Developing bones can adapt their shape in response to mechanical stresses from neighbouring growing organs. In a new study, Koichi Matsuo and colleagues examine how bone-forming osteoblasts and bone-resorbing osteoclasts coordinate growth in the mouse fibula. They describe the process called ‘endo-forming trans-pairing’, where bone resorption by osteoclasts in the outer periosteum is paired with bone formation by osteoblasts in the inner endosteum to shape the growing bone. To learn more about the story behind the paper, we caught up with first author Yukiko Kuroda and the corresponding author Koichi Matsuo, Professor at the School of Medicine, Keio University, Japan.
Yukiko Kuroda (left) and Koichi Matsuo (right) in front of the nano-CT used
Koichi, can you give us your scientific biography and the questions your lab is trying to answer?
KM: I was an osteoclast biologist when I joined Keio University School of Medicine in the early 2000s. Transcriptional target gene screening led me to osteoclast-osteoblast interaction. Then, in 2009, I was introduced to X-ray microscopy and have been collaborating with an engineering professor ever since. The current biological question the lab is trying to answer is how the skeleton forms its characteristic shape.
Yukiko, how did you come to work in the lab?
YK: In my previous studies on calcium signalling, I had to use viral vectors to transfer genes into osteoclasts, so I came to the lab to learn the technique and naturally became interested in bone shape in vivo.
What is the background of the field that inspired your work?
KM: Observations down to the cellular level using X-ray microscopy at the SPring-8 synchrotron radiation facility in Japan have made it possible to correlate cells and bone morphology. To optimise resolution and field of view for experiments using X-ray microscopy, we decided to study small bones such as the fibula and auditory ossicles.
Can you give us the key results of the paper in a paragraph?
KM: During the course of a series of descriptive observations, it was after the intervention experiment of sciatic nerve transection that we realised there is a functional correlation between the intracortical canal structures and the endo-forming trans-pairing, as both canals and trans-pairing extended over the entire cortical bone.
Yukiko, when doing the research, did you have any particular result or eureka moment that has stuck with you?
YK: The eureka moment came when we found that the structure of cortical bone undergoing endo-forming and peri-forming trans-paring is different. That is, cortical bone formed by endo-forming trans-paring is rich in canal structures, whereas that formed by peri-forming trans-paring is poor in canal structures, as seen by X-ray and fluorescence light sheet microscopy.
Endo-forming trans-pairing involving periosteal osteoclasts (magenta) and endocortical osteoblasts (yellow). Nuclei (cyan) are also visualised.
Endo-forming trans-pairing involving periosteal osteoclasts (magenta) and endocortical osteoblasts (yellow). Nuclei (cyan) are also visualised.
And what about the flipside: any moments of frustration or despair?
YK: The frustration came when it was not clear how to understand the experimental results of the sciatic nerve transection. The fact that endo-forming trans-paring spread circumferentially despite reduced muscle compression was initially very difficult to explain.
Why did you choose to submit this paper to Development?
KM: The phenomenon of endo-forming trans-paring was described at the end of the 19th century and has fascinated scientists for more than 100 years. The 3D analysis of morphology has advanced dramatically with recent developments in computer technology, allowing the handling of large giga datasets. The combination of different modalities – such as X-ray tomography, fluorescence light sheet microscopy, classical histology and mouse genetics – allows quantitative and robust analysis. We wanted to publish our cutting-edge results in bone morphology in the prestigious journal Development.
The phenomenon of endo-forming trans-paring was described at the end of the 19th century and has fascinated scientists for more than 100 years
Yukiko, what is next for you after this paper?
In the present study, the fibula was used for high-resolution analysis, but generalisation to bones other than the fibula, and to bones from animals with osteons will be attempted. We will also investigate whether the nervous system is involved in trans-pairing.
Koichi, where will this story take your lab next?
This story can go some way to explaining the left-right symmetric morphogenesis of the skeleton. In the future, we would like to clarify the mechanism of symmetric morphogenesis of the skeleton.
Finally, let's move outside the lab – what do you like to do in your spare time?
YK: I like cooking.
KM: I enjoy gardening.
Laboratory of Cell and Tissue Biology, Keio University School of Medicine, 35 Shinanomachi, Shinjuku, Tokyo 160-8582, Japan.
E-mail: [email protected]
