Bone is never mechanically quiet. Walking, muscle activity and ordinary movement expose the skeleton to repeated loading, and bone cells respond to those physical cues as part of normal tissue maintenance. That makes mechanical loading of bone organoids an interesting problem for researchers trying to build more realistic in vitro models. A bone model can reproduce cell types, matrix formation and mineral deposition, but without a mechanical input it leaves out an important part of the environment those cells normally experience.
Estrogen adds another layer. Its decline after menopause is closely tied to changes in bone remodelling, osteoblast and osteocyte activity, vascular behaviour and mechanosensitivity. So the response to a given loading regime may not be the same when estrogen is withdrawn.
A 2026 study in the Journal of Biomechanics from Muhammad M.M. Bukhari, Syeda M. Naqvi and Laoise M. McNamara at the University of Galway examined that interaction using an advanced 3D vascularized bone organoid. The team compared static and mechanically stimulated constructs under continued estrogen supplementation and estrogen withdrawal. A CellScale MechanoCulture TX (MCTX) was used to apply cyclic compression during culture.
Rather than asking only whether loading increased mineral formation, the study followed changes in collagen, osteogenic markers, hypertrophy, apoptosis and vascularization. The pattern was not especially simple, which is part of what makes the work useful for bone organoid mechanobiology.
Why mechanical loading of bone organoids matters in an estrogen deficiency model
Mechanical loading and estrogen signalling overlap in bone biology. Bone cells sense deformation and fluid movement through mechanotransduction, then alter signalling, matrix production and remodelling behaviour in response. Estrogen can affect that sensitivity, so removing it changes more than a single biochemical variable.
This is relevant to bone tissue engineering as models become more biologically complex. A vascularized bone construct may contain several of the cell interactions researchers want to study, but its behaviour can still depend on how it is mechanically cultured. Mechanical loading of bone organoids gives researchers a way to examine that part of the environment directly rather than treating mechanics as an endpoint measurement.
The Galway group had previously developed a human vascularized and mineralized 3D bone model that reproduced aspects of estrogen withdrawal. In the new study, they kept the same general model but added controlled cyclic compression. That allowed them to ask a narrower question: does the tissue respond to mechanical stimulation differently when estrogen remains present versus when it is removed?
Building a vascularized model for mechanical loading of bone organoids
The model was built over several stages. Human bone marrow mesenchymal stem cells were first differentiated into a chondrogenic template. The template was then incorporated into a vascular construct containing additional bone marrow stromal cells and human umbilical vein endothelial cells, followed by culture in osteogenic media with estrogen.
This developmental sequence is worth noting. The researchers were not simply compressing freshly seeded cells in a scaffold. By the time mechanical loading of bone organoids began, the constructs had already spent 56 days progressing through chondrogenic and osteogenic culture.
That also puts the work close to two areas CellScale sees increasingly often: stem cell mechanobiology and organoid and tissue-mimetic systems. In both cases, the mechanical environment becomes another experimental variable layered onto cell differentiation, matrix formation and longer-term culture.
How the MechanoCulture TX was used for mechanical loading of bone organoids
At day 0 of the experimental phase, the mineralized constructs were split between static and stimulated groups, with estrogen either maintained or withdrawn. The stimulated samples were placed in the MechanoCulture TX (a six-well compression stimulation bioreactor) and loaded at 0.5% cyclic compressive strain.
The loading frequency was 1 Hz for one hour per day, five days per week, over 21 days. Each cycle included compression, a hold period and recovery, with the protocol reaching up to 1,800 cycles during the one-hour session.
This distinction between compression testing and mechanical stimulation is useful. The MCTX was not being used here to run a one-time mechanical characterization test. It provided repeated compression during culture so the researchers could compare biological responses after a defined loading history. For researchers working with similar constructs, CellScale’s compression testing overview covers the broader mechanics of compressive loading, while this study shows how repeated compression can be incorporated into an incubator-based culture protocol.
The 0.5% value also refers to the imposed construct-level strain. As the authors note later in the paper, the organoids mineralized and changed mechanically over time, so the local strain experienced by individual cells would not necessarily have remained uniform across the construct.
Mechanical loading of bone organoids with estrogen still present
The estrogen-supplemented groups provide a useful reference before looking at estrogen withdrawal.
Under continued estrogen supplementation, mechanical stimulation increased calcium content at both day 10 and day 21 compared with static controls. By day 21, ALP activity was also higher in the stimulated group. Changes in osteogenic gene expression suggested that the response evolved over time rather than simply increasing every marker associated with bone formation.
That is a small but useful point. Early osteogenic markers are not expected to rise indefinitely as cells mature. In these constructs, the loading response looked more like a progression through stages of osteogenesis and mineral production.
This is the more familiar side of mechanical loading of bone organoids: a cyclic mechanical input is added to a developing bone-like tissue, and matrix and mineral-related readouts change over culture. The estrogen-withdrawal groups were less straightforward.
How did mechanical loading of bone organoids change after estrogen withdrawal?
When estrogen was removed, mechanically stimulated constructs produced substantially more collagen I than static estrogen-withdrawal controls at both measured time points. By day 21, the gene-expression pattern had also shifted. Runx2 and Opn, which are associated with earlier stages of osteogenic differentiation, were lower with stimulation, while DMP1 was higher.
The authors interpreted that pattern as accelerated osteoblast maturation. It does not mean that every osteogenic readout simply went up with loading. In fact, some of the clearest changes were decreases in earlier markers as the cells moved toward a more mature phenotype.
