Final answer:
The differentiation of pre-osteoclasts into osteoclasts is stimulated by parathyroid hormone (PTH) and requires the presence of macrophage colony-stimulating factor (M-CSF) as well.
Step-by-step explanation:
The differentiation of pre-osteoclasts into osteoclasts is primarily stimulated by the secretion of parathyroid hormone (PTH) from the parathyroid gland. When calcium levels are low and fail to bind to receptors on the parathyroid gland's cells, PTH is released. This hormone plays a crucial role in the regulation of calcium levels by stimulating osteoclast proliferation, enhancing bone resorption, and thereby raising calcium concentration in the blood. In addition, PTH stimulates the synthesis of vitamin D, which further aids in calcium absorption from digested food in the small intestine. Another factor that plays a part in this differentiation process is macrophage colony-stimulating factor (M-CSF), which promotes the production of monocytes and is key to the maturation of pre-osteoclasts into active osteoclasts.