Porotic paradox: distribution of cortical bone pore sizes at nano- and micro-levels in healthy vs. fragile human bone

Plant Tissue Parenchyma and Vascular Bundles Selectively Regulate Stem Cell Mechanosensing and Differentiation

Introduction

Plant tissues are plentiful, diverse, and due to convergent evolution are structurally similar to many animal tissues. Decellularized plant tissues feature microtopographies that resemble cancellous bone (porous parenchyma) and skeletal muscle (fibrous vascular bundles). However, the use of plant tissues as an inexpensive and abundant biomaterial for controlling stem cell behavior has not been widely explored.

Methods

Celery plant tissues were cut cross-sectionally (porous parenchyma) or longitudinally (fibrous vascular bundles) and decellularized. Human mesenchymal stem cells (MSCs) were then cultured atop plant tissues and confocal imaging of single cells was used to evaluate the early effects of microtopography on MSC adhesion, morphology, cytoskeletal alignment, Yes-associated protein (YAP) signaling, and downstream lineage commitment to osteogenic or myogenic phenotypes.

Results

Microtopography was conserved post plant tissue decellularization and MSCs attached and proliferated on plant tissues. MSCs cultured on porous parenchyma spread isotropically along the periphery of plant tissue pores. In contrast, MSCs cultured on vascular bundles spread anisotropically and aligned in the direction of fibrous vascular bundles. Differences in microtopography also influenced MSC nuclear YAP localization and actin anisotropy, with higher values observed on fibrous tissues. When exposed to osteogenic or myogenic culture medium, MSCs on porous parenchyma had a higher percentage of cells stain positive for bone biomarker alkaline phosphatase, whereas myoblast determination protein 1 (MyoD) was significantly upregulated for MSCs on fibrous vascular bundles.

Conclusions

Together, these results show that plant tissues are an abundant biomaterial with defined microarchitecture that can reproducibly regulate MSC morphology, mechanosensing, and differentiation.

Supplementary Information

The online version of this article contains supplementary material available 10.1007/s12195-022-00737-9.

Criteria, Challenges, and Opportunities for Acellularized Allogeneic/Xenogeneic Bone Grafts in Bone Repairing

As bone grafts become more commonly needed by patients and as donors become scarcer, acellularized bone grafts (ABGs) are becoming more popular for restorative purposes. While autogeneic grafts are reliable as a gold standard, allogeneic and xenogeneic ABGs have been shown to be of particular interest due to the limited availability of autogeneic resources and reduced patient well-being in long-term surgeries. Because of the complete similarity of their structures with native bone, excellent mechanical properties, high biocompatibility, and similarities of biological behaviors (osteoinductive and osteoconductive) with local bones, successful outcomes of allogeneic and xenogeneic ABGs in both in vitro and in vivo research have raised hopes of repairing patients' bone injuries in clinical applications. However, clinical trials have been delayed due to a lack of standardized protocols pertaining to acellularization, cell seeding, maintenance, and diversity of ABG evaluation criteria. This study sought to uncover these factors by exploring the bone structures, ossification properties of ABGs, sources, benefits, and challenges of acellularization approaches (physical, chemical, and enzymatic), cell loading, and type of cells used and effects of each of the above items on the regenerative technologies. To gain a perspective on the repair and commercialization of products before implementing new research activities, this study describes the differences between ABGs created by various techniques and methods applied to them. With a comprehensive understanding of ABG behavior, future research focused on treating bone defects could provide a better way to combine the treatment approaches needed to treat bone defects.

A simplified homogenization model applied to viscoelastic behavior of cortical bone at ultrasonic frequencies

Cortical bone is a complex multiscale medium and its study is of importance for clinical fracture prevention. In particular, cortical attenuation is known to be linked with shock energy absorption and ability to resist fracture. However, the links between cortical bone absorption and its multiscale structure are still not well understood. This work is about the use of homogenized tensors in order to characterize the viscoelastic behavior of cortical bone at ultrasonic frequencies, i.e., about 0.1 to 10 MHz. Such tensors are derived from the cell problem via two-scale homogenization theory for linear elastic and Kelvin-Voigt viscoelastic descriptions. The elliptic formulations obtained from the cell problems are implemented within the range of medically-observed porosities. Microstructure is assessed considering cubic cells with cylindrical inclusion and transverse isotropic assumption. A simplified model, adding one temporal parameter τ per phase, allows a good agreement with experimental data. The corresponding attenuation is proportional to the square of the frequency, in agreement with Kramer-Kronig relations. This development is proposed in the context of robust clinical inverse problem approaches using a restricted number of parameter. Two main properties for the material filling the pores are adjusted and discussed: absorption and shear contribution. Best agreement with experimental data is observed for material inside the pores being solid and highly attenuating.

