Bone research reveals skeleton as active organ communicating with body

Bone research reveals skeleton as active organ communicating with body

For decades, the human skeleton was viewed primarily as a structural framework that provides support and stores minerals. However, research from the last two decades is shifting this perspective, revealing that bones are dynamic, living tissues that engage in complex chemical conversations with other organ systems, including the brain, kidneys, and gut microbiome.

Beyond its role as a physical scaffold, bone functions as an active tissue that regulates mineral balance and supports the production of blood cells. Within the bone marrow, the body produces platelets for clotting, white blood cells for fighting infection, and red blood cells to transport oxygen. This makes the skeleton intrinsically linked to the immune and circulatory systems, as the marrow adjusts blood cell production in response to inflammation, infection, and blood loss.

The Cellular Engine of Bone

Bone is continuously being broken down, rebuilt, and reshaped through a process called remodeling. This activity is managed by three primary types of cells:

• Osteoblasts: Cells that build new bone.
• Osteoclasts: Cells that remove old or damaged bone.
• Osteocytes: Embedded cells that sense mechanical strain and coordinate the skeletal response.

While osteocytes make up 90 percent or more of bone cells, their complex roles were largely overlooked until roughly 20 years ago. According to Lynda Bonewald of Indiana University, these cells do more than sense mechanical load. Her research, first reported in 2006, found that osteocytes produce a growth factor called FGF23. This molecule travels through the bloodstream to the kidneys; if there is too much FGF23, the kidneys release excessive phosphorus into urine, which can lead to dental problems and softened bones.

A Two-Way Conversation with Organs

The skeleton does not only receive signals; it sends them. Gerard Karsenty of Columbia University discovered that bones produce a protein called osteocalcin, which influences energy metabolism. In studies involving mice, Karsenty found that those lacking osteocalcin had difficulty regulating blood sugar. He also linked the protein to improved learning and memory through brain neurotransmitter alterations, boosted muscle function during exercise, and male fertility via sex hormone production.

Karsenty suggests this may be part of a survival mechanism, stating:

"Bone may be an organ defining a physiology of danger,"

Gerard Karsenty, Columbia University, via Smithsonian Magazine

Other organs similarly influence the bone. Fat cells produce the hormone leptin, which Karsenty found can put the brakes on bone remodeling via the brain. In experiments with children who had a genetic mutation causing a lack of fat cells and leptin, their bones appeared months or years older in X-rays, possessing higher density than typical for their age.

Muscle and Microbiome Interactions

The relationship between bone and muscle is both physical and chemical. While muscles tug on bone to make it stronger, muscle cells also release myostatin to keep bone mass in check. During exercise, muscles produce a molecule called beta-aminoisobutyric acid (BAIBA), which Lynda Bonewald found protects osteocytes from reactive oxygen species. Another exercise-related molecule, irisin, promotes bone remodeling and helps osteocytes survive.

The gut microbiome also plays a surprising role. A 2012 study of sterile mice showed they had fewer osteoclasts and higher bone mass. When microbes were introduced, bone mass returned to normal. Microbes produce short-chain fatty acids, such as butyrate, which facilitate the conversation between the parathyroid glands and the bone. Furthermore, Christopher Hernandez of Cornell University found that antibiotics can make bones brittle by reducing the amount of vitamin K produced by gut bacteria, which alters the shape of mineral crystals in the bone.

Clinical Implications and Future Treatments

These discoveries are opening new therapeutic avenues for conditions like osteoporosis, which affects nearly 13 percent of Americans over 50, according to the Centers for Disease Control and Prevention. Because current medications can have side effects, physician-scientists are looking for "creative" alternatives.

Potential new strategies include:

• Probiotics: Using bacteria like Lactobacillus reuteri to protect against bone loss.
• Beta Blockers: Sundeep Khosla of the Mayo Clinic is studying whether drugs that block beta-adrenergic receptors, typically used for blood pressure, can keep bones strong. He is currently running a study with 420 post-menopausal women using atenolol.
• Senolytics: Drugs that eliminate old, senescent osteocytes that produce inflammation. Khosla is testing these in a trial with 120 women aged 70 or older.

While doctors treat more than 6 million bone fractures in the United States annually, the focus of skeletal health is expanding. Researchers are moving beyond simple bone density to study bone quality and the integrated way the skeleton communicates with the rest of the body to maintain overall health.