Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Exploration of bone metabolism status in the distal femur of mice at different growth stages

The mouse femur, particularly the distal femur, is commonly utilized in orthopedic research. Despite its significance, little is known about the key events involved in the postnatal development of the distal femur. Therefore, investigating the development process of the mouse distal femur is of great importance. In this study, distal femurs of CD-1 mice aged 1, 2, 4, 6, and 8 weeks were examined. We found that the width and height of the distal femur continued to increase till the 4th week, followed with stabilization. Notably, the width to height ratio remained relatively consistent with age. Micro computed tomography analysis demonstrated gradual increases in bone volume/tissue volume, trabecular number, and trabecular thickness from 1 to 6 weeks, alongside a gradual decrease in trabecular separation. Histological analysis further indicated the appearance of the secondary ossification center at approximately 2 weeks, with ossification mostly completed by 4 weeks, leading to the formation of a prototype epiphyseal plate. Subsequently, the epiphyseal plate gradually narrowed at 6 and 8 weeks. Moreover, the thickness and maturity of the bone cortex surrounding the epiphyseal plate increased over time, reaching peak cortical bone density at 8 weeks. In conclusion, to enhance model stability and operational ease, we recommend constructing conventional mouse models of the distal femur between 4 and 8 weeks old.

Sustained release of MAPK14-targeting siRNA from polyelectrolyte complex hydrogels mitigates MSC osteogenesis in vitro with potential application in growth plate injury

The growth plate is a cartilage structure at the end of long bones which mediates growth in children. When fractured, the formation of bony repair tissue known as a "bony bar" can occur and cause limb deformities. There are currently no effective clinical solutions for the prevention of the bony bar formation or regeneration of healthy growth plate cartilage after a fracture. This study employs previously developed alginate/chitosan polyelectrolyte complex (PEC) hydrogels as a sustained release vehicle for the delivery of short-interfering RNA (siRNA). Specifically, the siRNA targets the p38-MAPK pathway in mesenchymal stem cells (MSCs) to prevent their osteogenic differentiation. In vitro experimental findings show sustained release of siRNA from the hydrogels for 6 months. Flow cytometry and confocal imaging indicate that the hydrogels release siRNA to effectively knockdown GFP expression over a sustained period. MAPK-14 targeting siRNA was used to knockdown the expression of MAPK-14 and correspondingly decrease the expression of other osteogenic genes in MSCs in vitro over the span of 21 days. These hydrogels were used in a rat model of growth plate injury to determine whether siMAPK-14 released from the gels could inhibit bony bar formation. No significant reduction of bony bar formation was seen in vivo at the one concentration of siRNA examined. This PEC hydrogel represents a significant advancement for siRNA sustained delivery, and presents an interesting potential therapeutic delivery system for growth plate injuries and other regenerative medicine applications.

Clinical-genetic analysis of selected genes involved in the development of the human skeleton in 128 Czech patients with suspected congenital skeletal abnormalities

BACKGROUND

Congenital skeletal abnormalities are a heterogeneous group of diseases most commonly associated with small or disproportionate growth, cranial and facial dysmorphisms, delayed bone maturation, etc. Nonetheless, no detailed genotype-phenotype correlation in patients with specific genetic variants is readily available. Ergo, this study focuses on the analysis of patient phenotypes with candidate variants in genes involved in bone growth as detected by molecular genetic analysis.

METHODS

In this study we used molecular genetic methods to analyse the ACAN, COL2A1, FGFR3, IGFALS, IGF1, IGF1R, GHR, NPR2, STAT5B and SHOX genes in 128 Czech children with suspected congenital skeletal abnormalities. Pathogenic variants and variants of unclear clinical significance were identified and we compared their frequency in this study cohort to the European non-Finnish population. Furthermore, a prediction tool was utilised to determine their possible impact on the final protein. All clinical patient data was obtained during pre-test genetic counselling.

RESULTS

Pathogenic variants were identified in the FGFR3, GHR, COL2A1 and SHOX genes in a total of six patients. Furthermore, we identified 23 variants with unclear clinical significance and high allelic frequency in this cohort of patients with skeletal abnormalities. Five of them have not yet been reported in the scientific literature.

