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The endocrine system produces and secretes hormones, which interact with the skeletal system. These hormones control bone growth, maintain bone once it is formed, and remodel it.
Hormones That Influence Osteoblasts and/or Maintain the Matrix
Several hormones are necessary for controlling bone growth and maintaining the bone matrix. The pituitary gland secretes growth hormone (GH), which, as its name implies, controls bone growth. This happens in several ways: first, it triggers chondrocyte proliferation in epiphyseal plates, increasing the length of long bones. GH also increases calcium retention, which enhances mineralization, and stimulates osteoblastic activity, which improves bone density.
GH not only stimulates bone growth and maintains osseous tissue, but it also stimulates the liver to produce insulin-like growth factors, which facilitate the activity of osteoblasts to make proteins necessary for bone formation.
Thyroxine, a hormone secreted by the thyroid gland, promotes osteoblastic activity and the synthesis of bone matrix. During puberty, the sex hormones (estrogen and testosterone) also come into play. They, too, promote osteoblastic activity and production of bone matrix and, in addition, are responsible for the growth spurt that often occurs during adolescence. They also boost the conversion of the epiphyseal plate to the epiphyseal line (i.e., cartilage to its bony remnant), thereby ending the longitudinal growth of bones. Additionally, calcitriol, the active form of vitamin D, is produced by the kidneys and stimulates the absorption of calcium and phosphate from the digestive tract.
Hormones That Influence Osteoclasts
Bone modeling and remodeling require osteoclasts to resorb unneeded, damaged, or old bone and osteoblasts to lay down new bone. Two hormones that affect osteoclasts are the parathyroid hormone (PTH) and calcitonin.
PTH stimulates osteoclast proliferation and activity. As a result, calcium is released from the bones into circulation, increasing the calcium ion concentration in the blood. PTH also promotes calcium reabsorption by the kidney tubules, which can affect calcium homeostasis.
The small intestine is also affected by PTH, albeit indirectly. Because another function of PTH is to stimulate the synthesis of vitamin D, and because vitamin D promotes intestinal absorption of calcium, PTH indirectly increases calcium uptake by the small intestine.
Calcitonin, a hormone secreted by the thyroid gland, has some effects that counteract those of PTH. Calcitonin inhibits osteoclast activity and stimulates calcium uptake by the bones, reducing the concentration of calcium ions in the blood. As evidenced by their opposing functions in maintaining calcium homeostasis, PTH and calcitonin are generally not secreted simultaneously.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 6.6: Exercise, Nutrition, Hormones, and Bone Tissue.
