Osteoblasts are derived from mesenchymal stem cells (MSCs), which initiate and regulate bone formation. New strategies for osteoporosis treatments have aimed to control the fate of MSCs. While functional disuse decreases MSC growth and osteogenic potentials, mechanical signals enhance MSC quantity and bias their differentiation toward osteoblastogenesis. Through a non-invasive dynamic hydraulic stimulation (DHS), we have found that DHS can mitigate trabecular bone loss in a functional disuse model via rat hindlimb suspension (HLS). To further elucidate the downstream cellular effect of DHS and its potential mechanism underlying the bone quality enhancement, a longitudinal in vivo study was designed to evaluate the MSC populations in response to DHS over 3, 7, 14, and 21 days. Five-month old female Sprague Dawley rats were divided into three groups for each time point: age-matched control, HLS, and HLS+DHS. DHS was delivered to the right mid-tibiae with a daily "10 min on-5 min off-10 min on" loading regime for five days/week. At each sacrifice time point, bone marrow MSCs of the stimulated and control tibiae were isolated through specific cell surface markers and quantified by flow cytometry analysis. A strong time-dependent manner of bone marrow MSC induction was observed in response to DHS, which peaked on day 14. After 21 days, this effect of DHS was diminished. This study indicates that the MSC pool is positively influenced by the mechanical signals driven by DHS. Coinciding with our previous findings of mitigation of disuse bone loss, DHS induced changes in MSC number may bias the differentiation of the MSC population towards osteoblastogenesis, thereby promoting bone formation under disuse conditions. This study provides insights into the mechanism of time-sensitive MSC induction in response to mechanical loading, and for the optimal design of osteoporosis treatments. Keywords: bone adaptation; mechanical loading; noninvasive stimulation; osteoporosis; osteopenia Bone Research (2013) 1: 98-104. doi: 10.4248/BR201301007 IntroductionBone loss due to functional disuse osteopenia, classified as secondary osteoporosis ( 1), leads to osteoporosisrelated fractures and high medical costs (2, 3). While millions of people are affected by these conditions, patients subjected to prolonged immobility or bed-rest (e.g., due to spinal cord injury, and nonunion), as well as (6,12). Moreover, more complex 3D systems have been used by tissue engineers to explore the role of mechanical forces on MSC adhesion and differentiation, as well as promoting the growth of both bone and cartilage (13)(14)(15)(16)(17)(18)(19). In vivo studies using mechanical stimulation, such as whole body vibration, have also demonstrated their effects on MSC proliferation and differentiation toward osteogenesis (20)(21)(22).Mechanical signals direct MSCs toward osteoblastogenesis and are anabolic to bone, while a reduction in mechanical forces on bone leads to adverse outcomes. In vitro simulated microgravity conditions, mimicking the reduced mecha...

Osteoporosis is a debilitating disease characterized as decreased bone mass and structural deterioration of bone tissue. Osteoporotic bone tissue turns itself into altered structure, which leads to weaker bones that are more susceptible for fractures. While often happening in elderly, long-term bed-rest patients, e.g. spinal cord injury, and astronauts who participate in long-duration spaceflights, osteoporosis has been considered as a major public health thread and causes great medical cost impacts to the society. Mechanobiology and novel stimulation on regulating bone health have long been recognized. Loading induced bone fluid flow, as a critical mechanotransductive promoter, has been demonstrated to regulate cellular signaling, osteogenesis, and bone adaptation [4]. As one of the factors that mediate bone fluid flow, intromedullary pressure (ImP) creates a pressure gradient that further influence the magnitude of mechanotransductory signals [5]. As for a potential translational development of ImP, our group has recently introduced a novel, non-invasive dynamic hydraulic stimulation (DHS) on bone structural enhancement. Its promising effects on inhibition of disuse bone loss has been shown with 2 Hz loading through a 4-week hindlimb suspension rat study followed by microCT analysis. At the cellular level, mesenchymal stem cells (MSCs) are defined by their self-renewal ability and that to potentially differentiate into the cells that form tissues such as bone [1]. To further elucidate the cellular effects of DHS and its potential mechanism on bone quality enhancement, the objective of this study was to measure MSC quantification in response to the in vivo mechanical signals driven by DHS.

This study attempted to compare the muscle fiber morphological responses to dynamic electrical muscle stimulation (DEMS) and dynamic hydraulic stimulation (DHS) in rats under hindlimb suspension (HLS). DEMS at 1 Hz, 50 Hz and 100 Hz for 10 min/day, 5 days/week were introduced to the animals’ right quadriceps. Static and 2 Hz DHS were introduced to the right tibiae of other animal groups on a "10 min on – 5 min off – 10 min on" loading regime for 5 days/week. In the end of the 4-week experiments, histological changes in the corresponding soleus, gastrocnemius and quadriceps of the stimulated sites were examined. Compared to age-matched, HLS led to muscle atrophy and strongly reduced muscle wet weights and averaged cross-sectional fiber areas. Among the tested DEMS frequencies, the averaged cross-sectional quadriceps fiber area in the 50 Hz group was 29 % larger than the 100 Hz group. In contrast, difference in the muscle fiber response to the static and 2 Hz DHS was not observed in either soleus or gastrocnemius. Muscle fiber morphological responses to the active DEMS was in a load frequency dependent manner under disuse condition. Relatively passive compressions, either via static or 2 Hz DHS, were unable to induce any difference in the muscle fiber responses under functional disuse.