Near the end of adolescence, chondroblasts divide less often; the ______ thins then is replaced by bone.

• A. Epiphyseal plate
• B. Medullary cavity
• C. Periosteum
• D. Endosteum

Answer: (A)

Long bones grow in length at the epiphyseal plate, a region of cartilage between the ends (epiphyses) and the shaft (diaphysis). Chondroblasts in this plate continually divide to push the ends apart and create new cartilage, which is then replaced by bone through endochondral ossification. Near the end of adolescence, chondroblasts divide less often, causing the cartilage to thin and be progressively ossified. This replaces the growth plate with bone, and afterward a bony epiphyseal line remains where the plate once was. The other options—medullary cavity, periosteum, and endosteum—are not growing zones governed by chondroblast activity and do not thicken or thin in this process.