Bone remodelling is a continuous, lifelong process where the skeleton demolishes and rebuilds itself. Discover the Basic Multicellular Unit (BMU), the RANKL axis, and why bone is never finished. ⏱ Chapters 0:00 Intro — a crew is cutting bone right now 0:27 Why bone remodels at all: structure, calcium, load 1:17 Modelling is not remodelling 2:20 Osteoclasts — sealing zone, ruffled border, Howship's lacuna 3:15 Osteoblasts — the build crew and osteoid 4:03 The osteocyte — the mechanosensor 4:55 The BMU and its five phases 5:58 Coupling — the crew that follows 6:49 Fast to demolish, slow to rebuild 7:58 Cortical tunnels, trabecular trenches 9:02 The control system: RANKL, RANK and OPG 10:02 Oestrogen holds the brake on 10:57 Every drug is an intervention in this cycle 12:04 Recap and sources This video provides a comprehensive breakdown of bone remodelling physiology, explaining how the skeleton handles structural maintenance, metabolic calcium demands, and mechanical adaptation. We explore the vital cellular anatomy of the BMU, differentiate remodelling from modelling, and explain how coupling imbalances drive metabolic bone disease and modern pharmacological targets. 🦴 IN THIS VIDEO YOU'LL LEARN • The three reasons bone remodels: removing fatigue damage, calcium supply, and mechanical adaptation • The critical distinction between coupled bone remodelling and independent bone modelling • The exact cellular anatomy and function of the Basic Multicellular Unit (BMU) • How osteoclasts create Howship's lacunae using a sealing zone and ruffled border • How osteoblasts deposit unmineralised osteoid at one to two micrometres per day, as a sheet • The ten-to-fifteen-day mineralisation delay of the osteoid seam — and why that lag matters • How the RANK/RANKL/OPG axis acts as the master regulator of bone resorption • How osteocytes sense load and change what they secrete — loading suppresses sclerostin • The clinical consequences of trabecular perforation and irreversible microarchitectural damage • Why understanding modelling explains the potent anabolic effects of modern osteoporosis drugs 📌 KEY TAKEAWAYS ✔️ Remodelling replaces bone without changing its shape, while modelling changes bone size and architecture ✔️ Bone resorption and formation are normally tightly coupled within the BMU to prevent net bone loss ✔️ The RANKL to OPG ratio strictly dictates the rate of osteoclast formation and bone resorption ✔️ Trabecular bone remodels at a significantly higher rate than cortical bone, making it more vulnerable to early osteoporosis ✔️ Uncoupling resorption from formation is the fundamental physiological mechanism underlying osteoporosis 👩⚕️ WHO THIS IS FOR This video is designed for orthopaedic trainees, junior doctors, and medical students seeking a deep understanding of bone physiology. It is also highly valuable for curious patients interested in how the skeletal system works. 📚 RELATED TOPICS: Osteoporosis and Metabolic Bone Disease Pathology · RANKL Pathway Pharmacology and Denosumab · Bone Composition and Macroscopic Structure · Osteocyte Mechanotransduction and Wolff's Law · Wnt Signalling and Sclerostin Inhibition · Calcium Homeostasis and Parathyroid Hormone 🔗 More free orthopaedic teaching at Orthovellum.com — subscribe for weekly videos. 🎨 CREDITS • Original Orthovellum production. ⚠️ DISCLAIMER: Educational only — not individual medical advice. Always consult a qualified clinician for diagnosis and treatment. #orthopaedics #boneremodelling #bmubiology #rankl #osteoporosis #bonemetabolism #osteoclast #osteoblast #orthovellum #medicaleducation #bonescience #wolffslaw #osteocytes