A 15-year-old boy presents with 6 weeks of progressive pain and swelling around the right knee, worse at night. Clinical examination reveals a firm, tender, warm mass over the distal thigh. Laboratory investigations show elevated LDH (720 U/L) and alkaline phosphatase (480 U/L). Plain radiographs of the right knee demonstrate a mixed lytic-sclerotic metaphyseal lesion of the distal femur with cortical destruction and a periosteal reaction. The structure marked **A** in the diagram shows the characteristic radiologic finding of periosteal elevation at the tumour margin. Which of the following best describes the pathologic basis of the radiologic appearance marked **A**?