Building bone marrow-on-a-chip

Researchers from Harvard University’s Wyss Institute for Biologically Inspired Engineering have created a new organ-on-a-chip - a microfluidic device that mimics the physiology of a living organism. The device reproduces the structure, functions and cellular make-up of bone marrow.

“Bone marrow is an incredibly complex organ that is responsible for producing all of the blood cell types of our body, and our bone marrow chips are able to recapitulate this complexity in its entirety and maintain it in a functional form in vitro,” said Dr Don Ingber, founding director of the Wyss Institute and senior author of the study.

Writing in the journal Nature Methods, the researchers stated, “Current in vitro hematopoiesis models fail to demonstrate the cellular diversity and complex functions of living bone marrow; hence, most translational studies relevant to the hematologic system are conducted in live animals.” But as co-author Dr Yu-suke Torisawa noted, human and animal responses are totally different. The team thus set out to engineer artificial bone with living marrow.

The task would be a complex one, as bone marrow has an integral relationship with bone. Marrow sits inside trabecular bone - a solid-looking type of bone with a porous, honeycombed interior. Inside, conditions vary between areas, and the dozen or so cell types each have their own preferred spots. Furthermore, bone marrow cells communicate with each other by secreting and sensing a variety of biomolecules, which act locally to tell them whether to live, die, specialise or multiply.

“In order for bone marrow to produce all of its functions, all these little areas and niches serve a role,” said co-author Catherine S Spina. “Certain cells line certain parts of the bone marrow, so if you don’t have the proper bone architecture, you will not have the proper physiology.”

“Most organs on chips, we simplify down to the most basic elements and we build back,” explained Dr Ingber. “Here, we have an incredibly complex system and we maintain the complexity in vitro.”

Rather than trying to reproduce such a complex structure cell by cell, the researchers enlisted mice to do it. Spina and Dr Torisawa packed dried bone powder into an open, ring-shaped mould the size of a coin battery and implanted the mould under the skin on a mouse’s back. After eight weeks, they surgically removed the disc-shaped bone that had formed in the mould. It revealed a honeycomb-like structure that looked identical to natural trabecular bone.

Microscopic view of the engineered bone with an opening exposing the internal trabecular bony network, overlaid with coloured images of blood cells and a supportive vascular network that fill the open spaces in the bone marrow-on-a-chip. Credit: James Weaver, Harvard’s Wyss Institute.

When the researchers stained the tissue and examined it under a microscope, the marrow was packed with blood cells, just like marrow from a living mouse. And when they sorted the bone marrow cells by type and tallied their numbers, the mix of different types of blood and immune cells in the engineered bone marrow was identical to that in a mouse thighbone.

To sustain the engineered bone marrow outside of a living animal, the researchers placed the engineered bone in a microfluidic device that steadily supplied nutrients and removed waste to mimic the circulation the tissue would experience in the body. The finished product, bone marrow-on-a-chip, gives scientists a much-needed new tool to test the effects of new drugs and toxic agents on whole bone marrow, remaining healthy for up to one week.

The device could be used to develop safe and effective strategies to prevent or treat radiation’s lethal effects on bone marrow without resorting to animal testing. In an initial test, the engineered bone marrow, like human marrow, withered in response to radiation unless a drug known to prevent radiation poisoning was present.

Bone marrow-on-a-chip could also generate blood cells, which could circulate in an artificial circulatory system to supply a network of other organs-on-chips. The Defense Agency Advanced Research Projects Agency (DARPA) is currently providing funds for the Wyss Institute to develop such a system - an interconnected network of 10 organs on chips to study complex human physiology outside the body.

“You can imagine a system in which we connect individual organs, including the heart and the lungs, and we use the bone marrow-on-a-chip to produce the blood that circulates through all the organs, thus connecting the system and really recreating human physiology,” said Spina.

Other potential uses include maintaining a cancer patient’s own marrow temporarily while they undergo marrow-damaging treatments such as radiation therapy or chemotherapy; and growing human bone marrow in immune-deficient mice.

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