MICROANATOMY

Detailed explanation-1: -Parathyroid hormone is directly involved in the bones, kidneys, and small intestine. In the bones, PTH stimulates the release of calcium in an indirect process through osteoclasts which ultimately leads to the resorption of the bones.

Detailed explanation-2: -Osteoblasts use excess calcium in blood to build new bone matrix. During this process, some bone cells become trapped as mature osteocytes. When blood calcium is low, PTH stimulates osteoclasts to break down bone matrix, releasing calcium into the blood.

Detailed explanation-3: -Parathyroid hormone (PTH) stimulates bone resorption by acting directly on osteoblasts/stromal cells and then indirectly to increase differentiation and function of osteoclasts. PTH acting on osteoblasts/stromal cells increases collagenase gene transcription and synthesis.

Detailed explanation-4: -The hormone calcitonin, which is produced by the parafollicular (or C) cells of the thyroid, has the opposite effect on blood calcium levels as PTH. Calcitonin decreases blood calcium levels by inhibiting osteoclasts, stimulating osteoblasts, and stimulating calcium excretion by the kidneys.

Detailed explanation-5: -PTH actions on osteoblasts An increase in osteoblast differentiation rather than osteoblast proliferation appears to be the main mechanism by which PTH stimulates osteoblastogenesis. Indeed, PTH arrests the cell cycle progression of osteoblasts, increasing their commitment to a differentiated osteogenic fate [69].