Mineralization continued as well. Calcium content in the stimulated estrogen-withdrawal constructs increased between day 10 and day 21. Viewed alongside the collagen and gene-expression data, mechanical loading of bone organoids under estrogen withdrawal appeared to push the tissue toward a different maturation state than the one seen under continued estrogen supplementation.
Why mechanical loading of bone organoids did not simply mean healthier bone
It would be easy to stop at the increase in collagen and mineral-related measurements and describe the mechanical loading as beneficial. The next part of the study makes that interpretation harder.
The researchers also measured collagen X, used here as a marker of hypertrophic differentiation, and cleaved caspase-3, associated with apoptosis. Under estrogen withdrawal, the constructs showed a pronounced hypertrophic and apoptotic phenotype as culture progressed. Those changes were present alongside the increased matrix production and osteoblast maturation.
So mechanical loading of bone organoids under estrogen deficiency did not recreate the same biological context as loading with estrogen present. The authors describe the estrogen-withdrawal response as being associated with pathological hypertrophy and apoptosis, rather than simply healthier or more mature bone.
That distinction matters for an in vitro osteoporosis model. Increased mineral deposition can sound inherently positive, but osteoporotic bone is not defined only by how much mineral is present. Mineral distribution, remodelling, cell survival and vascular behaviour can all change. This model appears to capture some of that messiness.
Mechanical loading of bone organoids also changed vascularization
Because this was a vascularized bone organoid, the researchers could also look at how the endothelial component responded.
CD31 staining was higher in mechanically stimulated constructs at day 21 under both estrogen-supplemented and estrogen-withdrawal conditions. The images suggest that mechanical loading was influencing the vascular compartment at the same time as the osteogenic compartment.
The signalling data were not a simple “more loading, more vascular growth” story. VEGF expression, for example, was lower in stimulated groups at day 21. That is another reason the model is interesting. Mechanical loading of bone organoids produced changes across several interacting cell populations, and not every marker moved in the same direction.
What this vascularized bone organoid model still leaves out
The model is more complex than a conventional monolayer or non-vascularized scaffold culture, but it is still a model.
For mechanical loading of bone organoids, one limitation here is the absence of osteoclasts. That means the system cannot reproduce the full balance between bone formation and bone resorption that becomes disrupted in osteoporosis. Estrogen withdrawal was also abrupt, whereas menopause involves a more gradual hormonal transition.
The loading protocol represents another simplification. The constructs received one hour of cyclic compression per day, five days each week. Native bone experiences a much less tidy mix of loading, unloading, rest and fluid movement. And because the constructs continued to mineralize and stiffen, a fixed macroscopic strain does not guarantee that cells throughout the organoid experienced the same local mechanical environment on day 21 as they did near the beginning.
Those limitations do not make mechanical loading of bone organoids less useful. They help define what this particular experiment can answer. Here, the setup isolates a controlled interaction between estrogen status and repeated compressive stimulation in a human vascularized bone model.
Related work in bone tissue engineering and mechanobiology
The study sits within a broader shift toward culturing engineered tissues under controlled mechanical conditions rather than measuring them only after culture. We have previously looked at an electrospun bone scaffold study in bone tissue engineering, where scaffold architecture and mechanical behaviour were central to the model, and at mechanical stimulation in tendon tissue engineering, where repeated loading was incorporated directly into the culture routine.
The tissue types and loading modes differ, but the broader question around mechanical loading of bone organoids and engineered tissues is related: when cells are building or remodelling tissue over days or weeks, the mechanical environment can affect what develops.
Using the MechanoCulture TX as a compression bioreactor
In this study, the MechanoCulture TX was used specifically for long-term cyclic compression of vascularized bone constructs. More generally, the MCTX is a six-well, incubator-compatible mechanical stimulation bioreactor for uniaxial compression of tissues, scaffolds, hydrogels and other 3D cultured constructs. Each well can be configured independently, and the system records force and displacement during stimulation so changes in construct behaviour can be followed over time.
For mechanical loading of bone organoids and other 3D cultures, the current MCTX supports cyclic compression up to 2 Hz with up to 2 mm of displacement. Researchers can use standard or perfusion well plates depending on how media handling fits into the culture workflow. It is primarily a stimulation bioreactor rather than an interactive mechanical tester, which is an important distinction when planning long-term mechanotransduction experiments.
The MCTX is one part of CellScale’s MechanoCulture series. The TX is used when the culture protocol calls for direct compression, while the J1 and T6 are used for tensile or cyclic strain stimulation and the TR applies hydrostatic pressure. In practice, the choice comes down to the mechanical cue the tissue model is meant to experience.
For bone organoids and other compression-responsive 3D constructs, mechanical loading of bone organoids with a system such as the MCTX can be incorporated into the culture period itself. The University of Galway study shows why the biological context still matters. The same nominal loading regime did not produce one universal “bone formation” response. Estrogen status changed the context in which the tissue responded, and the resulting differences appeared across matrix production, maturation, apoptosis and vascularization.
Citation
Muhammad M.M. Bukhari, Syeda M. Naqvi, Laoise M. McNamara, Mechanical loading under estrogen deficiency enhances mineralization and osteoblast maturation in an advanced 3D vascularized bone organoid driven by hypertrophy and apoptosis, Journal of Biomechanics, Volume 206, 2026, 113529, ISSN 0021-9290, https://doi.org/10.1016/j.jbiomech.2026.113529. (https://www.sciencedirect.com/science/article/pii/S0021929026003842)
