Effect of vitamin K2 and growth hormone on the long bones in hypophysectomized young rats: a bone histomorphometry study

Trabecular bone organoids: a micron-scale 'humanised' prototype designed to study the effects of microgravity and degeneration

Bone is a highly responsive organ, which continuously adapts to the environment it is subjected to in order to withstand metabolic demands. These events are difficult to study in this particular tissue in vivo, due to its rigid, mineralised structure and inaccessibility of the cellular component located within. This manuscript presents the development of a micron-scale bone organoid prototype, a concept that can allow the study of bone processes at the cell-tissue interface. The model is constructed with a combination of primary female osteoblastic and osteoclastic cells, seeded onto femoral head micro-trabeculae, where they recapitulate relevant phenotypes and functions. Subsequently, constructs are inserted into a simulated microgravity bioreactor (NASA-Synthecon) to model a pathological state of reduced mechanical stimulation. In these constructs, we detected osteoclastic bone resorption sites, which were different in morphology in the simulated microgravity group compared to static controls. Once encapsulated in human fibrin and exposed to analogue microgravity for 5 days, masses of bone can be observed being lost from the initial structure, allowing to simulate the bone loss process further. Constructs can function as multicellular, organotypic units. Large osteocytic projections and tubular structures develop from the initial construct into the matrix at the millimetre scale. Micron-level fragments from the initial bone structure are detected travelling along these tubules and carried to sites distant from the native structure, where new matrix formation is initiated. We believe this model allows the study of fine-level physiological processes, which can shed light into pathological bone loss and imbalances in bone remodelling.

Conditional Deletion of Prolyl Hydroxylase Domain-Containing Protein 2 (Phd2) Gene Reveals Its Essential Role in Chondrocyte Function and Endochondral Bone Formation

The hypoxic growth plate cartilage requires hypoxia-inducible factor (HIF)-mediated pathways to maintain chondrocyte survival and differentiation. HIF proteins are tightly regulated by prolyl hydroxylase domain-containing protein 2 (Phd2)-mediated proteosomal degradation. We conditionally disrupted the Phd2 gene in chondrocytes by crossing Phd2 floxed mice with type 2 collagen-α1-Cre transgenic mice and found massive increases (>50%) in the trabecular bone mass of long bones and lumbar vertebra of the Phd2 conditional knockout (cKO) mice caused by significant increases in trabecular number and thickness and reductions in trabecular separation. Cortical thickness and tissue mineral density at the femoral middiaphysis of the cKO mice were also significantly increased. Dynamic histomorphometric analyses revealed increased longitudinal length and osteoid surface per bone surface in the primary spongiosa of the cKO mice, suggesting elevated conversion rate from hypertrophic chondrocytes to mineralized bone matrix as well as increased bone formation in the primary spongiosa. In the secondary spongiosa, bone formation measured by mineralizing surface per bone surface and mineral apposition rate were not changed, but resorption was slightly reduced. Increases in the mRNA levels of SRY (sex determining region Y)-box 9, osterix (Osx), type 2 collagen, aggrecan, alkaline phosphatase, bone sialoprotein, vascular endothelial growth factor, erythropoietin, and glycolytic enzymes in the growth plate of cKO mice were detected by quantitative RT-PCR. Immunohistochemistry revealed an increased HIF-1α protein level in the hypertrophic chondrocytes of cKO mice. Infection of chondrocytes isolated from Phd2 floxed mice with adenoviral Cre resulted in similar gene expression patterns as observed in the cKO growth plate chondrocytes. Our findings indicate that Phd2 suppresses endochondral bone formation, in part, via HIF-dependent mechanisms in mice.

Effects of growth hormone on bone modeling and remodeling in hypophysectomized young female rats: a bone histomorphometric study

Growth hormone (GH) deficiency causes decreased bone mineral density and osteoporosis, predisposing to fractures. We investigated the mechanism of action of GH on bone modeling and remodeling in hypophysectomized (HX) female rats. Thirty female Sprague-Dawley rats at age 2 months were divided into three groups with 10 rats each: control (CON) group, HX group, and HX + GH (3 mg/kg daily s.c.) group, for a 4-week study. Hypophysectomy resulted in cessation of bone growth and decrease in cancellous bone mass. Periosteal bone formation decreased and bone turnover rate of endocortical and trabecular surfaces increased as compared to the CON group. GH administration for 4 weeks restored weight gain and bone growth and mitigated decrease in bone density after hypophysectomy. However, trabecular bone mass in the proximal tibial metaphysis remained lower in group HX + GH than in group CON. Dynamic histomorphometric analysis showed that bone modeling of periosteal bone formation and growth plate elongation was significantly higher in group HX + GH than in group HX. New bone formed beneath the growth plate was predominately woven bone in group CON and group HX + GH. Bone remodeling and modeling-remodeling mixed modes in the endocortical and PTM sites were enhanced by GH administration; both bone formation and resorption activities were significantly higher than in group HX. In conclusion, GH administration to HX rats reactivated modeling activities in modeling predominant sites and increased new bone formation. GH administration also increases remodeling activities in remodeling predominant sites, giving limited net gain in the bone mass.