Which mechanism best explains PTH-induced activation of osteoclasts leading to bone resorption?

Which mechanism best explains PTH-induced activation of osteoclasts leading to bone resorption?

• A. Paracrine stimulation of osteoclasts by osteoblasts
• B. Paracrine stimulation of osteoblasts by osteoclasts
• C. Direct PTH activation of osteoclasts
• D. Elevated calcitonin promoting osteoclast activity

Answer: (A)

PTH promotes bone resorption by signaling to osteoblasts, which in turn activate osteoclasts in a paracrine fashion. When PTH binds to its receptors on osteoblasts/stromal cells, it increases the production of RANKL and decreases osteoprotegerin (OPG). RANKL then binds to RANK on osteoclast precursors, driving their differentiation and activating mature osteoclasts to resorb bone. This indirect pathway explains the effect of PTH on bone, since osteoclasts themselves do not have PTH receptors. Calcitonin would counteract osteoclast activity, not promote it, so that option isn’t correct.

PTH promotes bone resorption by signaling to osteoblasts, which in turn activate osteoclasts in a paracrine fashion. When PTH binds to its receptors on osteoblasts/stromal cells, it increases the production of RANKL and decreases osteoprotegerin (OPG). RANKL then binds to RANK on osteoclast precursors, driving their differentiation and activating mature osteoclasts to resorb bone. This indirect pathway explains the effect of PTH on bone, since osteoclasts themselves do not have PTH receptors. Calcitonin would counteract osteoclast activity, not promote it, so that option isn’t correct.