The skeleton's secret endocrine life. Bone remodels itself under hormonal control — and talks back, releasing osteocalcin and FGF23 as hormones of its own.
The remodeling unit is the endocrine stage: osteoclasts resorb, osteoblasts refill, osteocytes sense mechanical load and orchestrate. Osteoblasts embed themselves as osteocytes in a lacunar network — the skeleton's wired sensor grid. This tissue's mechanical role is covered at the musculoskeletal system; its hormonal output is the story here.
Hormones & Regulation
Inbound control: parathyroid hormone raises calcium by bone resorption; calcitonin modestly opposes.
Estrogen restrains remodeling — its menopausal loss releases osteoclasts into the osteoporosis pattern.
Outbound signals: osteocalcin from osteoblasts and FGF23 from osteocytes, both hormones in their own right.
Physiological Role
Calcium banking: 99 percent of body calcium exchanged with blood on hormonal command.
Osteocalcin acts on pancreas and fat — in mice, better insulin secretion and sensitivity; the human physiology is under study.
FGF23 tells the kidney to excrete phosphate — bone as an endocrine regulator of mineral balance.