CONCLUSION

Congenital skeletal abnormalities may lead to a number of musculoskeletal, neurological, cardiovascular problems. Knowledge of specific pathogenic variants may help us in therapeutic procedures.

ANGPTL4 binds to the leptin receptor to regulate ectopic bone formation

Leptin protein was thought to be unique to leptin receptor (LepR), but the phenotypes of mice with mutation in LepR [db/db (diabetes)] and leptin [ob/ob (obese)] are not identical, and the cause remains unclear. Here, we show that db/db, but not ob/ob, mice had defect in tenotomy-induced heterotopic ossification (HO), implicating alternative ligand(s) for LepR might be involved. Ligand screening revealed that ANGPTL4 (angiopoietin-like protein 4), a stress and fasting-induced factor, was elicited from brown adipose tissue after tenotomy, bound to LepR on PRRX1+ mesenchymal cells at the HO site, thus promotes chondrogenesis and HO development. Disruption of LepR in PRRX1+ cells, or lineage ablation of LepR+ cells, or deletion of ANGPTL4 impeded chondrogenesis and HO in mice. Together, these findings identify ANGPTL4 as a ligand for LepR to regulate the formation of acquired HO.

3D-printed PCL scaffolds with anatomy-inspired bionic stratified structures for the treatment of growth plate injuries

The growth plate is a cartilaginous tissue with three distinct zones. Resident chondrocytes are highly organized in a columnar structure, which is critical for the longitudinal growth of immature long bones. Once injured, the growth plate may potentially be replaced by bony bar formation and, consequently, cause limb abnormalities in children. It is well-known that the essential step in growth plate repair is the remolding of the organized structure of chondrocytes. To achieve this, we prepared an anatomy-inspired bionic Poly(ε-caprolactone) (PCL) scaffold with a stratified structure using three-dimensional (3D) printing technology. The bionic scaffold is engineered by surface modification of NaOH and collagen Ⅰ (COL Ⅰ) to promote cell adhesion. Moreover, chondrocytes and bone marrow mesenchymal stem cells (BMSCs) are loaded in the most suitable ratio of 1:3 for growth plate reconstruction. Based on the anatomical structure of the growth plate, the bionic scaffold is designed to have three regions, which are the small-, medium-, and large-pore-size regions. These pore sizes are used to induce BMSCs to differentiate into similar structures such as the growth plate. Remarkably, the X-ray and histological results also demonstrate that the cell-loaded stratified scaffold can successfully rebuild the structure of the growth plate and reduce limb abnormalities, including limb length discrepancies and angular deformities in vivo. This study provides a potential method of preparing a bioinspired stratified scaffold for the treatment of growth plate injuries.

Disruption of Endochondral Ossification and Extracellular Matrix Maturation in an Ex Vivo Rat Femur Organotypic Slice Model Due to Growth Plate Injury

Postnatal bone fractures of the growth plate (GP) are often associated with regenerative complications such as growth impairment. In order to understand the underlying processes of trauma-associated growth impairment within postnatal bone, an ex vivo rat femur slice model was developed. To achieve this, a 2 mm horizontal cut was made through the GP of rat femur prior to the organotypic culture being cultivated for 15 days in vitro. Histological analysis showed disrupted endochondral ossification, including disordered architecture, increased chondrocyte metabolic activity, and a loss of hypertrophic zone throughout the distal femur. Furthermore, altered expression patterns of Col2α1, Acan, and ColX, and increased chondrocyte metabolic activity in the TZ and MZ at day 7 and day 15 postinjury were observed. STEM revealed the presence of stem cells, fibroblasts, and chondrocytes within the injury site at day 7. In summary, the findings of this study suggest that the ex vivo organotypic GP injury model could be a valuable tool for investigating the underlying mechanisms of GP regeneration post-trauma, as well as other tissue engineering and disease studies.

