Bone Marrow Model Created Entirely from Human Cells

Typically, bone marrow research relies heavily on animal models and oversimplified cell cultures in the laboratory. Now, researchers from the Department of Biomedicine at the University of Basel and University Hospital Basel have developed a realistic model of bone marrow engineered entirely from human cells. Derived using human induced pluripotent stem cells (hiPSCs) and macro-scale porous hydroxyapatite scaffolds, the engineered vascularized osteoblastic niche (eVON) model may become a valuable tool not only for blood cancer research, but also for drug testing and potentially for personalized therapies. The researchers suggest the novel system could reduce the need for animal experiments for many applications.

The research team, headed by Professor Ivan Martin, PhD, and Andrés García-García, PhD, reported on their achievement in Cell Stem Cell. In their paper, titled “Macro-scale, scaffold-assisted model of the human bone marrow endosteal niche using hiPSC-vascularized osteoblastic organoids,” the team stated, “The described eVON model addresses some of the current limitations in the development of uniform, durable, and reproducible human organoids toward enhanced relevance in disease modeling and drug screening.”

Our body’s “blood factory” consists of specialized tissue made up of bone cells, blood vessels, nerves, and other cell types. The bone marrow usually works quietly in the background. It only comes into focus when something goes wrong, such as in blood cancers. In these cases, understanding exactly how blood production in our body works and how this process fails becomes critical.

Bone marrow is not uniform; it is made up of several specialized microenvironments, also known as niches. “The bone marrow (BM) niches regulate blood cell production by providing signals that influence hematopoietic stem and progenitor cell (HSPC) maintenance, differentiation, self-renewal, and migration,” the authors explained. One niche that is particularly important for blood formation and is related to blood cancer’s resistance to therapies is located close to the bone surface. This endosteal niche includes blood vessels, bone cells, nerves, and immune cells.

Research in murine models has indicated the important contribution that BM endosteal niches play in a diverse range of processes, including blood cancers and solid tumor metastasis, aging, and aberrant osteogenesis, the authors noted. However, they pointed out, “Despite this cumulative evidence, human BM endosteal niches and the role of their associated vasculature remain poorly investigated. This is partly because until now, there had been no human bone marrow model that included all these cellular components. “A critical hurdle is the scarcity of standardized bioengineered models recapitulating this specific microenvironment in entirely human settings,” the investigators continued.

Martin, García-García, and colleagues have now successfully created such a model. The basis for this complex tissue was an artificial bone structure made from hydroxyapatite, a natural component of bones and teeth, and human-induced pluripotent stem cells. These artificially developed stem cells can produce different specialized cell types depending on the signals they receive in their environment.

In their paper, the team pointed out that the first two BM organoid models based on hiPSCs had recently reported and shown to successfully recapitulate key features of human adult BM while modeling both healthy and malignant hematopoiesis. They further commented, “Despite their innovative character, these BM organoids lack the bone compartment and are thus not suitable to properly model human BM endosteal niches.” The models were also only created at the micrometer scale, “rendering the vascular structures non-physiological in shape and size,” and used Matrigel, “… which introduces mouse-derived proteins into the system.”

To generate their new human cell-based eVON model, the researchers integrated the stem cells into the artificial bone structure and guided them through specific differentiation processes to produce a wide range of bone marrow cell types in a reproducible and controlled way.

Subsequent analyses confirmed that this three-dimensional construct closely resembles the human endosteal niche and is larger than previous systems, measuring 8 mm in diameter and 4 mm in thickness. The described model also allowed the researchers to sustain human blood formation in the laboratory for weeks. “… we present a developmentally guided approach combining hiPSC-derived organoids with macro-scale hydroxyapatite scaffolds to generate a standardized model capturing compositional, structural, and functional features of the human BM endosteal niche (hereafter referred to as engineered vascularized osteoblastic niche [eVON]),” they wrote in summary. “The system recapitulates key physiological features of native endosteal niches, whereby diverse cellular subsets exchange specific molecular signals regulating HSPC maintenance and lineage specification.”

Martin added, “We have learned a great deal about how bone marrow works from mouse studies. “However, our model brings us closer to the biology of the human organism. It could serve as a complement to many animal experiments in the study of blood formation in both healthy and diseased conditions”. The system could also be used in drug development in the future. “However, for this specific purpose, the size of our bone marrow model might be too large,” explained García-García. In order to test multiple compounds and doses in parallel, the model would need to be miniaturized.

In the long term, the use of the model in the definition of personalized treatments for blood cancers is also conceivable, with individual bone marrow models being generated using patients’ cells in order to test different therapies and select the most effective one for each patient. However, the researchers acknowledge that this will also require further development.

“Our results showed that the eVON can be robustly generated from at least three different hiPSC lines in chemically defined and entirely human settings,” the investigators noted. “The combination of this robust long-term culture with gene editing technologies could provide a platform to study human malignant hematopoietic processes that are challenging to replicate in animal models due to the lack of a human BM microenvironment.”