Bones provide the scaffold for all movements of vertebrate animals. Bone has been studied extensively, typically with the hope to understand how to better heal and grow this tissue; however, little is understood about its evolutionary origin. While our bones have osteocytes, which are cells embedded in our bones, not all species’ bones do. This leads paleontologists to wonder why early bony vertebrates developed osteocytic bone.
Yara Haridy, graduate student at the Museum für Naturkunde (Germany). Image credit: Pablo Castagnola.
Tell us about the laboratory where you are doing your graduate research.
Our overarching goals are to understand deep time development. We work on everything from early fish evolution to limb regeneration in amphibians. We use fossils as well as modern animals to bridge the gaps in our understanding of evolutionary changes in physiology. I study how our bones became the way they are and what steps in evolution contributed to the abilities our bones have today. I mainly focus on fossils, which means the organic components are long gone. I use cell spaces that are left behind to understand cell structure and I use rare pathologies in the bone to understand ancient healing processes.
Bothriolepis was a widespread, abundant and diverse genus of antiarch placoderms that lived approximately 420 million years ago. Image credit Brian Engh – dontmesswithdinosaurs.com
What led you to use focused ion beam scanning electron microscopy (FIB-SEM) to study ancient osteocytes?
Bone cells live inside caves created by the bone mineral. These caves are the exact replicas of the cell shape and are called lacunae. We were hoping to image and understand early osteocyte lacunae and overall early bone evolution in our ancient fish ancestors, Bothriolepis and Tremataspis mammillata, which lived approximately 420 million years ago.
However, the imaging technologies commonly used in the paleontological world – transmitted light microscopy, 2D transmission and scanning electron microscopy and micro CT – did not provide the necessary, high resolution 3D visualization I required. I was frustrated that this project would not progress and was considering abandoning this line of research.
Dr. Markus Osenber, Helmholtz Centre for Materials and Energy (HZB) (Germany) with a ZEISS Crossbeam FIB-SEM
At the same time, I was working with our collaborators in Dr. Ingo Manke’s group at the Helmholtz Zentrum Berlin (HZB). This group works in a completely unrelated research field – energy materials, fuel cells and batteries – but they also run the imaging facility and have been consulting with Dr. Witzmann for some time on micro CT scanning of our fossils. I was walking down the hall at the HZB and saw some amazing, high resolution, 3D images that looked topographically very similar to osteocyte caves. It turns out they were actually images of fuel cells and the technology used was FIB-SEM. This is what inspired me to try this technology on our fossils – before which had never been done.
What did you discover using focused ion beam scanning electron microscopy (FIB-SEM) on your fossils?
We found evidence of mineral metabolism in the earliest cellular bone tissues. One of the things that osteocytes can do is eat the bone around them and redistribute that mineral back into the bloodstream – a process called mineral metabolism. Your skeleton is a mineral storage unit and, if your organs are starving for certain minerals, it can be broken down to share those minerals. Using FIB-SEM, we were able to find evidence of mineral metabolism in this ancient fish. It is very likely this evolutionary advantage that bones with osteocytes became the prevailing model for millions of years after that.
Osteocyte image created from fossils of ancient fish bones. Sample courtesy of Yara Haridy and Dr. Florian Witzmann, Museum für Naturkunde (Germany). Image courtesy of Dr. Markus Osenberg, Helmholtz Centre for Materials and Energy (HZB) (Germany)
Where will this research go next?
We hope to apply FIB-SEM to other early fossil material to look at early tooth making cells, we also want to continue understanding ancient bone and cartilage better, and we even want to look at fossil feather structure. The only limit is the preservation of the fossil and the field of view, so we are looking at small specimens that can answer big questions.
Learn More
Read the full article: Bone metabolism and evolutionary origin of osteocytes: Novel application of FIB-SEM tomography Link
Learn more about focused ion beam solutions with ZEISS Crossbeam.