SummaryMatrix vesicle-mediated mineralization is an orchestrated sequence of ultrastructural and biochemical events that lead to crystal nucleation and growth. The influx of phosphate ions into the matrix vesicle is mediated by several proteins such as TNAP, ENPP1, Pit1, annexin and so forth. The catalytic activity of ENPP1 generates pyrophosphate (PPi) using extracellular ATPs as a substrate, and the resultant PPi prevents crystal overgrowth. However, TNAP hydrolyzes PPi into phosphate ion monomers, which are then transported into the matrix vesicle through Pit1. Accumulation of Ca2+ and PO43− inside matrix vesicles then induces crystalline nucleation, with calcium phosphate crystals budding off radially, puncturing the matrix vesicle’s membrane and finally growing out of it to form mineralized nodules.

Objectives: The aim of this study is a biological application of focused ion beam-scanning electron microscopy (FIB-SEM) to demonstrate serial sectional images of skeletal tissues, here presenting the ultrastructure of 1) cartilaginous extracellular fibrils and 2) osteoblastic cytoplasmic processes.Methods: Seven weeks-old female wild-type mice were fixed with half-Karnovsky solution and subsequent OsO4, and the tibiae were extracted for block staining prior to observation under transmission electron microscope (TEM) and FIB-SEM.Results: TEM showed the fine fibrillar, but somewhat amorphous ultrastructure of the intercolumnar septa in the growth plate cartilage. Alternatively, FIB-SEM revealed bundles of stout fibrils at regular intervals paralleling the septa's longitudinal axis, as well as vesicular structures embedded in the cartilaginous matrix of the proliferative zone. In the primary trabeculae, both TEM and FIB-SEM showed several osteoblastic cytoplasmic processes on the osteoid, with numbers higher than those seen in the bone matrix. FIB-SEM revealed the agglomeration of cytoplasmic processes beneath the osteoblasts, which formed a tubular continuum extending from those cells. Based on these findings, we postulated that osteoblasts not only extend their cytoplasmic processes through to the bone matrix, but also stack these cell processes on the osteoid of the primary trabeculae. Conclusion:Taken together, it is likely that FIB-SEM imaging strategy on serial sections may successfully deliver new insights on the ultrastructure of cartilage and bone tissues. words

Minodronate is highlighted for its marked and sustained effects on osteoporotic bones. To determine the duration of minodronate's effects, we have assessed the localization of the drug in mouse bones through isotope microscopy, after labeling it with a stable nitrogen isotope ([(15)N]-minodronate). In addition, minodronate-treated bones were assessed by histochemistry and transmission electron microscopy (TEM). Eight-week-old male ICR mice received [(15)N]-minodronate (1 mg/kg) intravenously and were sacrificed after 3 hr, 24 hr, 1 week, and 1 month. Isotope microscopy showed that [(15)N]-minodronate was present mainly beneath osteoblasts rather than nearby osteoclasts. At 3 hr after minodronate administration, histochemistry and TEM showed osteoclasts with well-developed ruffled borders. However, osteoclasts were roughly attached to the bone surfaces and did not feature ruffled borders at 24 hr after minodronate administration. The numbers of tartrate-resistant acid phosphatase-positive osteoclasts and alkaline phosphatase-reactive osteoblastic area were not reduced suddenly, and apoptotic osteoclasts appeared in 1 week and 1 month after the injections. Von Kossa staining demonstrated that osteoclasts treated with minodronate did not incorporate mineralized bone matrix. Taken together, minodronate accumulates in bone underneath osteoblasts rather than under bone-resorbing osteoclasts; therefore, it is likely that the minodronate-coated bone matrix is resistant to osteoclastic resorption, which results in a long-lasting and bone-preserving effect.

To assess the chronological participation of sclerostin and FGF23 in bone metabolism, this study tracked the immunolocalization of sclerostin and FGF23 in the metaphyses of murine long bones from embryonic day 18 (E18) through 1 day after birth, 1 week, 2 weeks, 4 weeks, 8 weeks, and 20 weeks of age. We have selected two regions in the metaphyseal trabeculae for assessing sclerostin and FGF23 localization: close to the chondro-osseous junction, i.e., bone modeling site even in the adult animals, and the trabecular region distant from the growth plate, where bone remodeling takes place. As a consequence, sclerostin-immunopositive osteocytes could not be observed in both close and distant trabecular regions early at the embryonic and young adult stages. However, osteocytes gradually started to express sclerostin in the distant region earlier than in the close region of the trabeculae. Immunoreactivity for FGF23 was observed mainly in osteoblasts in the early stages, but detectable in osteocytes in the later stages of growth in trabecular and cortical bones. Fgf23 was weakly expressed in the embryonic and neonatal stages, while the receptors, Fgfr1c and αKlotho were strongly expressed in femora. At the adult stages, Fgf23 expression became more intense while Fgfr1c and aKlotho were weakly expressed. These findings suggest that sclerostin is secreted by osteocytes in mature bone undergoing remodeling while FGF23 is synthesized by osteoblasts and osteocytes depending on the developmental/growth stage. In addition, it appears that FGF23 acts in an autocrine and paracrine fashion in fetal and neonatal bones.By establishing a communication network between osteocytes and osteoblasts, osteocytes are situated at the center of bone turnover and help determine how bones adjust to mechanical stress through bone remodeling. Osteocytes exist inside osteocytic lacunae and connect to neighboring osteocytes and osteoblasts on the bone surfaces via fine cytoplasmic processes that run through osteocytic canaliculi. Osteocytes interconnect their cytoplasmic processes via gap junctions (2,6,7,33), thereby building functional syncytia referred to as osteocytic lacunar-canalicular system (OLCS) (5,15,16,31). In the last few decades, many studies have reported on important osteocyte-derived factors, such as sclerostin (which regulates osteoblastic activities) and fibroblast growth factor 23 (FGF23, which affects the proximal renal tubules to reduce serum

No abstract