What triggers an increase in the number of osteoclasts in the body?

An increase in the number of osteoclasts is primarily triggered by low calcium levels in the blood. Osteoclasts are specialized cells responsible for bone resorption, which is the process of breaking down bone tissue to release calcium and phosphate into the bloodstream. When calcium levels drop, the parathyroid glands sense this deficiency and secrete parathyroid hormone (PTH). PTH then stimulates osteoclast differentiation and activity, leading to increased bone resorption, which helps elevate calcium levels back to a normal range.

In contrast, high calcium levels do not stimulate osteoclast formation; they would typically lead to a decrease in osteoclast activity, as the body aims to maintain calcium homeostasis. A surplus of vitamin D generally promotes the activity of osteoclasts but is not the initial trigger—its role is more about enhancing bone resorption in the presence of supportive signals like PTH. Growth hormone primarily influences growth and metabolism rather than directly regulating osteoclast numbers, although it does have indirect effects on bone remodeling.

Therefore, low calcium levels are the critical factor that drives an increase in osteoclasts, highlighting the body's intricate regulatory mechanisms for maintaining calcium homeostasis.