Which type of cells appear on the bone surface during the reversal phase of bone remodeling?

• A. Osteoclasts
• B. Mononuclear cells
• C. Osteoblasts
• D. Chondrocytes

Answer: (B)

During the reversal phase of bone remodeling, mononuclear cells are essential for the process. These cells are primarily derived from the monocyte/macrophage lineage and play a critical role in setting the stage for new bone formation. After the resorption phase, where osteoclasts break down bone matrix, the presence of mononuclear cells indicates that the area is prepared for subsequent bone formation.

Mononuclear cells function by secreting signaling molecules and growth factors that help recruit osteoblasts, which are responsible for new bone formation. This makes them pivotal to the transition between bone resorption and formation, as they help ensure that the process of remodeling is coordinated effectively.

In contrast, osteoclasts are involved in the resorption phase, osteoblasts are responsible for new bone formation in the following phases, and chondrocytes are primarily associated with cartilage rather than directly involved in the remodeling of bone. Therefore, the correct identification of mononuclear cells as key players during the reversal phase underscores their role in the continual cycle of bone remodeling.