Decoupling the effects of nanopore size and surface roughness on the attachment, spreading and differentiation of bone marrow-derived stem cells

The nanopore size and roughness of nanoporous surface are two critical variables in determining stem cell fate, but little is known about the contribution from each cue individually. To address this gap, we use two-dimensional nanoporous membranes with controlled nanopore size and roughness to culture bone marrow-derived mesenchymal stem cells (BMSCs), and study their behaviors such as attachment, spreading and differentiation. We find that increasing the roughness of nanoporous surface has no noticeable effect on cell attachment, and only slightly decreases cell spreading areas and inhibits osteogenic differentiation. However, BMSCs cultured on membranes with larger nanopores have significantly fewer attached cells and larger spreading areas. Moreover, these cells cultured on larger nanopores undergo enhanced osteogenic differentiation by expressing more alkaline phosphatase, osteocalcin, osteopontin, and secreting more collagen type I. These results suggest that although both nanopore size and roughness can affect BMSCs, nanopore size plays a more significant role than roughness in controlling BMSC behavior.

Compromised bone strength in patients with alcoholic liver cirrhosis

Osteoporosis and/or osteopenia are frequently observed complications in alcoholic liver disease patients, especially in its irreversible stage (alcoholic liver cirrhosis - ALC). Current literature review regarding osteoporosis incidence increased fracture risk, bone mineral density changes, micro-structural alteration and pathogenetic mechanisms leading to ALC - induced decrease in bone quality is presented. It has been reported that osteoporosis is influenceing significant number of ALC patients, with particular accent on lumbar spine, as most commonly affected skeletal site. Several risk factors and ethio-pathogenic mechanisms have been associated with the loss of bone mineral density, in patients with ALC and other cirrhosis. Certain authors claim that dominant mechanism in ALC-induced bone loss is decrease in bone formation and osteoblastic function, while other results suggest increased bone remodeling as underlying cause of increased bone fragility in ALC patients. Increased bone fragility and susceptibility to osteoporotic fractures significantly affect life expectancy and quality of life in patients with ALC. Thus, osteological screening and individual fracture risk assessment are highly recommended for all patients with alcoholic liver cirrhosis.

Constricted migration modulates stem cell differentiation

Tissue regeneration at an injured site depends on proliferation, migration, and differentiation of resident stem or progenitor cells, but solid tissues are often sufficiently dense and constricting that nuclei are highly stressed by migration. In this study, constricted migration of myoblastic cell types and mesenchymal stem cells (MSCs) increases nuclear rupture, increases DNA damage, and modulates differentiation. Fewer myoblasts fuse into regenerating muscle in vivo after constricted migration in vitro, and myodifferentiation in vitro is likewise suppressed. Myosin II inhibition rescues rupture and DNA damage, implicating nuclear forces, while mitosis and the cell cycle are suppressed by constricted migration, consistent with a checkpoint. Although perturbed proliferation fails to explain defective differentiation, nuclear rupture mislocalizes differentiation-relevant MyoD and KU80 (a DNA repair factor), with nuclear entry of the DNA-binding factor cGAS. Human MSCs exhibit similar damage, but osteogenesis increases-which is relevant to bone and to calcified fibrotic tissues, including diseased muscle. Tissue repair can thus be modulated up or down by the curvature of pores through which stem cells squeeze.

Two step porosification of biomimetic thin-film hydroxyapatite/alpha-tri calcium phosphate coatings by pulsed electron beam irradiation

Here we show a new and effective methodology for rapid/controllable porosification of thin-film ceramics, which may be applied in medical devices/electronics and membrane nano-filtration. Dense hydroxyapatite applied to Ti6Al4V by plasma-assisted PVD was electron-beam irradiated to induce flash melting/boiling. Deposited coatings contained amorphous and nano-crystalline/stoichiometric hydroxyapatite (~35 nm). Irradiation (voltages 13-29 kV) led to ablation (up to 45% mass loss) and average/maximum pore areas from (0.07-1.66)/(0.69-92.53) μm2, mimicking the human cortical bone. Vitrification above 1150 °C formed (~62-30 nm) crystallites of α-Tri Calcium Phosphate. Unique porosification resulted from irradiation-induced sub-surface boiling and limited thermal conductivity of hydroxyapatite, causing material to expand/explode through the more quickly solidified top surface. Commercially applicable, roughened Ti6Al4V exacerbated the heating and boiling explosion phenomenon in certain regions, producing an array of pore sizes. Scaffold-like morphologies were generated by interconnection of micron/sub-micron porosity, showing great potential for facile generation of a biomimetic surface treatment for osseointegration.