What feature helps distinguish reactive bone from neoplastic invasion of the endosteal region?

• A. A single layer of plump osteoblasts lining reactive bone
• B. Multiple layers of osteoblasts
• C. A thick, pleomorphic neoplastic cell population
• D. Absence of osteoblasts

Answer: (B)

Reactive bone formation is driven by osteoblastic activity along the endosteal surface. When this response is robust, you can see several layers of osteoblasts lining the endosteal rim, indicating a strong, non-neoplastic repair process. Neoplastic invasion disrupts the normal architecture and is characterized by a thick, pleomorphic population of tumor cells with little to no organized osteoblastic lining. So, observing multiple layers of osteoblasts along the endosteal surface best distinguishes reactive bone from malignant invasion. A single layer can occur with reactive bone but is less definitive, and a thick pleomorphic neoplastic population or absence of osteoblasts points toward neoplasia rather than a reactive process.