Bones are the skeleton that supports us and allows us to perform nearly every daily task, whether it is going on a morning run or hugging someone we love. They are the structural scaffold that protects our organs and provides a surface for our muscles to move upon, but there is a hidden life of bones that most of the world is unaware of. Instead of picturing bone as a rigid frame that simply provides structure to our bodies, imagine a living organ that stores nutrients, creates blood, and a tissue with cell populations in constant communication and continuously adapting to its environment.
The hardness of bone comes from a calcified matrix that is secreted by bone cells. However, there are several types of bone cells, not all of which build bone. Some people may have heard of bone-forming osteoblasts and bone-resorbing osteoclasts, but the most numerous bone cells, comprising over 90% of all the cells in bone, are the osteocytes. Osteocytes are fully matured osteoblasts that become embedded inside the mineralized bone matrix and have developed long, thin, dendritic processes (cellular extensions) emanating from their cell bodies. Osteocytes have around 50 dendrites per cell, and these dendrites form a complex and interconnected network for cell-to-cell signaling and communication, creating a neuronal-like environment inside our bones (see graphic).
For many years osteocytes were considered merely placeholder cells, living out their long lifespan caught in the meshwork of bone matrix, contributing minimally to the functioning of bone. However, we now know that osteocytes live a secret, incredibly critical life inside of our bones.
If osteoblasts are the foundation laying team and osteoclasts are the demolition team, osteocytes are the construction site foremen. They are able to sense small changes in their environment, like a spider sensing a fly caught in their web. Within the hardened web of bone matrix, osteocytes reside in a fluid-filled network of channels (known as the lacunocanalicular network), surrounded by a thin layer of fluid that changes its flow when pressure or force is applied to the bone. Fluid flow is extremely important for nearly all cells to function, as it provides a means of obtaining nutrients and removing waste products. This is also true for osteocytes, but fluid flow has an additional role for these specialized cells. Osteocytes have numerous sensitive proteins that link the inner cytoplasm of the cell to the mineralized matrix; when fluid flow increases or decreases near the cells, osteocytes are able to sense that difference. Exercise increases the fluid flow within your bones, while bedrest or microgravity decreases it. Ever wonder why astronauts lose bone mass while they are in space? Osteocytes and their reduced mechanical stimulation due to low gravity is the answer!
As mechanical sensors, or mechanosensors, osteocytes translate physical force to biochemical signaling, changing their gene expression and secreted molecules in response to small changes in applied force. While this skill in itself is impressive, how does this mechanosensitivity impact other cells that are the major bone formers and resorbers? The osteocyte dendritic network does not just connect osteocytes to each other but also to osteoblasts and osteoclasts on the surface of the bone. Molecules secreted by osteocytes can be transported to other bone cells through this network and can signal directly to these cells. Orders from the construction foremen direct the actions of building and destruction on the bone worksite. Constant signaling is necessary to conserve the balance between bone growth and resorption. Breakdowns in communication from osteocytes to the other bone cells can lead to disease, including osteoporosis and osteopetrosis. When the radios on the construction site are only providing static, the workers don’t know the directions from the foreman and might accidentally demolish an important foundation. This loss of communication is one way you can end up with weak, osteoporotic bones or a heavy, overbuilt skeleton.
So, as you appreciate your ability to stand up out of bed in the morning and hit the weights at the gym, remember the secret lives of osteocytes and how your daily actions help to maintain the health and strength of your bones. Exercise regularly with fast, high-intensity activities like jumping jacks, and make sure that you consume a well-rounded diet with vitamins and minerals often found in fish and vegetables. Most of all, marvel at the wonder and secrets of the body and how all the hidden components, including bone cells, work to keep us alive and thriving.
Further reading:
If you are interested in learning more, The Amazing Osteocyte by Lynda Bonewald is a great place to start! (https://pubmed.ncbi.nlm.nih.gov/21254230/)
Author
Melia Matthews is a PhD candidate in Biomedical and Biological Science at Cornell University. She studies bone cell mechanobiology in the lab with advanced nanoparticle and imaging technology. Outside the lab, she loves communicating about science, spending time outdoors, and taking photos while scuba diving!
Editors
Sumbul Jawed Khan is the Assistant Editor-in-Chief at Club SciWri. She received her Ph. D. from the Indian Institute of Technology Kanpur, where she studied the role of the microenvironment in cancer progression and tumor formation. During her post-doctoral research at the University of Illinois at Urbana-Champaign, she investigated the gene regulatory networks important for tissue regeneration. She is committed to science outreach activities and believes it is essential to inspire young people to apply scientific methods to tackle the challenges faced by humanity. As an editor, she aims to simplify, translate, and excite people about current advances in science.
Roopsha Sengupta is the Editor-in-Chief at Club SciWri. She did her Ph.D. at the Institute of Molecular Pathology, Vienna, and post-doctoral research at the University of Cambridge UK, specializing in Epigenetics. During her research, she was involved in many exciting discoveries and had the privilege of working and collaborating with many inspiring scientists. As an editor for Club SciWri, she loves working on diverse topics and presenting articles coherently while nudging authors to give their best.
Illustrator
Andreia Rocha did her M.Sc. at Universidade do Algarve in Faro, Portugal, in Oncobiology and moved to Vienna to complete her thesis at IMBA where she studied stem cells and focused on working with organoids while using them as cancer models. Currently, she is a research assistant at JLP Health, a startup company based in Vienna, Austria. She is also passionate about communicating science through art and illustration and wishes to combine the two careers in the future. You can visit her website and follow her on instagram.
Cover Image- Andrea Rocha
Inset Image- Melia Matthews
This article was initially submitted to the ComSciCon Flagship 2023 Create-a-thon workshop and further improved by the editorial process at Club SciWri. Club SciWri and ComSciCon have a shared mission of helping scientists make science accessible for all.
