNA, indicating inhibition of chondrocyte hypertrophy. An arrest of differentiation at the (pre)hypertrophic chondrocyte stage was also indicated by the large area of cells expressing MGP mRNA in the OP-1-treated rudiments. We conclude that OP-1 affected the expression of marker genes of chondrocyte differentiation by acting on two steps in endochondral ossification. First, cell proliferation was enhanced, particularly so in the perichondrium where cells started to express the chondrocyte phenotype. Second, the terminal differentiation of mature chondrocytes into hypertrophic chondrocytes was inhibited. These results, combined with the known pattern of OP-1 ligand and BMP receptor expression in the embryo, suggest that OP-1 plays a local role in the cascade of events during endochondral ossification.