Research on solute transport behaviors in the lacunar-canalicular system using numerical simulation in microgravity

Numerical simulation on mass transfer in the bone lacunar-canalicular system under different gravity fields

The bone lacunar-canalicular system (LCS) is a unique complex 3D microscopic tubular network structure within the osteon that contains interstitial fluid flow to ensure the efficient transport of signaling molecules, nutrients, and wastes to guarantee the normal physiological activities of bone tissue. The mass transfer laws in the LCS under microgravity and hypergravity are still unclear. In this paper, a multi-scale 3D osteon model was established to mimic the cortical osteon, and a finite element method was used to numerically analyze the mass transfer in the LCS under hypergravity, normal gravity and microgravity and combined with high-intensity exercise conditions. It was shown that hypergravity promoted mass transfer in the LCS to the deep lacunae, and the number of particles in lacunae increased more significantly from normal gravity to hypergravity the further away from the Haversian canal. The microgravity environment inhibited particles transport in the LCS to deep lacunae. Under normal gravity and microgravity, the number of particles in lacunae increased greatly when doing high-intensity exercise compared to stationary standing. This paper presents the first simulation of mass transfer within the LCS with different gravity fields combined with high-intensity exercise using the finite element method. The research suggested that hypergravity can greatly promote mass transfer in the LCS to deep lacunae, and microgravity strongly inhibited this mass transfer; high-intensity exercise increased the mass transfer rate in the LCS. This study provided a new strategy to combat and treat microgravity-induced osteoporosis.

Multi-scale numerical simulation on mechano-transduction of osteocytes in different gravity fields

A three-dimensional model for osteocyte was established to research the mechanisms of mechano-transduction and amplification of primary cilium and osteocyte process in every gravity field. The results showed that significant stress concentration was observed in the area of physical connection between TES and the osteocyte process, where the fluid shear stress (FSS) was around two orders of magnitude higher than that in other areas. Due to the significant amplification effect of the TES structure on mechanical stimulation, making osteocyte process the "optimal mechanical receptor". In microgravity, the mechanical signal conduction ability of the osteocyte decreased significantly.. HighlightsAt the micro-nano scale, a 3D model for single bone lacunae-osteocyte system is established.The stress amplification mechanism of the transverse element is verified.Compared with the primary cilium, osteocyte process is the 'optimal mechanical receptor'.In microgravity, the mechanical signal conduction ability of osteocyte system decreased.

Pathophysiological mechanism of acute bone loss after fracture

BACKGROUND

Acute bone loss after fracture is associated with various effects on the complete recovery process and a risk of secondary fractures among patients. Studies have reported similarities in pathophysiological mechanisms involved in acute bone loss after fractures and osteoporosis. However, given the silence nature of bone loss and bone metabolism complexities, the actual underlying pathophysiological mechanisms have yet to be fully elucidated.

AIM OF REVIEW

To elaborate the latest findings in basic research with a focus on acute bone loss after fracture. To briefly highlight potential therapeutic targets and current representative drugs. To arouse researchers' attention and discussion on acute bone loss after fracture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Bone loss after fracture is associated with immobilization, mechanical unloading, blood supply damage, sympathetic nerve regulation, and crosstalk between musculoskeletals among other factors. Current treatment strategies rely on regulation of osteoblasts and osteoclasts, therefore, there is a need to elucidate on the underlying mechanisms of acute bone loss after fractures to inform the development of efficacious and safe drugs. In addition, attention should be paid towards ensuring long-term skeletal health.

Mechano-immunology in microgravity

Life on Earth has evolved to thrive in the Earth's natural gravitational field; however, as space technology advances, we must revisit and investigate the effects of unnatural conditions on human health, such as gravitational change. Studies have shown that microgravity has a negative impact on various systemic parts of humans, with the effects being more severe in the human immune system. Increasing costs, limited experimental time, and sample handling issues hampered our understanding of this field. To address the existing knowledge gap and provide confidence in modelling the phenomena, in this review, we highlight experimental works in mechano-immunology under microgravity and different computational modelling approaches that can be used to address the existing problems.

Changes in interstitial fluid flow, mass transport and the bone cell response in microgravity and normogravity

In recent years, our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research, with hundreds of astronauts spending months of their time in space. A recent shift toward pursuing territories farther afield, aiming at near-Earth asteroids, the Moon, and Mars combined with the anticipated availability of commercial flights to space in the near future, warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment. Acute skeletal loss, more severe than any bone loss seen on Earth, has significant implications for deep space exploration, and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity. The removal of gravity eliminates a critical primary mechano-stimulus, and when combined with exposure to both galactic and solar cosmic radiation, healthy human tissue function can be negatively affected. An additional effect found in microgravity, and one with limited insight, involves changes in dynamic fluid flow. Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells. Furthermore, the cell cytoplasm is not a simple liquid, and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function. In microgravity, flow behavior changes drastically, and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood. This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions: normogravity and microgravity.

Study on mass transfer in the bone lacunar-canalicular system under different gravity fields

INTRODUCTION

The bone lacunar-canalicular system (LCS) is an important microstructural basis for signaling and material transport in bone tissue, guaranteeing normal physiological processes in tissues. Spaceflight astronauts and elderly osteoporosis are related to its function, so it is necessary to reveal the mass transfer laws in bone microstructure under different gravity fields to provide insight for effective clinical treatment.