3D bioprinted hydrogel/polymer scaffold with factor delivery and mechanical support for growth plate injury repair

Introduction: Growth plate injury is a significant challenge in clinical practice, as it could severely affect the limb development of children, leading to limb deformity. Tissue engineering and 3D bioprinting technology have great potential in the repair and regeneration of injured growth plate, but there are still challenges associated with achieving successful repair outcomes. Methods: In this study, GelMA hydrogel containing PLGA microspheres loaded with chondrogenic factor PTH(1-34) was combined with BMSCs and Polycaprolactone (PCL) to develop the PTH(1-34)@PLGA/BMSCs/GelMA-PCL scaffold using bio-3D printing technology. Results: The scaffold exhibited a three-dimensional interconnected porous network structure, good mechanical properties, biocompatibility, and was suitable for cellchondrogenic differentiation. And a rabbit model of growth plate injury was appliedto validate the effect of scaffold on the repair of injured growth plate. The resultsshowed that the scaffold was more effective than injectable hydrogel in promotingcartilage regeneration and reducing bone bridge formation. Moreover, the addition ofPCL to the scaffold provided good mechanical support, significantly reducing limbdeformities after growth plate injury compared with directly injected hydrogel. Discussion: Accordingly, our study demonstrates the feasibility of using 3D printed scaffolds for treating growth plate injuries and could offer a new strategy for the development of growth plate tissue engineering therapy.

Roles of Local Soluble Factors in Maintaining the Growth Plate: An Update

The growth plate is a cartilaginous tissue found at the ends of growing long bones, which contributes to the lengthening of bones during development. This unique structure contains at least three distinctive layers, including resting, proliferative, and hypertrophic chondrocyte zones, maintained by a complex regulatory network. Due to its soft tissue nature, the growth plate is the most susceptible tissue of the growing skeleton to injury in childhood. Although most growth plate damage in fractures can heal, some damage can result in growth arrest or disorder, impairing leg length and resulting in deformity. In this review, we re-visit previously established knowledge about the regulatory network that maintains the growth plate and integrate current research displaying the most recent progress. Next, we highlight local secretary factors, such as Wnt, Indian hedgehog (Ihh), and parathyroid hormone-related peptide (PTHrP), and dissect their roles and interactions in maintaining cell function and phenotype in different zones. Lastly, we discuss future research topics that can further our understanding of this unique tissue. Given the unmet need to engineer the growth plate, we also discuss the potential of creating particular patterns of soluble factors and generating them in vitro.

The role of chondrocyte-to-osteoblast trans-differentiation in fetal bone dysplasia of mice caused by prenatal exposure to dexamethasone

Maternal exposure to dexamethasone can cause developmental toxicity of long bones in offspring. However, the effect of dexamethasone on the trans-differentiation of growth plate chondrocytes into osteoblasts and its role in bone dysplasia of fetuses caused by prenatal dexamethasone exposure (PDE) remains unclear. In this study, pregnant mice were treated with different doses, stages, and courses of dexamethasone according to clinical practice to reveal the phenomenon. Further, growth plate chondrocytes were treated with dexamethasone in vitro to clarify the phenomenon and mechanism. The results showed that PDE caused dysplasia of fetal long bones in female and male mice, accompanied by the delayed formation of the primary ossification center and the widening hypertrophic zone of growth plate cartilage. Meanwhile, PDE increased the number of hypertrophic chondrocytes at growth plate cartilage and decreased the number of osteoblasts at the primary ossification center. Moreover, PDE significantly decreased the expression of osteogenic transcription factor Runx2 but increased the expression of hypertrophic chondrocytes marker Col10. These above phenomena were more significant in the high dose, early stage, and double courses of dexamethasone exposure groups, and the male fetal mice showed more obvious than the female fetal mice. In vitro, dexamethasone significantly inhibited the trans-differentiation of growth plate chondrocytes into osteoblasts, accompanied by a decrease in Runx2 expression and an increase in Col10 expression. In conclusion, this study revealed the phenomenon and mechanism of fetal bone dysplasia caused by PDE from the new perspective of trans-differentiation disorder of growth plate chondrocytes to osteoblasts.