Les hormones et le tissu osseux

Growth hormone from the pituitary gland controls bone growth by triggering chondrocyte proliferation in epiphyseal plates, increasing long bone length. It also enhances calcium retention for mineralization, stimulates osteoblastic activity to improve bone density, and triggers liver production of insulin-like growth factors that facilitate osteoblast protein synthesis necessary for bone formation.

During puberty, testosterone in males and estrogen in females promote osteoblastic activity and bone matrix production, causing the growth spurt typical of adolescence. These hormones also convert the epiphyseal plate to the epiphyseal line, transforming cartilage into its bony remnant and ending longitudinal bone growth.

Parathyroid hormone regulates calcium homeostasis by stimulating osteoclast proliferation and activity, releasing calcium from bones into circulation. PTH also promotes calcium reabsorption by kidney tubules and stimulates vitamin D synthesis, indirectly increasing calcium uptake by the small intestine to maintain blood calcium balance.

Calcitonin and parathyroid hormone have opposing functions in maintaining calcium homeostasis. While PTH stimulates osteoclasts to release calcium into blood, calcitonin inhibits osteoclast activity and stimulates calcium uptake by bones, reducing blood calcium concentration. These hormones are generally not secreted simultaneously.

Growth hormone and thyroxine from the thyroid gland are essential for bone matrix synthesis. Thyroxine promotes osteoblastic activity and bone matrix production. Sex hormones during puberty also enhance osteoblastic activity and matrix production, while calcitriol, the active form of vitamin D, stimulates calcium and phosphate absorption necessary for mineralization.

In pregnant women, calcitonin secreted by the thyroid gland's C-cells facilitates calcium ion transfer from the mother's body to the fetus, supporting fetal skeletal development and bone formation. This hormone ensures adequate calcium availability for proper mineralization and growth during pregnancy.

Parathyroid hormone and calcitonin are the primary hormones regulating bone remodeling and resorption. PTH stimulates osteoclast proliferation and activity to resorb old or damaged bone, while calcitonin inhibits osteoclast activity. Together, these hormones balance bone resorption by osteoclasts with new bone formation by osteoblasts.