A temporary decrease in mineral density in perinatal mouse long bones

Normal trabecular vertebral bone is formed via rapid transformation of mineralized spicules: A high-resolution 3D ex-vivo murine study

At birth, mouse vertebrae have a reticular fine spongy morphology, yet in the adult animal they exhibit elaborate trabecular architectures. Here, we characterize the physiological microstructural transformations in growing young female mice of the widely used C57BL/6 strain. Extensive architectural changes lead to the establishment of mature cancellous bone in the spine. Vertebrae were mapped in 3D by high resolution microcomputed tomography (µCT), backed by conventional histology. Three different phases are observed in the natural bony biomaterial: In a prenatal templating phase, early vertebrae are composed of foamy, loosely-packed mineralized spicules. During a consolidation phase in the first 7 days after birth, bone material condenses into struts and forms primitive trabeculae accompanied by a significant (>50%) reduction in bone volume/tissue volume ratio (BV/TV). After day 7, the trabeculae expand, reorient and increase in mineral density. Swift growth ensues such that by day 14 the young lumbar spine exhibits all morphological features observed in the mature animal. The greatly varied micro-morphologies of normal trabecular bone observed in 3D within a short timespan are typical for rodent and presumably for other mammalian forming spines. This suggests that fully structured cancellous bone emerges through rapid post-natal restructuring of a foamy mineralized scaffold. STATEMENT OF SIGNIFICANCE: Cancellous bone develops in stages that are not well documented. Using a mouse model, we provide an observer-independent quantification of normal bone formation in the spine. We find that within 14 days, the cancellous bone transforms in 3 phases from a scaffold of spicules into well organized, fully mineralized trabeculae in a functional spine. Detailed knowledge of the physiological restructuring of mineralized material may help to better understand bone formation and may serve as a blueprint for studies of pharmaceuticals effects, tissue healing and regeneration.

In ovo feeding with minerals and vitamin D3 improves bone properties in hatchlings and mature broilers

The objective of this study was to examine the effect of in ovo feeding (IOF) with inorganic minerals or organic minerals and vitamin D3 on bone properties and mineral consumption. Eggs were incubated and divided into 4 groups: IOF with organic minerals, phosphate, and vitamin D3 (IOF-OMD); IOF with inorganic minerals and phosphate (IOF-IM); sham; and non-treated controls (NTC). IOF was performed on embryonic day (E) 17; tibiae and yolk samples were taken on E19 and E21. Post-hatch, only chicks from the IOF-OMD, sham, and NTC were raised, and tibiae were taken on d 10 and 38. Yolk mineral content was examined by inductively coupled plasma spectroscopy. Tibiae were tested for their whole-bone mechanical properties, and mid-diaphysis bone sections were indented in a micro-indenter to determine bone material stiffness (Young's modulus). Micro-computed tomography (μCT) was used to examine cortical and trabecular bone structure. Ash content analysis was used to examine bone mineralization. A latency-to-lie (LTL) test was used to measure standing ability of the d 38 broilers. The results showed that embryos from both IOF-OMD and IOF-IM treatments had elevated Cu, Mn, and Zn amounts in the yolk on E19 and E21 and consumed more of these minerals (between E19 and E21) in comparison to the sham and NTC. On E21, these hatchlings had higher whole-bone stiffness in comparison to the NTC. On d 38, the IOF-OMD had higher ash content, elevated whole-bone stiffness, and elevated Young's modulus (in males) in comparison to the sham and NTC; however, no differences in standing ability were found. Very few structural differences were seen during the whole experiment. This study demonstrates that mineral supplementation by in ovo feeding is sufficient to induce higher mineral consumption from the yolk, regardless of its chemical form or the presence of vitamin D3. Additionally, IOF with organic minerals and vitamin D3 can increase bone ash content, as well as stiffness of the whole bone and bone material in the mature broiler, but does not lead to longer LTL.

Long bone maturation is driven by pore closing: A quantitative tomography investigation of structural formation in young C57BL/6 mice

During mammalian growth, long bones undergo extensive structural reorganization, transforming from primitive shapes in the limb buds into mature bones. Here we shed light on the steps involved in structural formation of the mineralized tissue in midshafts of C57BL/6 femurs, shortly after birth. By combining 3D micrometer-resolution X-ray microtomography with 2D histology, we study the transformation of the tissue from a partially-mineralized scaffold into a compact bone structure. We identify three growth phases that take place during murine long bone maturation: During a patterning phase (I) mineralized struts form a loosely connected foam-like cortical network. During a transitioning phase (II), the extensive non-mineralized tracts vanish, transforming the foam into a fully continuous mass, by 14 days of age. Concomitantly, closed porosity increases to about ∼ 1.4%, and stays at this level, also found in maturity. During a shaping phase (III), the bones gradually attain their characteristic intricate adult form. Architectured mineral depositioning--first in open foamy scaffolds, and later into solid bone material--is presumably a compromise between the mechanical needs of providing support to the body, and the biological requirements of vascularization and extensive nutritional needs in the early stages of bone formation.