MATERIALS AND METHODS

Using the natural LCS structure of bovine tibial cortical bone as the object, the mass transfer experiments on cortical bone were conducted by using sodium fluorescein tracer through different frequency pulsating pressure provided by dynamic perfusion loading device and different high G environments provided by high-speed centrifuge to analyze the mass transfer laws under different gravity fields and different pulsating pressures.

RESULTS

The fluorescence intensity of lacunae within the osteon was lower the farther away from the Haversian canal. As the gravity field magnitude increased, the fluorescence intensity within each lacuna enhanced, and the more distant the lacunae from the Haversian canal, the greater the fluorescence intensity enhancement. High-frequency pulsating pressure simulated high-intensity exercise in humans can improve mass transfer efficiency in the LCS.

CONCLUSION

High-intensity exercise may greatly increase solute molecules, nutrients, and signaling molecules in osteocytes and improve the activity of osteocytes. Hypergravity can enhance the transport of solute molecules, nutrients, and signaling molecules in the LCS, especially promoting mass transfer to deep layer lacunae. Conversely, mass transfer to deep layer lacunae may be inhibited under microgravity, causing bone loss and ultimately leading to osteoporosis.

Finite Element Models of Osteocytes and Their Load-Induced Activation

PURPOSE OF REVIEW

Osteocytes are the conductors of bone adaptation and remodelling. Buried inside the calcified matrix, they sense mechanical cues and signal osteoclasts in case of low activity, and osteoblasts when stresses are high. How do osteocytes detect mechanical stress? What physical signal do they perceive? Finite element analysis is a useful tool to address these questions as it allows calculating stresses, strains and fluid flow where they cannot be measured. The purpose of this review is to evaluate the capabilities and challenges of finite element models of bone, in particular the osteocytes and load-induced activation mechanisms.

RECENT FINDINGS

High-resolution imaging and increased computational power allow ever more detailed modelling of osteocytes, either in isolation or embedded within the mineralised matrix. Over the years, homogeneous models of bone and osteocytes got replaced by heterogeneous and microstructural models, including, e.g. the lacuno-canalicular network and the cytoskeleton. The lacuno-canalicular network induces strain amplifications and the osteocyte protrusions seem to be stimulated much more than the cell body, both by strain and fluid flow. More realistic cell geometries, like minute constrictions of the canaliculi, increase this effect. Microstructural osteocyte models describe the transduction of external stimuli to the nucleus. Supracellular multiscale models (e.g. of a tunnelling osteon) allow to study differential loading of osteocytes and to distinguish between strain and fluid flow as the pivotal stimulatory cue. In the future, the finite element models may be enhanced by including chemical transport and intercellular communication between osteocytes, osteoclasts and osteoblasts.

MULTI-SCALE MECHANICAL BEHAVIOR ANALYSIS ON FLUID–SOLID COUPLING FOR OSTEONS IN VARIOUS GRAVITATIONAL FIELDS

Background: Compact bone mainly consists of cylindrical osteon structures. In microgravity, the change in the mechanical microenvironment of osteocytes might be the root cause of astronauts’ bone loss during space flights.

Methods: A multi-scale three-dimensional (3D) fluid–solid coupling finite element model of osteons with a two-stage pore structure was developed using COMSOL software based on the natural structure of osteocytes. Gradients in gravitational fields of [Formula: see text]1, 0, 1, 2.5, and 3.7[Formula: see text]g were used to investigate the changes in the mechanical microenvironment on osteocyte structure. The difference in arteriole pulsating pressure and static compression stress caused by each gravity gradient was investigated.

Results: The mechanical response of osteocytes increased with the value of g, compared with the Earth’s gravitational field. For instance, the fluid pressure of osteocytes and the von Mises stress of bone matrix near lacunae decreased by 31.3% and 99.9%, respectively, in microgravity. Under static loading, only about 16.7% of osteocytes in microgravity and 58.3% of osteocytes in the Earth’s gravitational field could reach the fluid shear stress threshold of biological reactions in cell culture experiments. Compared with the Earth’s gravitational field, the pressure gradient inside osteocytes severely decreased in microgravity.

Conclusion: The mechanical microenvironment of osteocytes in microgravity might cause significant changes in the mechanical microenvironment of osteocytes, which may lead to disuse osteoporosis in astronauts.

The lack of mass transfer in bone lacunar-canalicular system may be the decisive factor of osteoporosis under microgravity

During spaceflight, astronauts experience 1-1.5% bone loss per month, especially in the lumbar spine, pelvis and lower limbs. The bone loss leads to osteoporosis and increased the risk of fracture. Current researches focus on anti-osteoporosis under microgravity mainly by inhibiting bone resorption of osteoclasts and / or increasing bone formation of osteoblasts. However, studies on the effects of mass transfer in the bone lacunar-canalicular system (LCS) on osteoporosis are lacking. Osteocytes reside in the lacunae and communicate with other osteocytes, osteoblasts and osteoclasts through the LCS in the bone matrix. Osteocytes are mainly responsible for mechanosensing and signal regulation in bone, and the LCS is the basic structure for signaling, mass transfer and mechanical stimulation. Microgravity causes deficient mass transfer in the LCS, especially in the outer layer of osteon. Osteocytes far away from the Haversian canals are inhibited or accelerated apoptosis to stimulate osteoclasts which result in bone loss. Deficient mass transfer in the LCS may be a determinant of human osteoporosis under microgravity, which will open up a new way to treat osteoporosis in space.