Fibrous Dysplasia Ground Glass

A 21-year-old female presents with 4 months of progressive left hip and thigh pain, worse on prolonged standing and relieved by rest. She has a subtle antalgic gait and a 1.2 cm leg length discrepancy. There are no café-au-lait macules, precocious puberty, or endocrine features. Plain radiograph of the left proximal femur shows an expansile, well-defined intramedullary lesion in the femoral neck and intertrochanteric region. The structure marked **A** in the diagram displays a characteristic hazy, ground-glass or smoky matrix appearance with gentle endosteal scalloping, a thick sclerotic rim, and no cortical destruction or periosteal reaction. Which of the following best describes the histological composition of the matrix lesion marked **A**?